Alice Lorenzoni • Ettore Seregni • Marco Maccauro • Andrea Maurichi • Alessandra Alessi • Flavio Crippa
Cutaneous melanoma is estimated to be the seventh most common malignant tumor, and a steady increase in incidence has been observed over the past 30 years.1 The incidence of malignant melanoma varies from 3 to 5/100,000/year in European countries to 12 to 20 in Australia, which has the highest incidence of cutaneous melanoma.2 Ultraviolet light exposure is only in part responsible for the increase in incidence that is related in part to improved screening programs. Consequently, the detection of earlier lesions with a more favorable prognosis is more frequent. The mortality rate is 3 to 5/100,000/year and has remained relatively stable over the last decade. The median survival time for patients with metastatic melanoma, however, is less than 1 year.
Risk factors include the presence of multiple atypical nevi, family history of melanoma, episodes of severe sunburn, and freckles. Thirty-five percent of patients with malignant melanoma will develop another cutaneous malignancy within 5 years.3 The natural history is characterized by hematogenous and lymphatic dissemination to regional and distant sites.
DIAGNOSIS AND STAGING
An atypical pigmented lesions suspicious for malignant melanoma is characterized by asymmetry, border irregularities, color heterogeneity, dynamics (dynamics in colors, elevation or size) (“ABCD rule”). The diagnosis is based on a full thickness excisional biopsy.
Histopathologic analysis includes evaluation of tumor thickness (Breslow measurement), presence of ulceration as well as of the mitotic rate/mm2, and the involvement of surgical margins. The updated version of the American Joint Committee on Cancer (AJCC) staging and classification system, which include sentinel node (SN) staging, is the internationally accepted classification system (Table 22.1).4
Tumor thickness thresholds were changed to reflect updated prognostic information (thresholds of 0.75, 1.50, and 4 mm were changed to 1, 2, and 4 mm), mitotic index was added as an independent prognostic factor (regardless of stage). Breslow thickness and the presence of ulceration are the two most important independent prognostic factors for localized melanoma, associated with an increased risk for distant metastases and poor survival. The value of mitotic rate as an independent determinant of prognosis was particularly evidenced in thin melanomas (≤1 mm in thickness).
The total number of metastatic lymph nodes influences subcategorization within stage III, and macroscopic nodal involvement is associated with worse prognosis compared to microscopic lymph node metastases. Lung metastases have a more favorable prognosis compared to other distant visceral lesions (M1b versus M1c disease).5 Consequently, accurate disease staging is important for appropriate patient management with melanoma.
Surgery with a wide excision of primary tumors is the potentially curative treatment for localized melanoma. Elective lymphadenectomy or irradiation of the regional lymph nodes does not significantly improve the overall survival. The role of adjuvant radiotherapy (RT) to the primary has not been defined in clinical trials. In stage IV melanoma, RT plays an important role in the treatment of brain metastases and may be useful for other systemic metastases as palliative treatment. The role of adjuvant whole brain RT or stereotactic radiosurgery is still controversial. Adjuvant therapy with dicarbazine and interleukin 2 has not been shown to improve survival.
AMERICAN JOINT COMMITTEE ON CANCER STAGING
An extent-of-disease workup in patients with melanoma should be considered to detect clinically occult disease and to define homogeneously staged patients for inclusion in clinical trials.
The utility of imaging studies in patients with melanoma generally depends on the stage of the tumor. In patients with early stage disease, all imaging methods have limited utility, with a very low sensitivity. The presence of clinical indications (e.g., suspected or palpable lymph nodes) should be considered for further imaging. Computed tomography (CT) imaging is not particularly helpful in the initial evaluation of patients with stage I or II disease because of the overall low frequency of patients with anatomically identifiable metastases.6 Neither is ultrasound (US) routine in the diagnostic workup of primary melanomas. In case of clinical suspicion of metastatic adenopathy, however, US of the lymph node with fine-needle aspiration (FNA) or biopsy can be useful to provide an indication for radical lymph node dissection. US of regional lymph nodes is able to accurately demonstrate metastases in 65% of SN-positive patients.7 Chest x-ray is commonly used for the evaluation of pulmonary lesions in newly diagnosed melanoma. However, this procedure has low sensitivity and specificity. In fact, in less than 1% of patients with abnormal radiography, other imaging modalities confirm the presence of metastatic disease.8 Patients with regional lymph node involvement (stage III) should undergo diagnostic imaging at the time of diagnosis to detect possible distant metastasis. The detection rate is low particularly in patients with microscopic lymph node involvement (stage IIIA). The use of CT in stage III melanoma can be particularly helpful in patients with groin or cervical adenopathy.9
In high-risk patients (e.g., those with thick primary tumors) or in patients with suspected or known stage IV disease, diagnostic imaging plays an important role. It may lead to an earlier diagnosis of regional or systemic relapses. In this setting, CT is most useful in the evaluation of pulmonary metastases but the sensitivity in the evaluation of intra-abdominal and distant nodal metastases is not well defined.10 MRI is usually reserved for the evaluation of suspected brain metastases and suspected or known liver metastases.11 However, the impact of radiologic examinations upon survival has not been demonstrated. Furthermore, no single modality is highly sensitive and specific for the detection of all types of local or distant lesions.
