Gang Cheng • Drew A. Torigian • Abass Alavi
In any specific region of the skin or subcutaneous tissues, there is a preferred lymphatic drainage to which afferent lymphatic vessels drain first. That is, there is one preferred lymph node (the sentinel lymph node [SLN]) that will be the first to receive lymphatic drainage from a given regional lymph node basin. This also applies to lymphatic drainage of malignant tumor cells that spread in a stepwise manner from the primary site to the regional lymph nodes. The underlying rationale of sentinel lymph node biopsy (SLNB) is that when lymph node metastases occur, metastatic tumor cells drain via afferent lymphatics to a limited number of SLNs that are at highest risk for nodal metastasis because these lymph nodes represent the first nodal sites along the path of regional lymph drainage from the tumor (Fig. 28.1). Accordingly, examination of the tumor status of the SLN reflects the tumor status of the entire lymphatic drainage basin: A negative finding of SLN typically indicates no other nodal metastasis in the same drainage basin with a high degree of confidence whereas a positive finding of SLN indicates the likely involvement of other nonsentinel lymph nodes (nonSLNs) as well. Because only a limited number of SLNs are biopsied and examined, this procedure allows more focused and more intensive pathologic assessment including more sections and immunohistochemistry (IHC) evaluation of the regional tumor-draining nodes.
Morton et al.1,2 reported their pioneering development of intraoperative lymphatic mapping in the early 1990s. Their work began in 1985 and relied solely on vital dyes to identify the SLN. The initial description of SLNB by Morton et al. was rejected by several major medical journals before eventually being published in the Archives of Surgery. Since then, their work has become one of the most frequently cited in surgical oncology. Their technique gained widespread acceptance in the early 1990s followed by continuous modifications and improvements. In the early 1990s, a radioactive tracer was incorporated into the intraoperative procedure as a second mapping agent so that an SLN can be identified by blue staining and/or by increased radioactivity. The addition of a hand-held γ-probe3 made it more convenient to identify SLNs containing radioactivity.
As a multidisciplinary procedure, SLNB requires close cooperation between nuclear medicine physicians (for lymphatic mapping), surgeons (for identification and biopsy of the SLN), and pathologists (for histologic evaluation of the biopsied SLN). Generally, 99mTc-sulfur colloid is injected intradermally at the site near the primary tumor, with or without local anesthesia. This is often performed in the nuclear medicine department a few hours before surgery. Serial lymphoscintigraphy is obtained in the next few hours by the nuclear medicine physician, who may also mark the location(s) of detected SLN on the skin. Figure 28.2 shows lymphatic drainage in the first 20 minutes after the injection of 99mTc-sulfur colloid. Surgery can be performed on the same day or on the following day. During surgery, a surgeon uses a hand-held γ-probe moving over the regional nodal basin (with the help of lymphoscintigraphic findings and skin marking) to detect a “hot spot” of SLN. A surgeon may also intradermally inject a vital dye at the beginning of surgery, at the site of the primary tumor, often after the induction of anesthesia. The injected dye will migrate in a manner similar to the injected radiotracer, concentrate at sites of SLN, and is identified visually during surgery. After intradermal injection of vital dye at the primary site at the beginning of surgery, incisions are made over the regional nodal basin to follow the migration of the dye and thus the lymphatic drainage channels, to identify blue-stained SLN. The major drawback of vital blue dye is that it is dependent on visual identification. Thus meticulous dissection of subcutaneous tissues is required to follow dye migration to find SLN. The use of a radiolabeled agent makes it easier to identify SLN, because a hand-held γ-probe is often able to pinpoint the location of a “hot” node, eliminates the need for meticulous dissection of subcutaneous flaps, and can identify additional SLN at the same time. Local excision of the primary tumor is performed after excision of the SLN. Rescanning of relevant nodal basins with the hand-held γ-probe is performed again to ensure removal of all SLNs, especially ectopic lymph nodes. All SLNs excised are evaluated by pathologists for the presence of metastases, first by frozen sectioning and H&E staining during surgery which will generate the first diagnosis, and later by permanent sectioning and H&E staining, also with IHC staining using appropriate antibodies, to generate a final diagnosis. For patients with negative SLNB, no further surgery for regional nodal basins is needed. SLNB procedure guidelines are available for breast cancer4 and melanoma.5
FIGURE 28.1. Model of lymphatic mapping. Schematic of afferent lymphatic channels draining from the primary tumor to sentinel nodes in the regional nodal basin. A: A sentinel lymph node (SLN) without metastatic cells identified by the injected blue dye or 99mTc-colloid. B: An SLN with metastatic cells identified by the injected blue dye or 99mTc-colloid. C: An SLN with overloaded metastatic cells is not identified, because of blockage of afferent lymphatic flow or depleted nodal space. D: An in-transit node with metastatic cells identified by the injected blue dye or 99mTc-colloid. Please note that second-tier nonSLNs may be occasionally identified by pass through of the injected blue dye or radiolabeled colloid.
FIGURE 28.2. Serial lymphoscintigraphy for a 56-year-old female with right breast cancer. This patient received periareolar subdermal injection of filtered 99mTc-sulfur colloids, and serial planar images are shown at 1-minute interval (from 6 to 20 minutes). Lymphatic drainage pathway to the axillary basin is visualized clearly in the first few minutes and is then cleared, whereas activity in a hot spot of a right axillary sentinel lymph node persists.
SLNB allows focused assessment of regional lymph node involvement by removal and examination of only a few SLNs receiving lymph drainage from the site of the primary malignancy. It allows more precise nodal staging than clinical evaluation, without morbidity and tissue damage associated with complete lymphadenectomy as previously performed. SLNB is cost effective and offers an improvement in health outcomes. Currently, it has become the standard of care in patients with melanoma and breast cancer. The procedure applied to other solid cancers is under investigation. Over the past two decades, SLNB has gained widespread acceptance as a model procedure of minimally invasive oncologic surgery. A great deal of experience with SLNB has accumulated. SLNB has undergone continuous fine-tuning with improved performance. There is no consensus, however, regarding how the procedure should be performed. Controversies exist with regard to the selection of agents (vital dyes or radiolabeled agents), the size of labeled particles, the optimal site of injection, and the value of preoperative lymphoscintigraphy.
TECHNICAL CONSIDERATIONS OF SENTINEL LYMPH NODE BIOPSY
An ideal radiolabeled agent to identify SLN should permit good visualization of the lymphatic channels leading from the primary tumor site to the corresponding regional lymph nodes. The radiolabeled agent should clear fast enough from the injection site to allow scintigraphic visualization of SLN in minutes to hours, but is retained in the SLN long enough to allow intraoperative identification via a hand-held γ-probe. The clearance of colloidal radiolabeled agents through lymphatic channels and lymph nodes is dependent on the particle size.
Radiolabeled particles must be small enough to enter the lymphatic channels, yet not so small that they diffuse out of the interstitium into venous capillaries. Particles that are too large will migrate too slowly through the lymphatic channels, preventing sufficient accumulation in lymph nodes before imaging. Numerous radiolabeled agents have been used to detect SLNs.6 In the United States, 99mTc-sulfur colloid is the most commonly used radiolabeled agent for lymphoscintigraphic SLN detection. The particles of 99mTc-sulfur colloid vary significantly in size (from 15 to 5,000 nm, depending on preparation methods), with an average size range of 305 to 340 nm. In practice, 99mTc-sulfur colloid is filtered through a 0.22-μm filter to produce particles between 100 and 220 nm in size. In Europe, 99mTc-nanocolloidal albumin (Nanocoll, GE Healthcare) is the preferred agent with a particle size of 5 to 100 nm. 99mTc-antimony trisulfide is the most commonly used agent in Canada and Australia with a particle size of 3 to 30 nm.
All of these agents are able to detect SLN effectively and reliably, and the selection of a radiolabeled agent is often based more on local availability rather than on differences in performance. Although the success rate in the identification of SLN is not significantly affected by the particle size, knowledge of the effect of particle size on the detection of SLN is useful, because drainage and clearance by the lymphatic system varies with particle size. Small particles are drained and cleared first, whereas large particles are drained slowly and may have long retention at the injection site.6 Smaller particles have the advantage of rapid (in minutes) visualization of SLN, whereas larger particles have the advantages of longer retention in SLN to permit intraoperative detection the following day. Thus, the timing of preoperative lymphoscintigraphy and intraoperative detection of SLN should be adjusted accordingly. It is believed that radiocolloids with 100 to 200 nm particles represent the best compromise between an efficient lymphatic drainage and early scintigraphic visualization versus the need for satisfactory retention in SLN for intraoperative detection.6
A vital blue dye was the first marker used to identify SLN in melanoma.1 It was later used in patients with breast cancer and other malignancies.7 Different vital dyes are currently used, and the common ones include patent blue V, isosulfan blue, and methylene blue. Multiple studies have confirmed vital dyes as valid markers for SLN identification with high detection rates, close to those achieved by radiolabeled agents.8 In most cases, the same SLNs are detected by both blue dyes and by radiolabeled agents.9
It should be recognized that the use of vital dyes is associated with a low but definite risk. Most reactions are mild and are noted in 1% to 2% of cases, including urticaria, hives, generalized rash, or pruritus.10 Hypotensive reactions or anaphylaxis (bronchospasm and respiratory compromise) are unusual after the injection of a blue dye. If it occurs, most patients can be treated with short-term vasopressor support.11 However, death, although extremely rare, has been reported.12 Use of methylene blue as an alternative to other vital dyes may help to decrease these adverse reactions. Preoperative prophylaxis with corticosteroids and antihistamines was tested to reduce these side effects but showed no significant difference and may lead to increased wound infection and dehiscence.13
The advantages of vital dyes are the convenience of use (without radiation safety issues and imaging facility requirements), as well as visual identification of positive nodes. The disadvantages are that they are less efficient in identifying distant and deep-lying nodes, and lead to more exploratory dissections and tissue damage. For example, blue dyes have very limited value in identifying extra-axillary nodes (internal mammary or supraclavicular nodes) in patients with breast cancer.14 Another disadvantage is that blue dyes may interfere with pulse oximetry readings. Use in certain patients, therefore, should be performed with caution.15 Finally, blue dyes are not recommended for use in pregnant women.16
SLN identification with isosulfan blue dye or patent blue dye is enhanced by the addition of radiolabeled sulfur colloid and intraoperative use of the hand-held γ-probe. Use of radiolabeled agents results in higher SLN identification rate compared to blue dyes alone, regardless of the injection methods.8,17 SLN identification rates in patients with cutaneous melanoma improved from 84% to 87% (blue dye alone) to 99% to 99.5% (blue dye plus sulfur colloid) ( p< 0.0001) with the combined technique at all anatomic sites examined. It is generally accepted that radiocolloids and blue dyes are complementary to each other for SLNB. The advantages of adding radiolabeled agents also include the ability to identify deep-lying and more distant nodes, “real time” efficiency in guiding the γ-probe to SLN, with reduced exploratory dissections and tissue damage, thus decreasing the morbidity associated with conventional nodal dissection. The addition of vital blue dyes to radiolabeled agents during lymphatic mapping may be helpful in the absence of “hot spots” on lymphoscintigraphy or where only weak hot spots are detected at lymphoscintigraphy.
Other Tracers Used for Sentinel Lymph Node Mapping
The disadvantages of 99mTc-sulfur colloid have been recognized, including a mixed particle size of 50 to 1,000 nm that cannot be maintained as a homogeneous suspension (and which is therefore not a true colloid), a relatively high level of free pertechnetate which can freely “leak” into lymphatic channels and cause high background activity, and relatively low extraction by SLN (thus with easy travel to second-tier nodes).18 Lymphoseek was developed by Neoprobe (Dublin, Ohio) to address these problems. Lymphoseek is composed of a dextran backbone attached to diethylene triamine penta-acetic acid (DTPA) and 99mTc. It has more uniform binding to the surface of reticuloendothelial cells compared to filtered 99mTc-sulfur colloid. Lymphoseek has faster clearance from the injection site and a lower mean number of SLN detected per study,19 and has been recently approved by FDA.
Indocyanine green absorbs light in the near-infrared range and emits maximum fluorescence at a wavelength of 840 nm. Motomura et al.20 at the Osaka University Medical Center first reported the use of indocyanine green as a tracer for SLN mapping in breast cancer patients in 1999, and later the combined use of 99mTc-sulfur colloid and indocyanine green. Use of indocyanine green for lymph node mapping was subsequently extended to melanoma and other cancers. Important advantages in the use of indocyanine green are high safety, and the ability to allow for simultaneous localization of green staining and fluorescent marking of lymphatic tissues using an infrared imaging device combined with a charge-coupled device (CCD) and a light-emitting diode (LED). A major limitation to the widespread use of indocyanine green is the need for portable, intraoperative imaging systems to allow for visualization of fluorescent structures. Although very promising, indocyanine green is still under investigational use. More in-depth studies are needed to fully evaluate its potential and limitations.21
For cutaneous melanoma, intradermal or subdermal radiolabeled agent injection is recommended using a 25- or 27-gauge needle. The number of radiocolloid aliquots and the sites of injection vary, depending on the size and location of the primary tumor and whether there is a wide excision scar. Total injected activity ranges from 0.2 to 1 mCi (7.4 to 37 MBq), often divided into 3 to 4 aliquots, and the injection volume should be small (0.1 to 0.2 mL). The injection sites are often 0.5 to 1 cm from the scar or tumor margin. Research has demonstrated that intradermally injected radiocolloid surrounding a biopsy excision site at distances of 1.5 or 0.5 cm showed similar detection rates for SLN.22 In general, injections should not be made into inflamed, infected, or scarred areas, and contamination of the patient’s skin and clothing should be avoided.
For breast cancer, peritumoral injection was the original method used for SLNB and was very successful.3 Later, multiple injection methods have been recommended for breast cancer SLNB, including deep subcutaneous or parenchymal (peritumoral, subtumoral, intratumoral) injections and superficial (intradermal or subdermal) periareolar or subareolar injections.17
Multiple studies demonstrate that all of these injection methods are similarly successful for the identification of SLNs. The underlying reason is likely that for most of the breast tissue and overlying skin, there seems to be a preferred drainage to the same few axillary SLNs, such that the identification of these few nodes is not affected by injection location.23,24 Still, some researchers believe that the superficial methods have the highest detection rate for SLN.
In addition to the success rate of SLN detection, other factors should be considered when choosing an injection technique. Superficial injection is easy to perform, results in less interference with scintigraphic imaging, and is more painful (addition of pH-balanced 1% lidocaine improves patient comfort without compromising SLN identification).25 Deep injection is more difficult to perform if the tumor is nonpalpable, such that ultrasound guidance may be needed. It may interfere with detection of SLN on preoperative scintigraphy. An important advantage of deep injection is the improved detection of extra-axillary nodes, especially for detection of internal mammary nodes (IMNs).26It is likely best to use a combination of a radiolabeled agent and a blue dye with both superficial and deep injections to further improve detection of SLN.8,26
Scintigraphic Imaging and Intraoperative γ-Probe Detection
The combination of preoperative scintigraphic mapping with intraoperative γ-probe detection proves to be highly efficient to identify SLN. The original method described by Morton et al. depended on the surgeon’s experience. Two of the three surgeons in the study who had performed over 30 procedures were able to locate SLN in the regional node field in only 75% of the time of the third surgeon.2 In current practice, the application of preoperative lymphoscintigraphy and an intraoperative hand-held γ-probe allows the surgeon to pinpoint to the location of SLN with identification rates close to 100%.
FIGURE 28.3. Lymphoscintigraphy for an 82-year-old male with right breast cancer. This patient received intradermal injection of filtered 99mTc-sulfur colloids, and planar anterior view (A, B, D, E ) and right lateral view (C, F )scintigraphic images are shown at 5 minutes (A–C) and at 20 minutes (D–F ) post injection. Addition of 57Co transmission imaging (B, E) provided information of body outline for the patient, indicating the lower neck and superior axillary regions were not well included on the early images (A–C), but were properly included after position adjustment (D, E).
FIGURE 28.4. Lymphoscintigraphy for a 61-year-old male with posterior right shoulder melanoma. This patient received intradermal injection of filtered 99mTc-sulfur colloids, and planar scintigraphic images are shown 20 minutes post injection. Multiple sentinel lymph nodes (SLNs) are visualized in the right axilla. A, C: Anterior views. B, D: Right lateral views. A 57Co transmission imaging was used to provide body outline for orientation of the SLNs (C,D).
Generally, after the injection of 99mTc-sulfur colloid, scintigraphic images of selected regions of the body are obtained to evaluate drainage of the radiocolloid and to detect SLN (Figs. 28.2–28.4). Transmission images of the patient body outline, using either a Cobalt-57 (57Co) flood source or a Gadolinium-153 (153Gd) line source, are often obtained at the same time to provide orientation and better localization of the SLN nodes (Figs. 28.3 and 28.4).27 A mark is often made on the skin overlying each detected radioactive focus to indicate the location of possible SLN. Preoperative lymphoscintigraphy can conveniently scan a large area of the body and facilitate surgical planning because SLNs may be located in unexpected distant regions or in multiple nodal basins, which is particularly true for melanomas of the trunk and head/neck. Preoperative scintigraphic imaging also helps in intraoperative searches for all radio-positive SLNs and may lead to more nodal removal.28 It is especially helpful if a node of interest is located close to an injection site, or if SLNs are located in deep tissues (e.g., to evaluate the status of IMN in breast cancer patients) or at a distance from the injection site (e.g., to evaluate the status of triangular intermuscular nodes in patients with back melanoma).
The timing of preoperative lymphoscintigraphy should be adjusted according to particle size, because the rate of migration along lymphatic channels is inversely related to particle size. For commonly used 99mTc-radiocolloids, SLNs are frequently seen within a few minutes, and the vast majority of SLNs are identified by 90 minutes. Babiera et al.29 demonstrated that the location and number of SLNs detected scintigraphically after injection of 99mTc-sulfur colloid were essentially the same whether the scintigraphic images were obtained early (15 minutes to 4 hours) or delayed (18 to 24 hours) after injection. In fact, 99mTc-sulfur colloid injection can be performed the day before surgery for SLN detection. Gray et al.30 found that the success of SLN mapping in breast cancer patients was very similar regardless of whether the lymphoscintigraphy was performed on the same day or on the following day after injection, although the mean number of SLNs was statistically significantly greater among those injected the day prior to surgery (2.71) than among those injected on the day of surgery (2.33).
FIGURE 28.5. Lymphoscintigraphy for a 67-year-old female with left breast cancer. This patient received 37 MBq (1 mCi) of filtered 99mTc-sulfur colloids (in four divided doses). Planar images at 30 minutes post tracer injection ([A, B]; [B] is the same image of [A] but with higher contrast) revealed no overt evidence of positive sentinel lymph nodes (SLNs). Single photon emission computed tomography (SPECT) images were subsequently obtained (C–E)and revealed two positive left axillary SLNs. C: Anterior view of a maximum projection SPECT image. D: A sagittal view of SPECT images showing one of the two positive SLNs. E: A coronal view of SPECT images showing both positive SLNs.
The use of a hand-held γ-probe was first described by Krag et al.3 in the identification of SLN in melanoma patients, and then in breast cancer patients. The hand-held γ-probe usually detects more SLN than preoperative scintigraphy,31 and helps to localize SLN located outside of the formal nodal basin. Even if preoperative lymphoscintigraphy is negative, SLN can still be identified intraoperatively by a hand-held γ-probe. Intraoperative SLN detection with a hand-held γ-probe can be performed successfully up to 16 to 18 hours after injection with radiocolloids of 200 to 1,000 nm.4 If the particle size of radiocolloids is small, thus favoring rapid clearance, reinjection of extra radiocolloids may be needed just before surgery.
Value of Single Photon Emission Computed Tomography
Single photon emission computed tomography (SPECT)/computed tomography (CT) is a more sensitive technique than conventional planar scintigraphy for detection of SLN because it provides higher resolution tomographic images with anatomic correlation (Fig. 28.5). van der Ploeg et al. reported that SPECT/CT visualized more SLN, identified additional SLN in patients with nonvisualized SLN based on planar scintigraphy, and decreased false-positive SLN based on planar images (related to skin contamination artifact) in patients with breast cancer. Based on the SPECT/CT findings, a more precise incision was made in 36% of patients, an extra incision was made in 4% of patients, and an incision was avoided in 1.5% of patients.32 The clinical value of SPECT/CT remains under investigation but is appealing in certain situations.
It is difficult to determine the exact locations of head and neck SLN on planar lymphoscintigraphy because a standard planar scintigraphic imaging provides limited anatomic and spatial resolution and does not show relationships between SLN and important anatomic structures of the head and the neck. This is partly because of the complex three-dimensional anatomy of the head and neck. A “hot” spot on a planar imaging may represent a deep or superficial SLN and there is no information of the position relative to other cervical structures such as muscles and vasculature. In addition, SLN may be missed on planar images because the SLN may be obscured by the high activity at the injection site if SLN is close to the primary tumor site and because the pattern of SLN drainage is often unpredictable in the head and the neck.33 These factors lead to considerably lower success rates of SLNB in the head and neck as compared to SLNB in the trunk and extremities.
The additional value of SPECT/CT to detect and localize SLN in the head and neck has been demonstrated in patients with oral/oropharyngeal squamous cell carcinoma (OSCC),33,34 melanoma,35 and other skin malignancies. SPECT/CT helps in surgical planning and facilitates surgical exploration in these patients.36 For example, Vermeeren et al.35 reported that SPECT/CT depicts an additional SLN in 16% of patients with melanoma, leading to modification of the surgical approach in 55% of patients. Bilde et al.34 reported that, compared with planar lymphoscintigraphy, SPECT/CT imaging demonstrates additional SLNs in 47% of patients and provides additional anatomical and spatial information about their locations. Similarly, SPECT/CT is helpful in patients with complex lymphatic drainage such as those with scapular melanoma, breast cancer patients who have had prior surgery or who have extra-axillary SLN, in patients with tumors draining to deep (pelvic) nodes,37 and in patients with prostate cancer.38 SPECT/CT is also indicated in any patient with nonvisualized SLN.39 However, if SPECT/CT is planned, it is performed in conjunction with conventional planar imaging, where sequential planar images should be obtained first because they remain critical to distinguish SLN from secondary nodes.
How Many Lymph Nodes Should Be Removed During SLNB?
Theoretically, an SLN is the first lymph node in the lymphatic chain that receives lymph drainage from a tumor basin. In SLNB, the SLN detected or removed could be any lymph node that has increased radioactivity or vital dye staining. It has been recognized that removal of just one or two nodes may miss potential metastases in regional lymph nodes,40 whereas indiscriminate removal of multiple nodes may cause soft tissue damage, increasing the morbidity of SLNB.
Efforts have been focused on determining a reasonable upper threshold for the number of SLNs that should be removed. Goyal et al.41 evaluated 803 patients with breast cancer, and found that the false-negative rate (FNR) was 10% if only one SLN was collected, versus 1% if three or more nodes were harvested. At the same time, 99.6% of positive nodal metastasis was detected within the first four SLNs removed,41 consistent with other reports.42 Similar findings were reported in patients with cutaneous melanoma. Abou-Nukta and Ariyan43 reported that excision of the three hottest nodes and all blue nodes detected 100% of nodal metastases. At the same time, removing nodes with radioactivity of less than 30% of the hottest nodes may be unnecessary, because most (98%) of the positive lymph nodes had radioactivity counts greater than 30% of the hottest node.43 Overall, it is believed that removal of three to four SLNs will significantly reduce the FNR and will identify 98% to 99% of patients with nodal metastases, whereas removal of more than five SLNs has limited value and is not necessary.