In the past, several radiotracers such as 67Ga citrate, labeled immunoreactive cells, or 99mTc-MIBI have been used for the detection of distant metastasis but the sensitivity of these techniques was limited and they were not used widely.
SENTINEL LYMPH NODE
The concept of sentinel lymph node (SLN) identification, the first node to which a tumor spreads, implies that nodal metastases progress in a nonrandom pathway explaining the pattern of metastatic disease in not only cutaneous cancers such as melanoma but also other organ-specific tumors such as breast, colon, and endometrium.
The relationship between the primary tumor location and the metastatic spread of cancer to specific regional lymph node was first described by Virchow in 1863.12 In 1955, radiolabeled colloidal gold was used by Seaman and Powers to describe metastatic spread to regional lymph node13 but the concept of “sentinel node” was first used in 1960 by Gould et al.14 to describe the presence of the first node in a nodal basin to be involved by cancer cells Sentinel lymph node biopsy (SLNB) provides important diagnostic data about the status of the nodal basin, as a negative SN correlates with the lack of metastatic involvement elsewhere. In fact, the negative predictive value for the remaining nodal basin has been shown to be 95%.15 Reintgen et al.16 validated the SN approach in melanoma by elucidating the concept of an orderly progression of lymph node metastases in melanoma. Preoperative localization of SLN has been used to identify draining nodal basins in patients with melanoma of the trunk to perform elective lymph node dissection. When the SLNB technique was introduced in the early 1990s, preoperative lymphoscintigraphy (LS) was of great assistance not only in identifying with certainty the draining lymph node field but also in providing precise information about the location and number of SLNs within that field. This led to the introduction of routine preoperative LS in patients scheduled for SLNB. It became apparent that SLNs were sometimes in clinically unexpected locations. Even for sites from which lymphatic drainage was thought to be completely predictable, surprising SLN locations were occasionally demonstrated.
SLN status has been shown to be the most important independent prognostic factor of melanoma stage I to II patients. In the past, elective lymph node dissection of the lymph node region draining the primary tumor site (based on Sappey’s anatomic description of cutaneous lymphatic pattern) was used as part of the staging procedure. However, in 80% of stage I to II melanoma, there are tumor-negative lymph nodes; consequently, the elective lymph node dissection associated with a significant morbidity is not an optimal procedure.
SLNB is a minimally invasive procedure for the staging of melanoma when there is no clinical evidence of regional nodal metastases. The term SLNB includes the preoperative node localization with a radiotracer, the intraoperative removal of the SLN, and pathological evaluation.
US-guided FNA is not a sufficient alternative to SLNB as it has low sensitivity to detect micrometastasis.17 In 1992, SLN has been defined as “the initial lymph node upon which the primary tumor drains.”18 More recently, the definition of SLN has been modified to “a node upon which a lymph vessel originating in the tumor drains directly” including all nodes in which the tumor directly drains. In fact, these nodes are at risk to receive malignant melanoma cells. The in-transit nodes (also called interval-node) are lymph nodes localized on a direct drainage pathway between the primary tumor and the draining lymph node basin.19 They are present in 3% to 10% of the cases and often are localized in subcutaneous fat.20 These nodes should also be considered as SLNs, because they directly receive the tumor lymphatic vessels.
Interval nodes can occur anywhere between a melanoma site and a recognized node basin, but they are more common in the midaxillary line on the lateral thorax and in the posterior loin area. Other common sites for interval nodes are on the upper back toward the base of the neck, in the upper arm, and over the costal margin.