Pregnancy itself is not regarded as a contraindication for radioactive scintigraphic SLN detection.4 Multiple studies have shown that the procedure of SLN detection is safe and successful in pregnant women with minimal risk to the embryo/fetus.44 To evaluate the risk to the embryo/fetus associated with SLNB and lymphoscintigraphy in pregnant patients, Keleher et al. calculated the absorbed dose to the embryo/fetus after injection of 92.5 MBq (2.5 mCi) of filtered 99mTc-sulfur colloid in two nonpregnant women with breast cancer. In the worst case scenario, one can assume that the entire injected radioactivity is instantaneously transported to the urinary bladder, where it remains and is eliminated only by physical decay. Under this assumption, this leads to a maximum absorbed dose (4.3 mGy) to the embryo/fetus (given the proximity of the urinary bladder to the uterus), but is still well below the 50 mGy level that is believed to be the threshold absorbed dose for adverse effects. The absorbed dose to the embryo/fetus will be 50-fold lower (0.0774 mGy) assuming that all of the injected radioactivity remains in the breast and is eliminated only by physical decay, and will be 12-fold lower (0.342 mGy) assuming that the injected radioactivity behaves as though it were administered intravenously (with predominant biodistribution in the liver, spleen, and bone marrow), as Keleher et al. reported.45 More direct measurement of radiation exposure in pregnant women and the embryo/fetus found that the uterine exposure to radioactivity from radiocolloids associated with SLNB and lymphoscintigraphy is much lower than Keleher et al. assumed (similar or less than the average daily background radiation), without significantly increased risk of prenatal death or developmental abnormalities.46 Overall, the patient is more likely to be adversely affected by the procedure of surgery or anesthesia rather than by the radiation exposure itself.
Please note that the safety of vital blue dyes has not been tested for pregnancy, and thus these dyes should not be used in pregnant patients.16
SENTINEL LYMPH NODE BIOPSY APPLICATION IN BREAST CANCER
Breast cancer is the most common malignancy diagnosed in women, with an estimated 230,480 new cases and 39,520 deaths in 2011 in the United States.47 The status of the axillary lymph nodes is the most important prognostic factor in patients with early stage breast cancer. Axillary lymph node dissection (ALND) has traditionally been a routine procedure in the management of early breast cancer. Before the introduction of SLNB in the 1990s, ALND was often the preferred method of treatment for removal of all of the axillary lymph nodes to reduce axillary recurrence. Although reliably identifying nodal metastases and maintaining regional control, ALND is associated with an indisputable high incidence of postsurgical complications including lymphedema and shoulder dysfunction, nerve injury, and a compromise in the quality of life.48
SLNB was first introduced in the early 1990s as an option to accurately stage axillary nodal involvement in breast cancer,3 and has since proved to be a great success and widely accepted as the standard of care in patients with early stage breast cancer without clinical evidence of nodal metastasis. SLNB is simple, accurate, and effective in the identification of potential nodal metastases in these patients and spares the majority of patients from the potentially devastating side effects of ALND, reduces the costs of hospital stays, and maintains the curative effects of surgery.49,50 With appropriate training, this technique leads to successful identification of SLN in >95% of patients with breast cancer. Approximately 20% to 35% of these patients with early breast cancer are expected to have positive tumor involvement in SLN16 and will be treated with elective lymph node dissection (ELND). Use of SLNB spares approximately 70% of early stage breast cancer patients from ALND and associated complications and morbidity, while providing similar staging accuracy and survival benefit. Kell et al. reported a meta-analysis involving a total of 9,608 breast cancer patients with no clinically palpable nodes to systematically appraise the outcome of SLNB when compared to ALND. It was found that SLNB is at least equivalent to ALND in detecting metastatic axillary nodes (the overall rate of axillary lymph node positivity was 27.6% for SLNB and 28.8% for ALND), but with significantly less risk of postoperative morbid effects (including infection, seromas, arm swelling, and numbness) relative to ALND.51
These findings were confirmed by other randomized research projects. The National Surgical Adjuvant Breast and Bowel Project trial B-32 (NSABP B-32) was a randomized controlled phase 3 trial performed at 80 centers in Canada and USA between 1999 and 2004, with 5,611 women with invasive breast cancer randomly assigned to either SLNB + ALND (group 1) or to SLNB alone with ALND only if the SLNs were positive (group 2). With a follow-up of 95.6 months (range: 70.1 to 126.7), this study demonstrated that the overall survival and disease-free survival were statistically equivalent between the two groups: The overall survival was 91.8% in group 1 and 90.3% in group 2; the disease-free survival was 82.4% in group 1 and 81.5% in group 2. Also, there was no statistical difference in regional node recurrences in these two groups (p = 0.22).52 Similar findings were reported in other randomized controlled studies.53 The equivalent overall survival, disease-free survival, and regional recurrence in negative SLN patients with or without ALND indicate that SLNB alone with no further ALND is an appropriate, safe, and effective therapy for breast cancer patients with clinically negative lymph nodes.
The adoption of SLNB leads to replacement of ALND as the standard node staging procedure for patients with breast cancer. ALND is spared in approximately 70% of patients when the diagnosis of negative SLNB is established. Accumulating evidence supports that SLNB is safe for these patients, because long-term follow-up studies in SLN-negative women treated without ALND reveal low rates of axillary recurrence. Recently, Andersson et al. reported their findings in a large-scale, prospective multicenter study of SLNB as a single axillary staging procedure for breast cancer. From a total of 2,195 SLN-negative patients with 2,216 breast tumors and with median follow-up of 65 months, 23 patients (1%) had isolated axillary recurrence. There was no difference in recurrence rates between different centers. The overall 5-year survival rate was 93.1% and the event-free 5-year survival rate was 88.8%.54 The low axillary recurrence in these patients demonstrated that SLNB can safely replace ALND in SLN-negative patients, and that false-negative findings of SLNB are not a major concern.
In current practice, SLNB is the standard of care for staging the axillary nodes in patients with clinically node-negative breast cancer.55,56 At the same time, ALND remains the standard approach for patients with positive axillary nodes confirmed by fine-needle aspiration (FNA) or SLNB.
SLNB has undergone significant and continuous modifications and refinements since its introduction, however, and the details of SLNB procedure have not been standardized. There are technical variables that are mainly selected by personal preference including the materials injected (radioactive colloid alone, blue dye alone, or a combination), the timing, location, and method of injection, and use of scintigraphic imaging as described above. There are also continuing discussions including indications for SLNB and the significance of positive findings on SLNB.
Patients with Palpable Axillary Nodes
SLNB should not be performed in breast cancer patients who have evidence of axillary node metastases. Based on this, clinically palpable axillary nodes were also considered as a contraindication to SLNB,40,57 assuming that the palpable nodes indicate metastatic disease. However, controversies exist in this regard because clinical axillary examination in breast cancer is subject to high false-positive results and because palpable axillary nodes do not always harbor metastases.58 Specht et al. reported a study designed to evaluate the accuracy of clinical examination of the axillary nodes in breast cancer patients with palpable axillary nodes. Clinical examinations of the axilla in 2,027 consecutive breast cancer patients with SLNB were classified as either moderately or highly suspicious for metastasis, and then correlated with pathologic findings on SLNB. It was found that clinical examination was inaccurate in 41% of patients overall, and was falsely positive in 53% of patients with moderately suspicious nodes and in 23% of patients with highly suspicious nodes. It was concluded that clinical axillary examination in breast cancer is subject to false-positive results, and that palpable axillary nodes alone does not provide sufficient justification for ALND.59 Lanng et al. performed another study involving 301 consecutive breast cancer patients undergoing either ALND or SLNB prospectively to evaluate the reliability of clinical axillary lymph node assessment by experts and to assess whether inaccuracy can be related to lymph node size. They also demonstrated inaccurate findings on clinical assessment of axillary lymph nodes. The risk of having axillary nodal metastasis was 40.4% if the preoperative clinical assessment was “nonpalpable,” 61.5% if the preoperative clinical assessment was “palpable but benign,” and 84.4% if there were “suspicious lymph nodes.” Furthermore, there was no significant association between lymph node size and palpability or nodal metastasis.60
The American Society of Clinical Oncology (ASCO) Guidelines do not recommend SLNB in the case of palpable axillary nodes in breast cancer patients, whereas ALND is recommended in the case of a failed or technically unsatisfactory SLNB procedure and in the case of detection of additional clinically suspicious axillary nodes after the removal of all SLNs.55 Another reasonable option for patients with palpable or other clinically suspicious axillary lymph nodes would be, as recommended recently by the National Comprehensive Cancer Network (NCCN) guidelines, to perform either a core biopsy or FNA of these nodes, followed by SLNB if biopsy results are negative.56 A formal ALND or axillary irradiation should be performed if SLNs cannot be identified or are positive for metastasis.
Reoperative Sentinel Lymph Node Biopsy with Prior Breast or Axillary Surgery
It is relatively common that patients with prior history of SLNB, ALND, or breast surgery develop ipsilateral breast cancer recurrence or a second primary breast cancer. Port et al.61 reported that 5% to 10% of patients with breast conservation therapy, with or without ALND, have local recurrence. Previous breast surgery or axillary biopsy had been considered as a relative contraindication to SLNB, because the pattern of lymphatic drainage may have been altered or interrupted. In current practice, however, it is convincingly demonstrated that reoperative SLNB is technically feasible and very effective after previous SLNB, ALND, or breast surgery. The successful detection rate of a reoperative SLNB in patients with prior breast surgeries ranges from 55% to 97% and has been successfully reported in patients with previous breast-conserving surgery,61,62 previous aesthetic breast surgery (either breast augmentation or reduction),63 and previous mastectomy.64 The successful detection rate of a second SLNB has been reported as high as 95% to 99% in patients who underwent prior SLNB65 or prior diagnostic excisional biopsy of breast cancer, comparable to primary SLNB. Port et al. reported that in breast cancer patients with local recurrence after breast conservation therapy with either SLNB or ALND more than 6 months prior, reoperative SLNB was successful in 55% of patients, and identified positive reoperative SLN in 16% of successful cases. These patients had no local or axillary recurrences at a mean follow-up of 2.2 years (although systemic recurrence was noted). The success rate of reoperative SLNB was inversely related to the number of nodes removed previously. It is more likely to succeed when less than 10 nodes were removed during the prior procedure and were more likely to be successful after a previous SLNB (rather than a previous ALND) as the lymph drainage is less interrupted.61 Preoperative lymphoscintigraphy is especially helpful in these patients, because nonaxillary drainage was significantly more common in reoperative SLNB than in primary SLNB.61,66 Taback et al.66 reported that in 73% of patients undergoing reoperative SLNB, migration to regional nodal drainage basins was noted in ipsilateral axillary, supraclavicular, internal mammary, interpectoral, and contralateral axillary nodes after prior breast-conservation surgery with SLNB and/or ALND.
More patients with breast cancer now receive neoadjuvant chemotherapy (NAC). NAC is used to treat not only patients with advanced inoperative breast cancer but also those with initially operable breast cancer. The timing and accuracy of SLNB in patients receiving NAC for breast cancer are controversial because there are concerns that the preoperative lymph drainage may not represent lymph drainage in the tumor basin before chemotherapy, therefore leading to false-negative results. The NCCN guidelines recommend prechemotherapy SLNB as the preferred option for surgical axillary staging in patients with clinically negative ipsilateral axillary nodes, because it provides additional information to guide local and systemic treatment decisions. If the SLN is histologically positive, level I and II axillary dissection is indicated at definitive surgical therapy.56
However, available data show that SLNB after NAC in breast cancer patients is feasible and accurate, with no significant differences in the success rate of SLNB according to clinical tumor size or clinical nodal status, and accurately selects patients who required complete level I and II axillary dissection.67,68 In a systematic review of 27 clinical trials of SLNB in 2,148 patients with invasive breast cancer who underwent SLNB after NAC followed by an ALND, van Deurzen et al.69 reported a pooled SLN identification rate of 90.9%, an FNR of 10.5%, and a negative predictive value (NPV) and accuracy of 89% and 94.4%, respectively. The data demonstrate the clinical value of SLNB after NAC, and the potential to reduce the morbidity associated with ALND for breast cancer patients after NAC.