Patients with positive SLNs have significantly decreased disease-free survival compared with those who have negative SLNs (5-year survival rate 72% versus 90%).21 Several factors are associated with the probability of having a positive SLN. They include age, mitotic rate, Breslow depth, ulceration, angiolymphatic invasion, and microsatellitosis.
SN localization and excision using radionuclide methods are indicated in patients with intermediate stage (Breslow 0.76 to 4 mm) and without clinical evidence of nodal involvement or distant disease. Melanoma with Breslow thickness greater than 4 mm and lymph nodes not clinically palpable have a very high risk of lymph node metastases (around 50%).
SLNB can be also an option in patients with melanocytic tumors of uncertain metastatic potential (MELTUMP). The contraindications to SN biopsy are as follows.
1. Extensive previous surgery in the primary tumor site with a possible significant modification in the drainage pathway or severe local inflammatory/infectious process.
2. Presence of known metastases.
3. Severe concurrent disease and poor patient compliance.
Pregnancy is not a contraindication to SLN mapping because of a low radiation exposure. In any case, the procedure should be modified to minimize radiation exposure to the fetus. The dose to the fetus from an SN examination is generally below 1 mSv.22 Breastfeeding should be discontinued prior to and 24 hours after the procedure.
SLN is evaluated with standard histological technique (hematoxylin and eosin staining) and immunohistochemistry (IHC). IHC, used to identify typical melanoma-associated antigens (S-100, HMB-45, MART-1/Melan-A), has been shown to increase the positive SLNs rate up to 34%.23
The lymphatic pathway may be altered after a wide local excision (WLE),24 thus SLNB ideally should be performed prior to WLE if possible. Leong et al.25 found that performing an SLNB in patients who had a previous WLE on the extremities still provides useful diagnostic data. Similar findings were reported by Kelemen et al.26
Lymphatic Drainage Pathway
In 1984, Sappey described lymphatic drainage pathway of the skin suggesting that it occurs in a predictable pattern. Therefore, the lymphatic drainage described never crossed the midline. Subsequently, cutaneous lymphatic pattern was found to be unpredictable in an area 5 cm wide along the midline and a circumferential area between the umbilicus and L2 termed Sappey’s line.27 The introduction of lymphoscintigraphy has radically changed previous descriptions of lymphatic pattern, determining that the entire head and neck area, as well as a wide area of 10 cm from the midline and 10 cm from Sappey’s line has unpredictable lymphatic drainage.
Lymphatic drainage pathway tends to be ipsilateral but the possibility of contralateral drainage should be evaluated especially in the region of the back and face.
The usual drainage from the upper limb is to the axilla, as well as the drainage from the leg and the thigh is to the groin. In some cases, popliteal nodes draining the leg can be identified. The lymphatic drainage patterns on different parts of the trunk are markedly variable (axillary, groin, and supraclavicular nodes) and cannot be predicted (Fig. 22.1). In posterior melanoma of the trunk, the triangular intermuscular space, lateral to the scapula should be considered. In fact, in 11% of posterior upper trunk, melanoma SLN is localized in this area. Lymphatic vessels after passing through triangular intermuscular space reach the axilla, thus the node in the axilla may be a second node, with the SN in intermuscular space. The node in the axilla may be a second node, with the SN in intermuscular space. In 3.4% of posterior trunk melanoma, the lymphatic pathway is directly anterior through the posterior abdominal or thoracic wall to paravertebral, para-aortic or retroperitoneal nodes. In a few patients with melanoma of the periumbilical area, SNs are localized to internal mammary chain with interval nodes localized along the costal margin.
For the lower and upper limb sites, popliteal fossa or epitrochlear nodes are occasionally observed in 3% to 5% of cases. Unexpected lymphatic drainage from upper limb site includes direct drainage from the forearm or supraclavicular nodes.
In head and neck melanomas, lymphoscintigraphy showed discordant results compared to clinical predictions in a high proportion of patients.
The drainage from the face is localized to preauricular and parotid nodes and level I to III cervical nodes. The drainage across the midline is observed in 15% of the patients. From the posterior scalp, the drainage is particularly unpredictable and it tends to occur to postauricular, occipital, cervical (level I to V), and supraclavicular nodes.
Atypical drainage pathways are not unusual especially regarding head and trunk regions.