Multifocal and Multicentric Breast Cancer
Multifocal breast cancer indicates breast cancer with separate foci of ductal carcinoma of more than 2 cm apart but within the same quadrant of the breast, whereas multicentric breast cancer indicates cancer with separate foci of carcinoma in different quadrants of the breast.70,71 SLNB had been regarded as contraindicated in patients with multicentric and multifocal breast cancers, because of the concern that it might be difficult to localize the true SLN, and that negative findings might not accurately represent the true nodal status.57
It has been recognized that the majority of the breast essentially has a single unit of functional lymphatic anatomy with lymph drainage to only a few designated lymph nodes in the axilla,23,24 which provides the scientific basis for successful SLNB in patients with multifocal or multicentric breast cancer. Multiple studies have demonstrated equal efficacy of SLNB for patients with multifocal or multicentric breast cancer as in patients with unicentric breast cancer, although there are no randomized trials in this regard. Gentilini et al.72 reported that for patients with multicentric breast cancer, SLN identification rate was 100% whereas the axillary recurrence rate was 2.2% after a median follow-up of 5 years (range: 17 to 134 months). A recently performed meta-analysis revealed that for “multiple breast cancer” (combining a total of 996 multicentric and multifocal cases), the overall success rate of SLN identification was 92% to 100%, the SLN positivity rate was 12% to 63%, the FNR was 0% to 25%, and the accuracy was 82% to 100%.73 The presence of nodal metastasis was significantly higher in SLN as well as in nonSLN in patients with multicentric breast cancer; however, the sensitivity, FNR, and overall accuracy of SLNB was similar to those in unicentric breast cancer patients.70,71
Internal Mammary Nodes
The status of the internal mammary nodes (IMNs) is an important prognostic factor in patients with breast cancer but is hard to evaluate because of the difficulty of access. There was great variation in the frequency of IMN visualization on SLNB procedure in early studies. Later, it was recognized that mapping of IMN requires deep injection of radiolabeled agents, either peritumoral or intratumoral.26,74 IMN is also more commonly identified in patients with medially located primary tumors.74,75 Radiolabeled agents must be used so that preoperative scintigraphic imaging can be performed to guide intraoperative detection. Injection with blue dyes does not help in identifying IMN because the medial chest wall is not routinely explored during the breast surgery.
Sentinel IMNs are much less commonly detected and harvested in breast cancer patients as compared with those for axillary nodes. IMNs are detected in approximately 20% to 34% of patients with breast cancer,75–77 of which about 63% to 92.5% can be harvested during surgery,74,76,78 and of which 11% to 28% harbor metastases.77–79 A prospective study on SLNB enrolled a total of 1,008 consecutive patients with clinically node-negative operable primary breast cancer. 20% of patients had IMN visualized on lymphoscintigraphy. Sampling of the internal mammary basin was successful in 71% of patients, where metastases were found in 22% of cases (29% of which had only positive nodes in the internal mammary chain [IMC]).76 Retrospective analysis of 2,203 breast cancer patients revealed that 426 (19%) patients had lymphatic drainage to the IMC, whereas exclusive IMC drainage was seen in 25 (1.1%) patients. In addition to exploration of the IMC, the axilla was explored for radioactive and/or blue-containing SLN, which led to three positive cases in the IMC and four positive cases on additional axillary exploration in these 25 patients.77
It has been reported that IMN mapping leads to stage migration and modification of postoperative treatment planning with respect to radiotherapy and systemic therapy in up to one-third of breast cancer cases.75,79 However, the significance of internal mammary SLNB is still under debate. Considering that there was no survival benefit from extended mastectomy compared with radical or modified radical mastectomy, the lack of data regarding a survival benefit from sentinel IMN biopsy, and the availability of systemic therapy, most centers do not perform IMN biopsy/dissection because of concerns about risk of added morbidity and lack of established survival benefit.80 Recent NCCN guidelines recommend that IMN excision is considered as optional if SLNs are visualized in the IMC.56
Micrometastasis or Isolated Tumor Cells
Micrometastasis indicates a deposit of metastatic tumor >0.2 mm but ≤2 mm, whereas isolated tumor cells indicate a lesion of ≤0.2 mm. Small metastases at this size range cannot be reliably detected by any imaging studies, and are often missed by routine sectioning and H&E staining of lymph nodes. However, as SLNB allows focused histopathologic examination on only a few lymph nodes, more detailed analysis with multiple sectioning becomes possible. In addition, IHC, reverse transcription polymerase chain reaction (RT-PCR), and flow cytometry also improve sensitivity, leading to more frequent diagnosis of micrometastasis or isolated tumor cells in clinical practice.81 However, it remains unclear if the presence of SLN micrometastases indicates increased risk of nonSLN metastases. Some clinicopathologic variables have been found to be associated with increased likelihood (odds ratio >2) of nonSLN metastases when the SLN is positive, including SLN metastases >2 mm in size, extracapsular extension in the SLN, >1 positive SLN, ≤1 negative SLN, ratio of positive sentinel nodes >50%, tumor size >2 cm, and lymphovascular invasion in the primary tumor.82
Available data have confirmed the prognostic importance of identifying SLN micrometastases,83 and the ASCO guidelines in 200555 and the NCCN guidelines in 200956 both recommend that a completion ALND should be performed for patients in whom SLN micrometastases are identified, regardless of the method of detection. However, the management of the axilla in patients with micrometastases or isolated tumor cells in SLNs without simultaneous macrometastases remains a topic of great debate. Multiple studies have demonstrated that detection of micrometastatic carcinoma in SLNs of invasive breast cancer patients is a major indicator of worse survival,84,85and others have observed no significant difference in the risk of distant disease,86 in regional axillary recurrence,87 or no statistically significant differences in overall or disease-free survival88 in patients with micrometastases compared to those with negative SLN. An analysis of 97,314 patients with breast cancer from the National Cancer Data Base (1998 to 2005) found an insignificant trend toward better outcomes (axillary recurrence and overall survival) for ALND (versus SLNB alone) in patients with macroscopic nodal metastases (n = 20,075), although there were no significant differences in axillary recurrence or survival between SLNB alone and complete ALND in patients with microscopic nodal metastases.89,90 It is likely that the incidence of nonSLN involvement is low in patients with micrometastatic SLN, especially in patients with a single SLN micrometastasis among four or more SLNs harvested.91 In addition, the postsurgery systemic therapy has an effective salvage effect. As the recent American College of Surgeons Oncology Group (ACOSOG) randomized trial Z0011 demonstrated that SLNB alone was similarly effective as SLNB followed by ALND in patients with breast cancer with positive SLN,92,93 the significance of micrometastases in SLN is likely not clinically critical.
Ductal Carcinoma In Situ
In the United States, the incidence of ductal carcinoma in situ (DCIS) rose significantly in the last 30 years, partly because of increased use of mammography. DCIS is defined as a proliferation of malignant epithelial cells within breast ducts without invasion through the basement membrane. Theoretically, DCIS indicates stage 0 noninvasive breast carcinoma with no metastatic lesions, and thus ALND is not indicated in these patients. However, some patients with DCIS develop regional or distant disease. This is likely as a result of sampling error upon pathologic examination, and many cases with the initial diagnosis of DCIS are actually invasive carcinomas with axillary metastases found postsurgically. Thus an accurate diagnosis (including assessment of DCIS size and grade) is critical to make a sound treatment plan, whether excision alone, excision plus radiation, or mastectomy.
About 15% of patients with DCIS initially diagnosed on core needle biopsy prove to have invasive breast cancer upon excision or mastectomy. Risk factors related to DCIS recurrence include younger age, positive surgical margins, palpability, tumor size and grade, comedo morphology, and necrosis.94 5.4% to 12% of high-risk DCIS patients with invasive ductal carcinoma at final diagnosis had positive SLN, the majority of which had micrometastases only.94 Ansari et al.95 performed a meta-analysis of 22 published studies and reached an estimate of 7.4% for the incidence of SLN metastases in patients with a preoperative diagnosis of DCIS, in contrast to 3.7% in patients with a definitive (postoperative) diagnosis of DCIS. SLNB is recommended for patients diagnosed with DCIS by needle biopsy when they have a high risk for harboring invasive ductal cancer.94 Because the rate of SLN metastasis is very low in pure DCIS, SLNB is not indicated in these patients.94
Prophylactic mastectomy is performed in high-risk patients to decrease the risk of breast cancer. There is no clear evidence of beneficial effect of SLNB at the time of prophylactic mastectomy. A recent meta-analysis (including six studies and a total of 1,251 patients who underwent 1,343 prophylactic mastectomies) indicated that the pooled rate of positive SLN was very low (1.9%, 95% CI 1.2% to 2.6%). As a result of SLNB findings at the time of prophylactic mastectomy, a significant change in surgical management was recorded in 2.8% of cases (95% CI 2% to 3.8%), about half because of negative SLN with invasive cancer (avoiding ALND) and half with positive SLN without invasive cancer (needing a subsequent ALND).96
Male Breast Cancer
Breast cancer in men accounts for approximately 1% of all breast cancer cases.97,98 SLNB is equally successful and accurate in male patients with breast cancer as it is in female patients, with a success rate of SLN detection ranging from 97% to 100%.97,98 Compared to breast cancer in women, breast cancer in men tended to be larger and was more likely to be associated with positive SLN,97,98 likely because of a higher stage of cancer at the time of diagnosis. A review of 7,315 SLNB procedures from September 1996 to July 2005 at the Memorial Sloan-Kettering Cancer Center (78 of which were in men) indicated that SLNB was successfully performed in 97% of male patients. Positive SLNs were found in 49% of patients, the majority of whom underwent immediate ALND based on node positivity determined intraoperatively, whereas patients with negative SLNs did not undergo ALND. There were no axillary recurrences at a median follow-up of 28 months (range: 5 to 96 months).98
Scintigraphic Nonvisualization of Sentinel Lymph Nodes
Preoperative scintigraphy visualizes SLNs in approximately 90% of cases of breast cancer patients. In the majority of patients with lymphoscintigraphic nonvisualization, SLNs will be identified intraoperatively either by γ-probe alone (detection rate of 84.6%) or by γ-probe combined with blue dye (detection rate of 88.4%).99 However, negative lymphoscintigraphy is associated with a lower intraoperative identification rate and fewer detected SLNs in breast cancer patients.99 SLN cannot be detected intraoperatively in approximately 1% to 2% patients, which is associated with old age, obesity, and tumor location in locations other than in the upper outer quadrant.99 Additional radiolabeled agent injection following an initial failure may increase the SLN detection rate scintigraphically without compromising accuracy. SPECT/CT is also advocated to detect “hidden” SLN in patients with nonvisualization on planar lymphoscintigraphy,37,39 and is receiving more interest in recent years.
Some studies suggest that preoperative scintigraphic nonvisualization of SLN indicates an increased risk for positive axillary node involvement. Brenot-Rossi et al.100 reported that metastases were found in 28.5% of cases with presurgically visualized SLN, compared to 63.3% of cases with lymphoscintigraphically nonvisualized SLN. Also, multiple nodal metastases are associated with more frequent scintigraphic nonvisualization of SLN.100,101 Rousseau et al.99 reported that these negative lymphoscintigraphy SLNs had fewer micrometastases and more macrometastases, and had larger size. It is likely that gross tumor involvement of a lymph node may interfere with the uptake of both radiocolloid and blue dye and deviate lymph flow to a node other than the true SLN, causing a false-negative finding. In addition, retention of a radioactivity in the SLN is dependent on the phagocytic activity of macrophages. As the amount of metastatic tumor in a lymph node increases, macrophages are replaced by tumor, leading to reduced or lack of macrophage function, and hence false-negative imaging findings. It is safer to perform ALND in these patients if no SLNs are identified during surgery.
False-Negative Sentinel Lymph Node Biopsy
The FNR of SLNB refers to the frequency of negative SLNB in patients with positive nodal metastases (i.e., the number of false-negative cases on SLNB/total positive cases on either SLNB or ALND), and varies widely,17 with an average 7.3% based on a meta-analysis of 69 trials before 2003.102 FNR is independent of experience and injection technique103 but appears more common with vital dye injection,102 with previous excisional biopsy,28 and when too few sentinel nodes are removed.40,104 In practice, the FNR cannot be determined if SLNB is adopted, because the majority of patients will not have ALND. It remains to be determined if a false-negative SLNB will compromise the therapeutic efficacy in breast cancer. However, the very low axillary recurrence rate in all breast cancer patients with negative SLN54 indicates that SLNB is a reliable procedure.