Lymphoscintigraphy is in the acquisition of the imaging pathways of lymphatic flow and lymph nodes after the injection of an appropriate radiopharmaceutical.
FIGURE 22.1. Lymphatic drainage pathway of trunk melanoma (whole-body anterior image). Node mapping shows the presence of sentinel nodes both in axillary and inguinal regions.
Several colloidal radiotracers have been used for these procedures, varying in different countries. In the 1950s, colloid gold-198 was the radiopharmaceutical used in the first clinical studies by Walker.28 This colloid (5 to 10 nm), however, has β-emission and a long half-life (2 to 3 days) within unacceptable radiation-absorbed dose. Thus, a 99mTc-radiocolloid was developed.
99mTc-stannous phytate and 99mTc-antimony were introduced in 1970s for lymphoscintigraphy.29
The optimal particle size for SN detection is 100 to 200 nm (Table 22.2). Larger particles (>200 nm) may have difficulty moving through the interstitial matrix to enter in the lymphatic capillaries. Most of the injected dose remains at the injection site. The mechanism of localization in the lymph node is more complex than mechanical filtration. It is caused by the phagocytosis by macrophages because of the delayed transit of these large tracers rather than particle’s trapping based on their size. 99mTc-human serum albumin (HSA) colloid (albumin nanocolloid, Nanocoll, Sentiscint) is currently used in Europe, 99mTc-sulfur colloid in North America, and 99mTc-antimony trisulfide colloid in Australia (1 to 30 nm diameter). The size range is 5 to 80 nm for Nanocoll, 100 to 600 nm for Senticint, and Nanocis has a median diameter of 100 nm. 99mTc-sulfide colloid is filtered (0.22-mm filtration) or unfiltered form. The size of the radiocolloids influences the timing of image acquisition.
The activity of injected radiocolloid varies from 5 to 120 MBq, depending also on the surgical timing (1-day or 2-day protocol). If 2-day protocol is planned, the injected activity (adjusted for physical decay) should exceed 10 MBq. Usually about 90% to 95% of the injected activity remains at the injection site. 99mTc-sulfur colloid has a prolonged injection site clearance half-life that can impede identification of the SLNs close to the primary lesion.
Injection Technique and Image Acquisition
Peritumoral intradermal injections of small (2 to 8) aliquots (0.1 to 0.2 mL each) of the radiocolloid should be administered by within 1 cm from the tumor or the excisional biopsy site of the melanoma. In patients with melanomas located in region with unpredictable drainage (e.g., the head, neck, and trunk), radiotracer injections should be performed equatorially around the lesion. The injection should be performed using a 25- or 27-gauge needle inserted in a direction as tangent as possible to the skin surface. The injection site should be covered to prevent leakage of tracer through the puncture site. Contamination of the skin can complicate image interpretation. The administration of radiocolloids has not been known to have interactions with drugs. Adverse effects are rare.
Sequential or continuous imaging begins immediately after completion of injections and continues up to 1 hour or until the SLN is identified. A large field-of-view camera head is preferable (Fig. 22.2).
The use of low-energy, high, or ultrahigh resolution collimators with parallel hole is recommended to reduce the septum penetration from the injection site.
A 10- to 30-minute dynamic image at 30 to 60 seconds per frame in a 128 × 128 matrix can help to determine the location of the lymphatic collectors. Anterior and posterior static images (256 × 256 matrix, acquisition time 5 to 10 minutes) over the identified node region should be acquired to identify and localize the SLNs. Lateral or oblique views are often helpful to correctly localize multiple nodes, especially in the head and neck, axilla, and groin areas. Dynamic imaging combined with delayed static imaging is superior to delayed static imaging only because the latter is more likely to fail in identifying SLNs.30 Delayed images should be performed in all node fields that could possibly receive lymphatic drainage to identify unusual drainage pathway.
FIGURE 22.2. Patient with anal melanoma. Anterior planar image of the pelvic region shows the presence of sentinel node in the bilateral inguinal region.
Single photon emission computed tomography (SPECT) or combined SPECT/CT imaging improves lesion detectability and the anatomic localization of nodes. In our opinion, SPECT/CT imaging, however, does not replace the conventional planar images. SPECT acquisition should be performed with a matrix size of 128 × 128, 180 degrees in the anterior L-mode rotation, and step-and-shoot of 20 to 30 seconds at 3-degree intervals. CT acquisition is usually performed as a low-dose CT (16 seconds for each transaxial slice). SPECT/CT is particularly helpful in the head and neck region and also in the pelvis where planar images cannot clearly identify lymph node locations. SPECT/CT identifies SNs missed on planar images, including nodes invaded by metastases in 43% cases of primary melanoma located in the head and neck or trunk region.31
Interval nodes should be distinguished from dilatation of lymphatic vessels. A lymphatic dilatation shows a rapid fade of radiotracer and usually disappeared within 1 hour.
Moreover, the patient’s body contour, useful to facilitate topographic localization, may be defined by transmission imaging (e.g., 57Co flood source) or by manual tracing with an external source. The surface location of the SLNs should be marked on the skin in the same position as for the surgical procedure.
SLNs can be missed at scintigraphy and/or surgery where the node is not visualized because it is obscured by another node or injection site or when the SLN contains only a small amount of radioactivity. False-positive interpretation includes lymphangioma or lymphatic lakes, skin folds, and other tissues containing radioactivity or skin contamination from the injection.
The use of blue dye before surgery may be useful as a visual confirmation of the SN, especially when a melanoma is in proximity to its regional nodal basin. In this case, the injection site of the radiotracer causes a high background radioactivity that interferes with the γ-probe localization. Blue dye (Isosulfan Blue or Patent Blue V) is injected intradermally (0.5 to 1 mL) 10 to 20 minutes prior to the surgical procedure around the site of primary tumor.
Intraoperative SN detection is performed with the use of a γ-probe. The first use of hand-held probe to identify SN intraoperatively was reported by Alex et al. in 199332 in 10 malignant melanoma patients. They reported a sensitivity equal to vital dye mapping, showing that these techniques correctly permits SLN identification and biopsy.
The hand-held probe, used to localize the SLNs, is a γ-radiation detector, either a crystal or a solid-state device. The collimation of the probe, resulting in a restricted field of view, is fundamental for the correct locations of focal radioactive accumulation, avoiding photons from sources that are not directly in front of the probe. Removal of the collimator provides increased sensitivity but a loss of spatial resolution. The unit connected to the probe provides a count rate from measured γ-rays, usually by audible sound volume variation and/or a visual display. A probe is sensitive in the range 650 to 900 cps/MBq for a 3-cm-deep node.
The number of SN removed depends on the anatomic location of the primary tumor and the number of draining nodal basins identified on preoperative lymphoscintigraphy.
Several probe criteria have been employed for identification of the SN (e.g., count rate of the SN compared with other nodes in vivo, ex vivo, or with background counts in vivo).
McMasters et al.33 suggested removal of all radioactive nodes until the background counts of the undissected nodes decrease below 10% of those of the hottest node. Other investigators have suggested that removing the 3 hottest nodes and all blue-staining nodes is sufficient to detect 100% of positive SLNs. In addition, removing lymph nodes with less than 30% of the radioactivity of that of the hottest node was found to be of no value.34
When a node occasionally is largely replaced by a tumor cell, the radioactive tracer may be diverted to a secondary node, resulting in lower counts in the SN. Removing only the hottest node may fail to remove the true SN in about 20% of patients. In fact, Martin et al.35 showed that the SLN with the highest counts is positive in 80% of patients with multiple SLN.
If preoperative lymphoscintigraphy identifies SNs that are not identified intraoperatively, a lymph node dissection should be performed.
The estimated local radiation dose varies depending on the administered activity and retention time. Nevertheless, the absorbed dose at the injection site (20 to 44 mGy/MBq) is below the threshold for deterministic radiation effects. The different radiopharmaceuticals used are not associated with substantial differences in terms of radiation exposure. The maximum effective dose has been calculated as 0.0019 mSv/MBq. Obviously, SPECT/CT imaging increases both the effective and local radiation dose.
Operating room staff receive less than 6 μSv/h; the activity in the removed SNs is usually less than 0.5 Bq/g.36 Necj et al.37 estimated that the maximum dose is 1,900 times less than the current 1-year dose limit recommended by the International Commission on Radiological Protection (ICRP).
Clinical Impact of Sentinel Lymph Node Biopsy
The SLNB is a diagnostic tool designed to assess the status of regional lymph nodes providing critical prognostic information and to determine further treatment.