False-negative SLNB results compromise patient outcome by missing positive nodes and understaging disease, which might lead to regional recurrence and/or distant metastases, and undertreatment with systemic therapy. The FNR should be reduced to as low as possible, and may be decreased by use of combined radiolabeled agents and a blue dye for SLN, use of preoperative lymphoscintigraphy (and SPECT/CT in difficult cases), biopsy of multiple SLN, application of multilevel sectioning and IHC in pathologic assessment of SLN, and assurance of adequate experience of surgeons who perform SLNB. The criteria used to define a “hot” SLN also affect the performance of SLNB. Song et al. performed a study to evaluate three different definitions of a “hot” SLN in patients with early stage breast cancer: (1) the node with the highest radioactivity, (2) any node with an in vivo hot spot-to-background activity ratio of ≥3 or an ex vivo SLN-to-nonSLN ratio of ≥10, and (3) all radioactive hot nodes. These three different definitions led to an FNR of 21.1%, 7.9%, and 2.6%, respectively, and an accuracy of 90.1%, 96.3%, and 98.8%, respectively (p < 0.05). Removing the first one, the first two, the first three, and the first four hottest SLNs identified 81.1%, 89.2%, 94.6%, and 97.4% of the positive-SLN patients, respectively.104
Another way to decrease FNR is to not miss suspicious nonSLNs. After excising the blue-stained or radioactive nodes, intraoperative palpation should be performed in the axillary node basin and all suspicious palpable SLNs that are not either blue-stained or radioactive should be excised.105 Choi et al.106 reported that biopsy of a suspicious palpable SLN should be done as part of SLNB in breast cancer patients and helps to reduce the number of false-negative findings of SLNB, because positive axillary nodal involvement was identified solely by biopsy of suspicious palpable nodes in 6.5% of patients with SLN metastases.
Intraoperative Pathologic Analysis
After the intraoperative identification and biopsy of SLNs, ALND is preferably performed as a synchronous procedure in patients with positive nodes (to avoid subsequent reoperation) if frozen section analysis of SLNs can be performed promptly. An important issue here is the accuracy of the intraoperative pathologic evaluation of SLNs in predicting nodal metastases.
It is generally believed that frozen section analysis is less accurate compared to permanent section analysis, and is associated with high false-negative findings. Intraoperative FNR is the proportion of patients undergoing SLNB with negative intraoperative pathologic findings among the total number of patients with positive nodes (i.e., the number of intraoperative SLN false-negative cases/total positive node cases). Most studies demonstrate that the sensitivity of intraoperative analysis for SLN metastases in breast cancer patients varies in the range of 60% to 75%.107 This variation is partially because of the different histopathologic techniques used, as imprint or smear cytology analysis is associated with higher false-negative findings compared to frozen sections.108 However, the specificity of intraoperative SLN evaluation is very high (close to 100%), allowing for synchronous ALND in those patients with positive SLN.107,109
It is important to note that the intraoperative FNR is associated with tumor stage and the size of metastatic tumor cells in the SLN.109,110 Pugliese et al.109 found that the sensitivity of intraoperative SLN evaluation for lymph node metastasis is 5% for N1mi disease and 63% for N1a-3a disease, although the specificity was 99.7%. Liu et al. performed a meta-analysis of 47 studies including 13,062 patients to determine the accuracy of intraoperative frozen section of SLN during breast cancer surgery. They found that intraoperative frozen section had a mean sensitivity of 73% and a mean specificity of 100%. Importantly, the mean sensitivity was 94% for the macrometastasis group and only 40% for the micrometastasis/isolated tumor cell group.111
A clinically important question is whether patients with a false-negative intraoperative SLN assessment need additional surgery with ALND. The ACOSOG trials Z0010 and Z0011112 and other studies113 demonstrated that delayed ALND is equally effective in patients with false-negative SLNB as ALND at the time of initial surgery. However, multiple studies have reported that axillary recurrence rates were low in patients who had SLN metastases, regardless of whether delayed ALND is performed.89,90 Also, there was similar recurrence-free survival of patients with negative SLNB and of patients with intraoperative false-negative SLNB without ALND (with a median follow-up period of 58.1 months).114 It is likely that the intraoperative SLNB FNR is more often observed in patients with early stage disease, less nodal involvement, and a higher rate of estrogen/progesterone receptor positivity,109,110 often with micrometastases rather than macrometastases,109–111 and more frequent use of adjuvant systemic therapy or radiation therapy.114 It is likely that ALND could be avoided in most patients with an intraoperative false-negative SLNB, if axillary radiotherapy or adjuvant systemic therapy is added.114
Is Axillary Lymph Node Dissection Really Indicated with Positive Sentinel Lymph Node Biopsy?
Some studies have questioned the value of ALND in breast cancer patients with SLN metastasis because SLNs are frequently the only involved nodes after an axillary dissection, indicating that SLNB may also provide a therapeutic benefit in controlling regional disease.115 Recently, the ACOSOG Z0011 trial demonstrated that SLNB alone had equivalent survival benefit compared to SLNB followed by ALND in selected patients with breast cancer and positive SLN.92,93
The Z0011 trial examined the outcome of patients who do not undergo ALND for positive SLN. In this prospective randomized trial, all patients (with T1/T2 breast cancer, but N0, M0) received lumpectomy, SLNB, adjuvant systemic therapy, and tangential-field whole breast radiation therapy. Patients with positive SLN metastasis were randomized to SLND alone (446 patients) or SLNB + ALND (445 patients). Patients with SLNB + ALND had more (median of 17) axillary nodes removed compared with patients with SLNB alone (median of 2). In the SLNB + ALND group, 27.3% of patients had additional metastasis in lymph nodes removed by ALND. However, there were no statistically significant differences in local recurrence or regional recurrence between these two groups. Local recurrence at a median follow-up of 6.3 years was only 1.8% in the SLND alone group versus 3.6% in the SLND + ALND group; local recurrences at 5 years were 1.6% and 3.1% in the SLND only and SLND + ALND groups, respectively (p = 0.11). Regional recurrences in the ipsilateral axilla were 0.9% in the SLND alone group and 0.5% in the SLND + ALND group.92 In addition, this study revealed no significant difference in overall survival or disease-free survival in these patients with T1/T2 breast cancer and positive SLN who were treated with lumpectomy, adjuvant systemic therapy, and tangential-field whole breast radiation therapy. The 5-year overall survival was 91.8% (95% CI, 89.1% to 94.5%) for the SLNB + ALND group and 92.5% (95% CI, 90% to 95.1%) for the SLND alone group, and the 5-year disease-free survival was 82.2% (95% CI, 78.3% to 86.3%) for the SLNB + ALND group and 83.9% (95% CI, 80.2% to 87.9%) for the SLND group.93 The findings indicate that SLNB alone can offer excellent regional control and may be a proper management option for selected patients with early stage breast cancer treated with breast-conserving therapy and adjuvant systemic therapy.
It is important to note that ALND removed additional positive nonSLN in 27% of patients, and that regional recurrence with SLNB alone was <1%. This indicated that the majority of the metastases in undissected axillary nodes (after SLNB but without ALND) did not develop into clinically detectable disease, or that adjuvant systemic therapy is effective as a second-line treatment. It is possible that use of systemic therapy was effective to treat occult metastases in these patients. As a result, removal of additional involved nodes with ALND did not result in fewer locoregional recurrences than did SLND alone.
Findings from the Z0011 trial indicate that in patients with clinical T1/T2 tumors undergoing lumpectomy and radiation therapy followed by systemic therapy, a positive SLN is not an indication for addition of ALND, because these patients do not benefit from ALND in terms of local control, disease-free survival, or overall survival. However, the Z0011 trial did not evaluate the value of ALND in other breast cancer patients such as those undergoing other surgical procedures such as mastectomy, those receiving neoadjuvant therapy, or those not receiving chemotherapy or radiotherapy. Future studies are needed to redefine the role of ALND and improve clinical outcomes in breast cancer patients to minimize the complications associated with ALND and to improve quality of life while maintaining survival.
SENTINEL LYMPH NODE BIOPSY APPLICATION IN MELANOMA
Melanoma is a relatively common malignancy with increased incidence every year and a suspected doubling of the incidence every 10 to 20 years. Based on the data from the American Cancer Society in 2011, melanoma is the fifth most common cancer in males and the seventh most common cancer in females, with 70,230 new cases and 8,790 deaths from melanoma predicted in 2011.47 On the basis of data from 2005 to 2007, the lifetime risk of developing melanoma is 2.73% (1 in 37) for men and 1.82% (1 in 55) for women.47 Although melanoma represents only 10% of all skin cancers, it accounts for at least 65% of deaths related to skin cancers.116
The prognosis for melanoma patients is determined by multiple factors related to the primary tumor such as the depth of tumor invasion, ulceration of the overlying skin, mitosis of tumor cells, and by the presence of regional and distant metastases. Melanoma thickness is a major factor in determining the T category and overall staging of cutaneous melanoma (Table 28.1). Nodal metastasis is a common initial manifestation of melanoma and is the most important prognostic factor for recurrence.117 The 5-year survival rate was 72.3% among patients with tumor-positive SLN versus 90.2% among those with tumor-negative SLN (p < 0.001).118 Accurate determination of nodal status is therefore important for staging, surgical/(adjuvant) therapeutic planning, and prognosis assessment. ELND had been a routine procedure in the surgical management of early melanoma patients with clinically normal regional lymph nodes. However, prospective randomized trials did not show an overall survival benefit for ELND versus observation for patients receiving wide local excision (WLE) of the primary malignancy.1 The limited beneficial effect of ELND was probably because of the observation that for early stage melanoma patients, only 20% have occult nodal disease (who might therefore benefit from ELND) whereas the remaining 80% have no nodal involvement (and who therefore do not need nodal dissection).1
Morton et al.2 pioneered vital blue dye detection of the status of SLN intraoperatively in patients with melanoma, and demonstrated that the pathologic findings of the SLN accurately reflected the status of the lymphatic basin. The addition of injection of radiolabeled agents with scintigraphic imaging and the addition of intraoperative hand-held γ-detector probes led to increased efficiency and enhanced identification rates of SLN. In the following 20 years, much knowledge and experience have been accumulated regarding the techniques, diagnostic performance, complications and morbidity, prognostic value, and appropriate patient candidate selection for SLN detection. In an analysis of over 2,000 patients with early stage melanoma, Morton et al.119 reported an overall SLN identification rate of over 95%. It is well established that SLNB can accurately stage regional lymph node basins in stage I and II melanoma patients with minimal morbidity, and is widely accepted as the standard procedure when melanomas are 1 mm or thicker. SLNB may also be appropriate for patients with thin melanomas if the tumor depth is at least 0.76 mm or if there are other high-risk factors such as ulceration or high mitotic rates.
Accuracy of Sentinel Lymph Node Biopsy for Nodal Staging in Melanoma
SLNB is a minimally invasive procedure that accurately detects nodal metastasis with high NPV in early stage melanoma patients.120,121 As with breast cancer, SLNB allows more focused pathologic examinations of limited SLN and can detect subclinical metastatic deposits. A negative SLNB spares a melanoma patient from further complete lymph node dissection (CLND). If the SLNB is positive, CLND can be performed immediately, before the development of clinically evident metastatic disease.122 The incidence of complications is low and the procedure is generally well tolerated.
SLNB in melanoma patients is usually performed with intradermal injection of 99mTc-sulfur colloid and vital blue dyes. Preoperative scintigraphic imaging is also employed. Although lymphatic drainage in breast cancer is more predictable, lymphatic drainage in melanoma is much more variable because melanoma may occur anywhere in the body. After intradermal injection of radiocolloid, lymphatic flow rates vary according to the location of the primary tumor. Lymph drainage is fastest from melanoma sites in the distal limbs, particularly in the lower limbs, and is slowest from the head and neck region and the proximal limbs.123 Multiple variations of lymphatic drainage pathways have been described. Radiocolloid injection at the site of cutaneous melanoma of the back may drain to SLN in the bilateral triangular intermuscular spaces, superior and lateral to the scapulae.124 There is direct lymphatic drainage through the posterior body wall to SLN in the retroperitoneal and paravertebral regions,125 and it is not infrequent to visualize drainage to interval nodes that lay outside standard nodal fields.126 These findings indicate that in previous (pre-SLNB) trials, elective dissection of draining lymph nodes in melanoma patients could be performed in the wrong field in up to 30% of patients.127
SIMPLIFIED STAGING OF CUTANEOUS MELANOMA
The incidence of lymph node metastasis is related to Breslow thickness of melanoma (7.3%, 19.7%, 33.2%, and 39.7% for primary lesions ≤1 mm, 1.01 to 2 mm, 2.01 to 4 mm, and >4 mm, respectively).122 In SLN-positive melanoma patients, approximately 20% of them have additional nonSLN involvement in the CLND specimen. ELND is performed if positive SLNs are detected, either immediately following SLNB or during a separate procedure, depending on whether intraoperative pathologic analysis is performed. ALND for melanoma should include all level I to III nodes.128 However, the management of inguinal lymph node metastases is controversial because patients with involved superficial inguinal nodes often have involved pelvic nodes. A superficial inguinal lymph node dissection should be considered in the presence of a single positive superficial inguinal or femoral triangle node either on SLNB or clinically. A pelvic lymph node dissection should be considered if there is evidence of more than one positive inguinal and/or femoral triangle node, either on clinical examination, CT, ultrasonography (US), or SLNB (including presence of micrometastases). A pelvic lymph node dissection is also indicated for microscopic or macroscopic involvement of Cloquet node,128 because its tumor status may serve as an indicator of the tumor status of the iliac lymph nodes. SLNB + ELND in patients with positive SLN prolongs disease-free survival. However, there is no convincing evidence of overall survival advantage for patients undergoing SLNB.118
There is clearly a learning period for surgeons performing this procedure. Morton et al. reported in the Multicenter Selective Lymphadenectomy Trial (MSLT-I) that even after a 30-case learning phase and 25 additional cases of SLNB, the accuracy of SLNB continued to improve with surgeon experience. The FNR was 10.3% for the first 25 cases versus 5.2% after 25 cases.119
Fewer Surgical Complications and Morbidity with Sentinel Lymph Node Biopsy
CLND used to be a common procedure in the treatment of melanoma, and delays nodal recurrence and prolongs disease-free survival for melanoma patients. However, as with breast cancer, CLND is associated with considerable and sometimes severe morbidity, without benefit of overall melanoma-specific survival. SLNB in melanoma patients is a minimally invasive procedure associated with many fewer complications, compared to CLND, as observed in breast cancer patients. Comparison of morbidity in 315 patients with SLNB versus CLND revealed that complication rates after SLNB were low, and that most complications of SLNB were minor and short-lived (with 13.8% overall incidence of at least one complication, a 11.3% short-term complication rate, and a 4.1% long-term complication rate), whereas complications after CLND were much more frequent and more often severe (with 65.5% overall incidence of at least one complication, a 50% short-term complication rate, and a 50% long-term complication rate).129 In fact, complications from groin dissection were more severe than those from axillary dissection, mandating additional surgery in 25.6% in groin dissection versus 8.5% in axillary dissection.130 Evaluation of 2,120 melanoma patients with a median follow-up of 16 months revealed an overall 4.6% incidence of major or minor complications in patients who underwent SLNB, compared to 23.2% in those who underwent SLNB + CLND.131 One important contributing factor is that 70% to 80% of patients will have negative SLN and require no further lymph node dissection. The reduced complications and morbidity associated with SLNB is among the major factors that have driven its wide acceptance in current practice. However, CLND remains indicated after identification of a metastatic lymph node by SLNB.
Survival Benefit of Sentinel Lymph Node Biopsy
It is well established that the status of SLN is an important predictor for survival in melanoma patients, that SLNB is highly accurate for nodal staging, and that SLNB significantly decreases morbidity and surgical complications compared with CLND. However, SLNB is recommended not because of a survival advantage (compared with WLE of the primary melanoma without SLNB) but because of its minimally invasive nature with little morbidity.
Multiple retrospective studies have reported conflicting findings, some reporting promising survival benefit for SLNB120,122 and others reporting limited or nonsignificant value.132 Morton et al. reported that patients who underwent immediate (after lymph node mapping and sentinel lymphadenectomy) dissection of nodal metastases had 5-, 10-, and 15-year survival rates of 73%, 69%, and 69%, respectively, in contrast to 51%, 37%, and 32%, respectively (p ≤ 0.001), for patients who underwent delayed (after observation) dissection of nodal metastases.122 A more recent analysis of 673 consecutive melanoma patients with or without SLNB and a median follow-up of 64 months showed a significantly better recurrence-free survival, distant metastasis-free survival, and overall survival for patients of the SLNB group than for nonSLNB group.121
The only randomized trial to evaluate the potential survival benefit of SLNB was reported by Morton et al., who demonstrated the contribution of SLNB (with immediate nodal dissection when positive SLNB was identified) to improved survival outcomes. The study analyzed 1,269 patients with intermediate-thickness (1.2 to 3.5 mm), newly diagnosed primary melanoma. These patients were randomly assigned to wide excision plus postoperative observation (with lymphadenectomy if nodal relapse subsequently developed), or wide excision plus SLNB (with immediate CLND if nodal micrometastases were detected on SLNB). Five-year melanoma-specific survival rates were similar between the observation group and SLNB group. However, among patients with nodal metastases, the 5-year survival rate was higher in the SLNB group (who underwent immediate lymphadenectomy) than in the observation group (who had lymphadenectomy when clinical nodal recurrence later occurred) (72.3% versus 52.4%, respectively; p = 0.004). This was likely because of disease progression during the observation period, because there were more nodal metastases in the observation group than in the SLNB group (the mean number of tumor-involved nodes was 1.4 in the SLNB group versus 3.3 in the observation group; p < 0.001).118 Pasquali et al.133 recently performed a meta-analysis that showed that although no significant survival difference was observed between early (SLNB-guided) and delayed lymphadenectomy in melanoma patients, the pooled data from multiple retrospective studies did suggest that SLNB-guided nodal dissection is associated with a significantly better outcome compared with CLND for clinically evident lymph node disease.
False-Negative Findings and Regional Recurrence
The FNR of SLNB is approximately 5% in melanoma patients, determined by concomitant CLND at the time of SLNB,134 and false-negative results were reported in up to 25% cases with long-term follow-up.135 However, the true FNR is difficult to determine and could be because of deficiencies in nuclear medicine, surgery, or pathology.
A prospective multi-institutional study with the largest patient population (2,451 patients with melanoma of thickness >1 mm who underwent SLNB with median follow-up of 61 months) demonstrated an FNR of 10.8%. The true-positive (TP) and true-negative (TN) SLN results were found in 19.8% and 77.8% of patients, respectively. The overall 5-year survival rate was significantly higher in the TN group (86.7%) compared with the TP (62.3%) and FN (51.3%) groups (p < 0.0001), although the overall survival rate was not significantly different between the TP and FN groups (p = 0.32).136
False-negative SLNB is associated with poor outcome in patients with melanoma, likely because of a delay in treatment.137 False-negative SLNB may occur if SLNs are close to the primary lesion (with high radioactivity) and is more likely to occur when only a single SLN is harvested. False-negative SLNB is more common for melanoma of the head and neck site and increased tumor thickness.138 Quantitative RT-PCR significantly increases positive findings in SLNB and may help to reduce FNR.122,135 As with breast cancer, there is a learning phase for SLNB in melanoma, and the percentage of false-negative sentinel node procedures in melanoma patients decreases as experience increases.139 In patients with negative SLNB, ulceration was a predictive factor for false-negative SLNB, and closer follow-up is recommended for these patients.134
Micromorphometric Parameters of Sentinel Lymph Nodes
In general, CLND is performed in patients with evidence of metastases in SLN. However, the majority of these patients do not have additional nodal metastases in other nonSLNs, because additional nodal metastasis is observed in approximately 20% patients with positive SLNB. Thus, it would be very helpful if one could determine which patients with positive SLNs are unlikely to harbor additional nodal metastases in the nonSLNs so that CLND can be avoided, and which patients are likely to harbor additional nodal metastases in the nonSLNs so that CLND and adjuvant treatment can be planned.
To avoid unnecessary surgery in these patients, multiple studies have tried to explore the predictive values of SLN micromorphometric features in a patient with positive SLN for identifying the risk of positive nonSLN and in determining the likelihood of developing regional recurrence within the nodal basin. It has been reported that 5-year overall survival rates are significantly correlated with SLN tumor burden (100%, 63%, and 35% for the patients with SLN tumor burden of <0.1 mm, 0.1 to 1 mm, and >1 mm, respectively), and that there is no additional nonSLN positivity for patients with <0.1 mm SLN micrometastases.140 Frankel et al. found that CLND was more likely to find additional nodal metastases in patients with primary melanoma on the head and neck or lower extremity ( p = 0.01), Breslow thickness >4 mm ( p = 0.001), presence of angiolymphatic invasion ( p < 0.0001), satellitosis ( p = 0.004), extranodal extension ( p = 0.0002), multiple positive SLN ( p = 0.02), and tumor burden within SLN >1% of the surface area ( p = 0.004). In contrast, finding an additional positive nonSLN had no association with gender, age, ulceration, Clark level, histologic subtype, regression, or location (capsular, subcapsular, or parenchymal) of metastases within a node.141 Multiple studies found that invasion depth of melanoma in sentinel node metastases and diameter of metastasis correlate best with the presence of additional nodal disease.142,143 Bogenrieder et al.144 demonstrated that a combination of <2-mm Breslow thickness and low SLN tumor load (Breslow thickness <2 mm and SLN tumor load <0.2 mm2 or tumor deposit <500 μm or tumor penetrative depth <600 μm) predicts the absence of nonSLN metastases in melanoma patients with positive SLN. However, no feature can reliably predict additional nodal involvement in nonSLN, and no long-term follow-up data are available in this regard. Thus, the biologic significance of these findings remains to be determined.
Sentinel Lymph Node Micrometastases
As the evaluation of the SLN is not well standardized, the detection of micrometastases varies and depends on the techniques employed (how many sections are examined, and if IHC is employed, etc.). Micrometastases are more common in melanoma than in breast cancer. Morton et al.118 reported an incidence of 16% of SLN micrometastases in a prospective study. However, the significance of positive findings of micrometastases in SLN remains unclear. Although some studies indicate that patients with minimal tumor burden (<0.1 mm) of SLN micrometastases have an overall 5-year survival rate similar to that of SLN-negative patients,145 other studies indicate that the prognosis is worse in patients with SLN micrometastases, even in those with lesions of less than 0.1 mm and invasion depth of ≤0.3 mm.146 According to the current 2009 American Joint Committee on Cancer (AJCC) staging system, all patients with microscopic nodal metastases, regardless of the extent of tumor burden (including nodal micrometastases at a microscopic level consisting of aggregates of only a few cells detected by IHC), are considered positive nodal metastases and classified as at least stage III.147
It remains unclear if early removal of micrometastases by radical lymph node dissection has any survival benefit. The currently ongoing prospective randomized study of Multicenter Selective Lymphadenectomy Trial II (MSLT-II) may provide a direct answer for this question. The International Sentinel Node Society 2008 guideline148 and the 2010 European Dermatology Forum guideline149 recommend that the presence of micrometastasis in SLN is an indication for CLND in patients with melanoma, even when node involvement is limited to micrometastasis in a single SLN. In current practice, it is prudent to treat patients with micrometastases as other positive SLN before more convincing evidence is available regarding the effect on long-term prognosis.
In-Transit Lymph Node Metastases
As experience with lymphoscintigraphy increased, it was gradually recognized that regional lymph node drainage is not well ordered and not confined to defined pathways, because some patients were found to have unexpected or aberrant drainage patterns.124,125,150 Isolated lymphatic nodes are often found in the area between the primary melanoma and a recognized regional basin, and could be anywhere along the pathway of a lymphatic channel. These nodes are called “in-transit nodes” or “interval nodes,” likely embryonic residuals of lymphatic tissue lying outside of an expected nodal basin. These “in-transit nodes” are very likely the first nodes receiving lymph drainage from the primary tumor/injection site and are therefore, by definition, true SLNs (Fig. 28.1). They are more common in patients with melanomas of the trunk than in those with melanoma of the lower limbs, and may be the only lymph nodes that contain metastatic disease. Nuclear medicine physicians should be able to recognize different lymphatic drainage patterns, and in-transit nodes should be suspected when a hot spot of increased radioactivity is seen along a lymphatic channel.