For thin melanomas (0.5 to 1 mm), the probability of a positive SLN is 6%; for intermediate thickness melanomas (1.01 to 2 mm), the risk is 16%; for intermediate thickness tumors (2.01 and 4 mm), the risk is 34%; and for thick melanomas (4 mm), the risk is about 55%.38
The presence of a small size (<0.2 mm) nodal micrometastasis characterized only by IHC may not have an impact on prognosis.39 However, SLNB technique can identify these very early micrometastases, raising the issue of the clinical relevance of these findings. In this scenario, some authors have advocated the “prognostic false-positivity” theory.40 Govindarajan et al.41 analyzed tumor characteristics, SLN tumor burden, and the location of the metastasis in the SLN, to serve as additional prognostic factors compared to non-SLN positive, disease-free survival, and overall survival. They found that submicrometastases (clusters of more than 10 cells, but <0.1 mm) may not be considered as metastatic melanoma because the estimated 5-year distant metastasis-free survival in these patients was comparable to the overall survival rate for SN-negative patients. Consequently, these patients are highly unlikely to benefit from lymph node dissection.
In contrast, a study performed by Scheri et al.42 demonstrated that 5-year melanoma-specific survival rate was significantly less in the presence of micrometastases compared to negative SLNs (89% versus 94%). It has been proposed that SLN with a micrometastasis less than 0.2 mm should be regarded as node negative [N0(i1)] and any positive lymph node with a greater than 0.2-mm focus of tumor be classified as stage III disease.43,44
Thin melanomas treated by early detection and resection result in excellent long-term survival. However, approximately 15% of patients develop recurrence; regional node involvement in 5% of cases and distant metastases in 3% to 4% of patients.45,46
SLNB could be effective in improving outcomes in selected high-risk patients with thin melanoma <1 mm. The presence of negative SLN, whereas another non-SN of the same basin contains metastatic cells, occurs in less than 1% of the cases (“skip metastases”) so the negative predictive value of SLNB is very high. To determine the incidence of skip metastasis, investigators evaluated patients with SLNB followed by complete lymphadenectomy. Other studies based on the long-term recurrences in previously mapped negative lymphatic basins suggested that the incidence of false-negative SN studies may be higher, ranging from 4% to 16%. Many of these studies utilized either frozen sections, or incomplete sampling of the node, or did not utilize IHC.
FIGURE 22.3. Lateral static image of the head in patient with left auricular melanoma. The sentinel node is localized to the ipsilateral cervical node.
The SLN approach is particularly useful in head and neck melanomas, in which lymphatic drainage is variable (Fig. 22.3). However, in 5% to 10% of patients, preoperative lymphoscintigraphy fails to identify any SLNs in patients with head and neck melanomas,47,48 making intraoperative localization of an SLN particularly difficult. In a large prospective study, Pathak et al.49 showed that the lymphatic drainage is not predictable in around 10% of patients with head and neck malignant melanoma. They correlated the anatomic distribution of pathologically involved lymph nodes with primary melanoma sites and compared these findings with clinically predicted patterns of metastatic spread. Furthermore, in 30% of patients the metastatic involvement affected more than one lymph node basin.50
The presence of a positive SLN is the most important prognostic factor in patients with head and neck melanomas. In fact, patients with negative lymph node involvement have a higher survival rate than patients with positive SN biopsy.50 The presence of recurrence in a previously node-negative basin by SLN, called false-negative rate, is approximately 3%.47 These false-negative results were initially attributed to skip metastases. Later, Thompson51 showed that 77% of node-negative SLN patients with recurrent nodal disease had undiagnosed micrometastases when the SLNs were reexamined. Similar results were reported by Gershenwald et al.52 estimating that with appropriate pathologic evaluation, the nodal recurrence rate was 3%. Failure to identify and subsequently remove all the SLNs represents an inadequate nodal staging procedure that may potentially place the patient at higher risk for nodal as well as visceral recurrences.
Iron oxide nanoparticles (SPIONs), used as magnetic resonance imaging (MRI) contrast agents, may represent an interesting tracer for SLN technique. These nanoparticles (diameter <10 nm) have several properties including favorable superparamagnetic properties and biodegradability as well as modifiable kinetics in vivo.53 SPIONs labeled with 99mTc have been investigated in animals to develop a new multimodality SPECT/MRI contrast agent.54 After subcutaneous injection, the radiotracer showed high accumulation in SLN, preferably in the cortical and subcapsular sinuses.
ROLE OF 18F-FDG PET
Positron emission tomography (PET) with 18F-FDG has been extensively investigated in patients with melanoma over the last two decades, establishing an important role in the staging and a potential role in assessing response to therapy. The rationale for the use of 18FDG was demonstrated in a study performed by Wahl et al.55 in 1991. Murine melanomas and human melanoma xenografts showed uptake of the radiotracer. Two years later, Gritters et al.56found a sensitivity and specificity of 100% to detect visceral and lymph node metastases in 12 patients with melanoma.
More recently, a significant relationship between [18F]-FDG uptake and the expression of GLUT-1 and GLUT-3 in malignant melanoma was observed,57 whereas HK-2 and Ki-67 were not related to 18FDG uptake of malignant melanoma.
In meta-analyses, the sensitivity, specificity, and accuracy of 18F-FDG PET to detect recurrent melanoma ranged from 70% to 100%.58–60
False-negative and false-positive rates of 18F-FDG PET can be reduced through the use of PET/CT compared to PET alone.61,62 In a study including 250 patients with melanoma (AJCC stages I to IV), PET/CT was found to be significantly more accurate than PET alone and CT alone for the staging of visceral and nonvisceral metastases. PET/CT was particularly helpful for the detection of lung metastases that are often missed by PET alone. These data also showed an incremental impact of PET/CT on treatment in the settings of restaging and therapy control in patients with melanoma.63
18F-FDG PET is not useful in the initial staging of primary cutaneous melanoma when there is no clinical evidence of local or distant metastatic lesions as detection of occult regional lymph node metastases is limited (Fig. 22.4). The basis for the low sensitivity is likely in the small mean tumor volume of lymph node metastases (<5 mm3) that is usually found in melanoma patients.64,65
In early stage disease, the sensitivity of 18FDG PET/CT for detection of tumor in regional nodes is unacceptably low, ranging from 0% to 22%. Consequently, for 18FDG PET/CT this imaging modality cannot replace the SLNB procedure.66,67 Whole-body MRI, with a sensitivity, specificity, PPV, NPV, and accuracy of 66%, 77%, 84%, 55%, and 67%, respectively, for the detection of lymph node metastases, has been shown to be equal in accuracy to whole-body CT68 and inferior to 18F-FDG PET/CT.69
FIGURE 22.4. 18F-FDG PET/CT (A: MIP image; B: PET/CT axial fused image; C: CT axial image; D: coronal PET/CT fused image; E: coronal CT image) in a patient with surgically treated posterior cervical melanoma. PET scans confirm avid FDG uptake adenopathy. Ultrasound shows the presence of suspicious lymphadenopathy in the right laterocervical region.
FIGURE 22.5. 18F-FDG PET/CT (A: MIP image; B: PET/CT axial fused image; C: CT axial image; D: coronal PET/CT fused image; E: coronal CT image) in patient with malignant melanoma of the right foot. The PET scan in (A) demonstrates cutaneous in-transit metastases and subcutaneous metastases in the right lumbar region (arrow ).
The utility of 18FDG PET in patients with known or suspected local recurrence, satellite lesions, or in-transit metastases (Fig. 22.5) is not established because of the lack of a sufficient number of conclusive studies. Current estimate of the sensitivity ranges from 50% to 93% and specificity from 50% to 100% to detect loco-regional metastatic disease.
Positive prognostic value of 18FDG PET/CT for the detection of recurrent metastases is stage dependent, yielding a higher PPV in high-risk patients (80%) than in intermediate-risk patients (63%) and low-risk patients (33%).70
Patients with suspected regional metastases based on physical examination or other imaging modalities have a high prevalence of detectable metastases on PET (80% or greater). Blessing et al.71 found a sensitivity of 74% and a specificity of 93% for the evaluation of 20 clinically suspicious lymph node basins. Crippa et al.72 showed an accuracy of 91% in the detection of metastases in 56 lymph node basins and a negative predictive value of 89%. Sensitivity was found to be very low for lymph nodes <5 mm but was 100% and 83% for nodes ≥10 mm and 6 to 10 mm, respectively.
18FDG PET is frequently used to evaluate patients with clinical, laboratory, or radiologic evidence of distant metastases (Fig. 22.6) and in patients with previously treated distant metastases to plan the management. Finally, 18FDG PET can detect unexpected metastases in patients with surgically treatable metastatic melanoma and should be considered as part of preoperative workup.