An evaluation of 2,045 patients with cutaneous melanoma with SLNB showed that the incidence of in-transit nodes was 7.2%. Micrometastasis was found in 14% of interval nodes that underwent biopsy, similar to that found in SLNs located in recognized nodal fields.126 Evaluation of 911 patients with a clinically localized primary cutaneous head and neck or truncal melanoma and a minimum of 10-year follow-up after SLNB and surgery revealed that visualization (but without removal) of in-transit nodes was associated with a significantly higher risk of recurrence than in patients without in-transit nodes.151 The melanoma patients with in-transit nodes tended to have slightly thicker primary tumors and slightly more frequent ulceration (although not statistically significant), and significantly more regular regional SLNs visualized at scintigraphy compared with the other patients with only regional nodes.
It was suggested that performance of an SLNB procedure in patients with cutaneous melanoma increases the incidence of in-transit metastasis (ITM),152 but this argument was not supported by the results of subsequent studies.153,154 Evaluation of 2,018 melanoma patients who were treated with WLE only, WLE + SLNB, and WLE + ELND revealed that there was no significant difference between these groups with regard to the total incidence of ITM or ITM as a first site of recurrent disease.153 An increased risk of developing ITM as a first recurrence after SLNB is associated with SLN status (but not the procedure itself), Breslow thickness, and extremity location of the primary melanoma (p = 0.005).154 It is more likely that the nature of the patient’s underlying melanoma, rather than the SLNB procedure itself, determines the likelihood of ITM.
Patients with Previous Local Excision of Primary Melanoma
It is known that previous surgical procedures can alter lymphatic drainage pathways, for both breast cancer and melanoma patients. However, SLNB can still be successfully performed and accurately reflects the status of the regional lymph node basin in selected melanoma patients with a previous WLE of the primary lesion. There was no significant difference in relapse-free survival and in overall survival whether SLNB is performed in patients with previous WLE or SLNB is performed concurrently with WLE.155 However, these patients may have unusual drainage pathways leading to more common visualization of SLN in unexpected locations.156 For these patients, inclusion of radiolabeled agents and preoperative scintigraphic imaging is critical, as vital blue dye injection may have only limited value.
The majority (65%) of newly diagnosed melanomas are ≤1 mm in thickness.157 Most patients with thin melanomas ≤1-mm Breslow depth have a very good prognosis after WLE alone, with recurrent disease observed in a small percentage of patients (4.3% with regional nodal recurrence and 3.9% with distant metastatic recurrence) as reported by Karakousis et al.158 in a study of 882 patients with a median follow-up of 16.4 years. In addition, the SLN status is significantly linked to overall survival in patients with thin melanoma, with a mean 10-year disease-free survival rate of 96% for patients with tumor-negative SLN versus 54% for patients with tumor-positive SLN (p < 0.001), at a median follow-up of 57 months for 631 patients.159
Efforts have been made to identify “higher-risk” patients with thin melanoma for regional nodal metastases. The incidence of positive SLNB in thin melanoma patients was 6.4%, in contrast to a 23.8% overall incidence of positive SLNB.160 However, all nodal metastases in thin melanoma were found in the group with a Breslow thickness of 0.76 to 1 mm, resulting in 12.8% of positive SLNB in this subgroup.160 Kesmodel et al.161 found that patients with a mitotic rate of >0 and a tumor thickness ≥0.76 mm had a 12.3% incidence of SLN metastases, in contrast to 5% of incidence of positive SLN in all thin melanoma patients, consistent with other studies.158,161–163
In addition to Breslow thickness and mitotic rate, many other factors are related to positive nodal metastases from thin melanoma, including ulceration,158,162,163 angiolymphatic invasion,162,163 microsatellitosis,162 vertical growth pattern,158 patient age,162 and male gender.158 Although some conflicting data exist, these findings indicate that SLNB should be performed in thin melanoma patients with a Breslow thickness ≥0.76 mm (but not in patients with a Breslow thickness <0.76 mm) and in patients with other high-risk factors.
Melanoma in the Head and Neck Region
SLNB in the head and neck region is a technically demanding procedure, and the number and location of SLNs are variable and difficult to predict. SLN identification rate is generally lower for the head and neck region as compared with other superficial lymph node basins. From the analysis of 2,001 patients with early stage melanoma, Morton et al.119 reported an overall SLN identification rate of 95.3% (99.3% for the groin, 95.3% for the axilla, and 84.5% for the neck basins). Preoperative lymphoscintigraphy is important for surgical planning and complete removal of all SLNs.164,165 Multiple studies have demonstrated that SPECT/CT has additional value for detection and localization of SLN in patients with a melanoma of the head and neck and is therefore recommended in these patients. Vermeeren et al.35 reported that SPECT/CT depicted additional SLNs in 16% of patients with better anatomic location correlation in all patients, leading to an adjusted surgical approach in 55% of cases. Although head and neck melanoma is associated with an increased number of positive SLNs, there is no association between number of SLNs removed and overall survival or recurrence-free survival in all patients or in patients with negative SLNs.166
Intraoperative Frozen Section Analysis
Controversies exist regarding the value of intraoperative analysis of SLNB to detect nodal metastases in patients with primary cutaneous melanoma. The advantage of intraoperative examination is that if evidence of metastases is found, regional lymph node dissection can be performed at the same time of SLNB so that the patient can avoid separate surgery and the related risks and additional costs. The disadvantage is that intraoperative examination is not as accurate as a permanent section examination.
Intraoperative pathologic evaluation can be performed either by frozen section analysis or more simply by imprint touch cytology (ITC) analysis. Both methods have consistently high specificity (100% positive predictive value [PPV]) in detecting lymph node metastases. That is, patients with a positive finding on frozen section or ITC will also have positive findings on permanent section analysis, so that these patients will benefit from a synchronous procedure of lymph node dissection. The problem is that neither frozen section nor ITC has satisfactory sensitivity in detecting lymph node metastases. ITC has a sensitivity of 33% to 47%, whereas frozen section analysis has a sensitivity of 59% to 75%, and the combination of frozen section and ITC has a sensitivity of 80%.167 These findings suggest that intraoperative frozen section analysis or ITC can be beneficial to some patients with cutaneous melanoma because there is no risk of over-treatment from false-positive results, and all patients with negative intraoperative frozen section or ITC should have permanent section analysis. However, data is limited and more research is indicated in this regard.
Role of Ultrasonography
Controversies exist regarding to the role of US in the evaluation of SLNs in melanoma patients. US using high-frequency probes provides detailed visualization of subcutaneous lymph nodes including their internal structure. The appearance of a normal hilum and subcapsular sinus as well as a smooth surface outline and shape often indicate a normal lymph node. However, deep lymph nodes (e.g., iliac and obturator nodes, and SLN in the retroperitoneal, paravertebral, and para-aortic regions) can be difficult to visualize. The axilla is also a difficult region to examine with US. More importantly, melanoma metastasis at presentation is frequently microscopic and below the detection resolution limit of US. The benefit of nodal US in combination with fine-needle aspiration cytology (FNAC) in identifying nodal involvement in melanoma patients remains to be determined.
The utility of US with FNAC to detect SLN metastases is currently being investigated but has a limited role at present, as the sensitivity of currently available US technologies is insufficient for detection of micrometastases (<2 mm in diameter), even though micrometastases are common in most melanoma patients with positive SLN.168
Data are conflicting regarding the sensitivity and specificity of US combined with FNAC in the analysis of SLN in melanoma patients. Rossi et al.169 reported that high-resolution US combined with FNAC in the analysis of SLN in melanoma patients during preoperative staging had a sensitivity and specificity of 39% and 100%, respectively, with an FNR of 61%, mainly caused by tumor deposits <2 mm in diameter. Another study revealed that the sensitivity of targeted US in the detection of positive SLN was very low (only 24.3%), although the specificity was as high as 96.8%.170 In an analysis of 127 patients with melanoma (median Breslow depth of 2.1 mm) who underwent SLNB as well as RT-PCR of SLN aspirates obtained by ultrasound-guided fine-needle aspiration cytology (US-FNAC), US-FNAC had a sensitivity of 82% and a specificity of 72% in predicting nodal involvement, and may eliminate the need for SLNB in 16% of all SLNB patients.171 More recently, Voit et al.172 reported that US-FNAC before SLNB had a sensitivity of 82% and PPV of 52%, and identified up to 65% of all SLN metastases.
It is likely that a positive finding by US-FNAC may eliminate the need for therapeutic lymph node dissection, although negative US (with or without FNA) of regional nodes is not a reliable substitute for SLNB. Also, the routine use of US-FNAC before SLNB is unlikely to be cost effective.148 At this time, there is no evidence to recommend US-FNAC as a standard procedure in melanoma patients.
SENTINEL LYMPH NODE BIOPSY APPLICATION IN OTHER MALIGNANCIES
The success of SLNB in melanoma and breast cancer has created great interest in the use of SLNB in the management of other solid cancers such as head and neck cancers, gynecologic cancers, thyroid cancers, gastric cancer, colon cancer, and prostate cancer, and has had variable success. For example, in the setting of gastric cancer, SLNB has an overall sensitivity of 85.4%, which is low, although the specificity (98.2%) and NPV (90.7%) are good,173 and in the setting of colorectal cancer, SLNB has a low sensitivity (76%) as well.174 As such, for most other tumor applications, SLNB is still a subject of research investigation. The value of SLNB in the management of thyroid carcinomas will be discussed in a separate chapter.
Oral/Oropharyngeal Squamous Cell Cancer
As with melanoma and breast cancer, the status of lymph node involvement is the single most important adverse prognostic factor for patients with OSCC. The current standard of care for a clinically node-negative (cN0) OSCC patient is to offer elective lymphadenectomy (or elective neck dissection). However, the risk of occult metastasis for most head and neck sites is approximately only 25% to 30%, which means that the majority of patients who have nonselective cervical node dissection do not harbor occult metastases, yet are exposed to additional costs and morbidity. Traditionally, the depth of tumor invasion has become widely regarded as a prognostic parameter of nodal metastases for patients with OSCC, and was used as a criterion in selecting low-risk patients for “watchful waiting” (where patients would be observed after removal of the primary tumor, with neck dissection being performed only if there was clinical evidence of developing metastases). However, evaluation of patients with oral cancer who underwent SLNB found that tumor depth and tumor thickness could not predict occult metastases in the context of SLNB. In contrast, poorly differentiated carcinomas, carcinomas with lymphovascular invasion, and carcinomas with invasive growth patterns had a high probability of positive SLNB, indicating that SLNB should be performed in patients with early squamous cell carcinoma of the oral cavity regardless of their tumor depth and thickness.175
SLNB with lymphoscintigraphy may provide more accurate staging than current elective neck dissection protocols, and may serve as an alternative to elective cervical node dissection for the management of OSCC. Many centers with adequate experience have abandoned routine elective neck dissection and offer SLNB in observational trials to appropriate patients with nodal negative T1 and T2 OSCC for accurate staging of the neck.176,177 Although not yet widely applied, available data demonstrate that SLNB has the ability to select patients with occult lymphatic disease for elective neck dissection with a reported SLN detection rate of more than 95% and an NPV of 95%,176,178 and to eliminate the costs and morbidity associated with nodal dissection in patients with negative SLN.
According to the “Joint Practice Guidelines for Radionuclide Lymphoscintigraphy for Sentinel Node Localization in Oral/Oropharyngeal Squamous Cell Carcinoma,” the most important clinical indication for SLNB in patients with OSCC was a cN0 neck by physical examination and imaging studies.179 SLNB is more likely to be successful in patients with cN0 necks, and is most commonly performed to stage the ipsilateral cN0 neck in patients with a unilateral primary tumor, but can also be performed to evaluate bilateral cN0 necks in primary tumors close to, or crossing, the midline. In general, SLNB is contraindicated if there is positive cervical node involvement, because gross lymphatic involvement can lead to distortion of the normal architecture, leading to aberrant drainage patterns and biopsy of false SLN.180 However, the Guidelines also recommend the use of SLNB to assess the contralateral cN0 neck in primary tumors close to the midline with an ipsilateral clinical node-positive (cN+) neck, to determine whether these patients need bilateral neck dissections or ipsilateral neck dissection alone.179 Patients who have received prior radiation therapy or surgical treatment to the neck may have distortion of the normal lymphatic pathways, which can give rise to unexpected patterns of metastasis,181 and are thus not candidates for SLNB.179
SLNB is easier to perform for tumors located in the oral cavity and accessible subsites of the oropharynx. However, for tumors located in other locations such as the hypopharynx and supraglottic larynx, poor access to these sites (via endoscopic guidance for radiolabeled agent injection) and close proximity of the primary tumor to the first-echelon lymph nodes (potentially obscuring the true location of SLN) make it difficult to perform the procedure and effectively detect SLNs on preoperative lymphoscintigraphy. In addition, the detection of SLNs may be less successful for floor-of-mouth tumors. Alkureishi et al. analyzed a total of 227 SLNB procedures in patients with early head and neck squamous cell carcinoma with a success rate of 93%, with upstaging in 34% of cases. The detection of SLN for floor-of-mouth tumors was 88% versus 96% for nonfloor-of-mouth tumors. The sensitivity and NPV were also lower for patients with floor-of-mouth tumors compared with other sites. The overall sensitivity of SLNB was 91% based on a minimum follow-up of 5 years.182 Although SLNB can be used as the sole staging tool for the majority of patients with OSCC, its value remains to be determined for patients with floor-of-mouth tumors. Additional imaging with SPECT/CT may help to improve SLN detection and localization in patients with head and neck cancers.33,34
Squamous Cell Vulvar Carcinoma
Squamous cell cancer of the vulva is a rare disease in which unrecognized positive inguinofemoral lymph nodes are often fatal. Radical excision of the tumor with inguinofemoral lymphadenectomy is the current standard surgical procedure in patients with early stage disease with tumors that have an invasion depth of ≥1 mm. Similar to breast cancer, there is morbidity associated with lymphadenectomy without proven benefit for the majority of patients, because two-thirds of patients do not have nodal involvement.183 A modified superficial inguinal lymphadenectomy (instead of a complete inguinofemoral dissection) has been proposed to reduce surgical complications but was later abandoned because of high regional recurrence rates. Application of SLNB in the management of patients with squamous cell cancer of the vulva, however, has produced very encouraging results.
The vulvar region drains into the inguinofemoral lymph nodes and then to the pelvic lymphatic nodes via external iliac pathway. The absence of inguinofemoral lymph node involvement is a reliable indicator of negative pelvic lymph node metastases. The SLNB procedure in vulvar cancer is highly accurate to identify lymph node metastases with a sensitivity of above 90% and an NPV approaching 100%.184 Van der Zee et al. reported a multicenter study of SLNB using a radiolabeled agent and blue dye, performed in 623 groins of 403 patients with T1/T2 (<4 cm) squamous cell cancer of the vulva. Two-hundred-and-fifty-nine patients with unifocal vulvar disease and a negative SLN received no inguinofemoral lymphadenectomy, and had 2.3% of inguinal recurrence and 97% of 3-year survival upon median follow-up of 35 months. In addition, these patients had significantly less short-term and long-term morbidities.183
It appears that SLNB is a safe and feasible alternative to CLND in early stage vulvar cancer patients. Although patient survival is excellent and treatment-related morbidity is minimal, there has been no large-scale randomized controlled trial to confirm these findings. In addition, there is a learning curve associated with the SLNB procedure, and an exposure of at least 5 to 10 SLNB procedures per year is recommended,183 although this is hard to achieve for a rare tumor such as vulvar cancer except for those surgeons practicing in a major oncology center.
The lymph node status is the most important prognostic factor for patients with cervical cancer, and the presence of node metastasis is associated with significantly reduced 5-year overall survival rates. Regional pelvic and/or para-aortic lymphadenectomy remains the gold standard in the treatment of early stage cervical cancer as an integral part of radical hysterectomy and/or radical cervicectomy, but is associated with unnecessary and significant morbidity, because more than 70% of patients with early stage disease have no lymph node metastases. SLNB is used in cervical cancer to reduce the morbidity of lymph node dissection while ensuring that lymph node metastases are accurately detected.
Radiolabeled agent injection for cervical cancer is more technically challenging. Generally, the injection of both 99mTc-radiolabeled agent and blue dye is given directly in divided doses to the area of the normal cervix–tumor interface (or to the bed of the cone in patients who have undergone a prior cone biopsy). As the cervix has a midline position and the cervical lymphatics drain bilaterally, lymphatic mapping should be performed accordingly. Using the combination of lymphoscintigraphy and blue dye injection followed by laparoscopic lymph node mapping, SLN removal, and lymph node dissection, Lecuru et al. reported that lymph node mapping in early stage cervical carcinoma was feasible with high SLN detection rate (97%) and with high sensitivity (92%) and NPV (98.2%) for metastasis detection, without false-negative results. Bilateral negative SLNB accurately excluded lymph node metastasis in early cervical cancer.185 Positive SLNs are frequently located at the external iliac, internal iliac, and obturator regions. Para-aortic metastasis without pelvic node involvement is rare. It is likely that bilateral pelvic CLND could be avoided in 75% of patients with early stage cervical cancer if SLNB is applied.186 In addition, SLNB allows for identification of lymph node metastases in atypical regions. SPECT/CT is helpful to delineate different lymph node groups in cervical cancer, especially external iliac versus internal iliac nodes, and parametrial nodal uptake.187
SLNB is not a standard procedure for cervical cancer patients in current practice, however, because uncertainties remain regarding the false-positive rate (FPR), the significance of parametrial nodes, and other relevant topics of interest. Large-scale, prospective randomized studies are needed to confirm the safety of omitting CLND in patients with negative SLN, before pelvic lymphadenectomy is replaced with SLNB as a standard of care in the surgical management of early stage cervical cancer.
Endometrial cancer is the most common gynecologic malignancy and the second most lethal gynecologic malignancy. In the majority of cases, the disease is confined to the uterus at the time of diagnosis with a relatively favorable prognosis. The surgical treatment of early stage endometrial cancer includes total hysterectomy and bilateral salpingo-oophorectomy with or without pelvic and para-aortic lymphadenectomy.
SLN mapping for endometrial cancer is technically more challenging because the lymphatic drainage of the uterine corpus is located bilaterally with multiple lymphatic pathways. The lower uterine segment drains into the pelvic lymph nodes whereas the upper segment of the uterine corpus drains into the para-aortic lymph nodes.188 Several injection strategies have been described for endometrial cancer SLNB, including injection into the cervix, the endometrium during hysteroscopy, or the uterine subserosa during laparoscopy/laparotomy.189 Cervical injection is the most convenient route for both blue dye and radiolabeled agents, and is correlated with an increased detection rate in the range of 80% to 100%.190 Ballester et al. recently reported a prospective, multicenter cohort study including 125 patients with early stage endometrial cancer. SLNB was performed via cervical combined injection, and SLNs were serial sectioned and examined by IHC. The SLN detection rate was 88.8%, with pelvic lymph node metastases in 17% of cases and para-aortic node metastases in 5% of cases. SLNB upstaged 10% of low-risk patients and 15% of intermediate-risk patients with endometrial cancer. SLNB had a sensitivity of 84% and an NPV of 97%.191
The role of SLNB remains investigational in endometrial cancer. In general, SLN detection rate remains low for patients with endometrial cancer compared to melanoma or breast cancer patients. A recent meta-analysis of the detection rate and/or sensitivity of SLNB in endometrial cancer (including 26 eligible studies and 1,101 SLN procedures) revealed an overall SLN detection rate of 78% (95% CI, 73% to 84%) and an overall sensitivity of 93% (95% CI, 87% to 100%).190 The current literature does not provide sufficient evidence to establish the value of SLNB in endometrial cancer because of very small sample sizes in the majority of available studies, and the lack of prospective randomized controlled trial design.
Regional node status is the most important prognostic indicator for patients with invasive breast cancer or melanoma. Since its first description in early 1990s, SLNB has become the standard for accurate staging of regional lymph node involvement in multiple malignancies, most commonly breast cancer and melanoma. SLNB has revolutionized the management of patients with breast cancer and melanoma by providing accurate lymph node staging, with the majority of patients being spared from a comprehensive lymphadenectomy and its associated morbidity. SLNB is routinely performed using a combination of blue dyes and radiolabeled agents, and preoperative scintigraphic imaging is commonly performed to assist in the intraoperative search for SLN, which is especially valuable in the setting of melanoma, for detecting IMN SLN in patients with breast cancer, and for detecting SLN in any patients who have undergone prior biopsy or surgery. SPECT/CT can also be employed in difficult clinical situations. When performed appropriately, the SLN identification rate is >95% in patients with breast cancer or melanoma.
INDICATIONS FOR SENTINEL LYMPH NODE BIOPSY IN CUTANEOUS MELANOMA
SLNB is indicated for the majority of patients with breast cancer without nodal or distant metastases. Only a small fraction of breast cancer patients should not have SLNB, including: (1) patients in very early stage breast cancer (low grade DCIS); (2) patients with histologically confirmed positive axillary or extra-axillary lymph nodes or distant metastases; (3) patients with inflammatory breast cancer; (4) patients who cannot tolerate the procedure; and (5) patients who are allergic to vital dyes or radiolabeled agents. Pregnancy, palpable axillary nodes, multifocal or multicentric tumor, and prior breast or axillary surgery are not contraindications for SLNB in breast cancer patients. The significance of micrometastatic nodes remains to be clarified, but there is a trend toward more conservative treatment.
SLNB should be offered to patients with clinical stage T1b to T4b cutaneous melanoma as long as there is no evidence of regional lymph node or distant metastases (Table 28.2). These include patients with intermediate thickness (1- to 4-mm) melanoma and thick (>4-mm) melanoma. For patients with thin melanoma (0.76 to 1 mm), SLNB is indicated if there are any adverse features that might portend a higher risk of nodal involvement, such as positive deep margins, ulceration, high mitotic rate, lymphovascular invasion, or Clark level IV to V. CLND is indicated if SLNB is positive for metastasis, and radical lymph node dissection is also recommended in melanoma patients with micrometastatic disease on SLNB.149
SLNB has also been used in the surgical treatment of other solid tumors, but is still under investigation. Many controversies remain regarding the technique itself, the pathologic evaluation of the SLN, the significance of microscopic nodal metastases, and the significance of false-negative SLN. Further studies are needed to refine the protocol of SLNB in patients with these malignancies, to establish the safety and accuracy of SLNB, and to clarify its value in terms of low regional nodal recurrence rates and long-term survival benefit.
Multiple prospective clinical research studies are currently ongoing. It is likely that a clearer picture regarding the survival benefit of SLNB in breast cancer and melanoma patients, as well as the significance of micrometastases and other controversial issues, will emerge. Progress regarding the value of SLNB in patients with other solid tumors. At the same time, SLNB techniques will continue to be modified and refined, making the identification of SLNB easier, and diagnosis of SLN metastasis more reliable. In particular, hand-held intraoperative portable γ-probes have been applied in clinical and experimental settings in the last 10 years192 and have been very promising for the detection of occult SLNs. These devices, dedicated to accurate and real-time intraoperative imaging and direct visual guidance, are able to provide visualization of the size, shape, and location of target “hot” spots in real time, and help to find additional SLNs in multiple malignancies including breast cancer,193 melanoma,193 gynecologic malignancies,193 urologic malignancies,194 head and neck melanoma, and oral cavity carcinoma.195
Another very promising potential improvement in SLNB techniques is intraoperative pathologic evaluation by RT-PCR. The pathologic evaluation is ideally performed immediately to determine if there is evidence of metastasis in the SLN to provide guidance for the subsequent surgery. However, intraoperative frozen section analysis of SLN has a high FNR, which cannot be improved significantly. RT-PCR is a highly sensitive method to identify specific mRNA markers of malignancy. Multiple targets of one or more tumor-associated antigens for a malignancy can be used for simultaneous RT-PCR analysis to increase both sensitivity and specificity. Addition of RT-PCR analysis significantly decreases the FNR of SLNB. For example, in cutaneous melanoma, the FNR was 3.6% with RT-PCR versus 17.9% without RT-PCR, when all lymph nodes were examined.196 In recent years, rapid highly automated real-time RT-PCR–based platforms have been developed which allow for incorporation of lymph node staging into the ongoing surgical procedure and avoid a separate delayed lymph node dissection in patients with breast cancer197or head and neck cancer.198 Importantly, the assay can be performed in a little more than half an hour. Because the technique of RT-PCR is much easier to improve, and multiple options exist for targeting different genes, more promising developments will likely be realized in this field in the near future.
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