In a study performed by Steinert et al.,73 PET/CT correctly identified 37/40 metastases with a sensitivity of 92%, in mixed group of patients with known metastatic melanoma or high-risk primary melanoma. In 18% of the cases, PET identified lesions not visualized with conventional staging modalities. Rinne et al.74 examined patients with thick primary melanoma and clinical or CT findings suggesting metastatic disease. Sensitivity, specificity, and accuracy were found to be 91.8%, 94.4%, and 92.1%, respectively, compared with 57.6%, 45%, and 55.7% for conventional imaging modalities. The superiority of 18FDG PET was confirmed by Holder et al.75 in a group of patients with different stages of disease. 18FDG PET had an overall sensitivity and specificity of 94.2% and 83.3% in the detection of liver, lymph nodes, and soft tissue lesions. CT showed overall sensitivity and specificity of 55.3% and 84.4%, respectively. The accuracy of detection of lung metastases was found comparable, but other studies have not confirmed these data. 18FDG PET is superior to conventional imaging to detect distant metastases, except in the lungs and brain, independent of the stage of the high-risk patient.
Three- and five-year overall survival is improved in patients who undergo resection of pulmonary metastases in the absence of extrapulmonary lesions excluded by 18FDG PET before surgery. In a study performed by Gulec et al.,76 PET identified more metastatic sites compared to CT and MRI, also visualizing metastases outside the field of view of these other imaging modalities. 18FDG PET findings changed patient management in 50% of cases.76
FIGURE 22.6. 18F-FDG PET/CT (A: MIP image; B: PET/CT axial fused image; C: CT axial image; D: coronal PET/CT fused image; E: coronal CT image) in patient with surgically treated malignant melanoma of the trunk. A follow-up PET scan shows hypermetabolic nodes at hepatic hilum (white arrow ) and peripancreatic pathological adenopathy (red arrow ).
In conclusion, 18FDG PET is the most accurate imaging modality to identify metastases in patients at high risk for developing distant lesions. 18F-FDG PET displays false-negative findings caused by small skin metastases or primary small skin lesions of melanoma.59,77 The cutaneous and subcutaneous lesions that were missed by 18F-FDG PET had a diameter between 1 and 10 mm.59
The use of 18FDG PET to evaluate response to therapy in patients with melanoma is not well established. In a retrospective study performed by Strobel et al.,78 responders to chemotherapy identified by 18F-FDG PET/CT have a longer progression-free and overall survival than nonresponders.
Integrated PET/MRI can be useful in melanoma restaging (N- and M-staging) and response assessment in high-risk patients (AJCC stages III to IV) but clinical data are not available at present. A hand-held 18F-FDG-sensitive intraoperative probe can be a valuable adjunct for the surgical localization of PET-positive tumors.79
A pilot study performed by Essner et al.80 showed the utility of a hand-held 18F-FDG-sensitive intraoperative probe in metastatic or recurrent melanoma in these patients. The PET probe detected all 18F-FDG PET-positive lesions and the smallest detectable lesion was 0.5 cm. In addition, the PET probe saw lesions that were not seen on the preoperative imaging study (retroperitoneal foci) or not immediately apparent at surgical exploration, particularly in a previously explored field (neck, axilla, and abdomen).
OTHER PET RADIOPHARMACEUTICALS
Several PET radiotracers have been developed in an attempt to identify melanoma metastases and to overcome problems associated with 18FDG including accumulation in inflammatory lesions resulting in false-positive results, decreased uptake in hyperglycemia, and lack of sensitivity to image brain metastases.
18F-thymidine is a pyrimidine analog phosphorylated by the enzyme thymidine kinase 1 (TK1), leading to intracellular trapping.81 During DNA synthesis, TK1 activity increases almost 10-fold and is, therefore, an accurate reflection of cellular proliferation.82
18F-thymidine PET/CT has been reported to have a sensitivity of 88% for the detection of regional lymph node metastases and a sensitivity of 60%. Brain metastases are readily detected with this radiopharmaceutical as there is no physiologic uptake of 18F-FLT in the brain.
11C-methionine PET detects primary lesions greater than 1.5 cm but fails to detect small pulmonary lesions in 50% of the cases.83
18F-DOPA (3,4-dihydroxyphenylalanine) has demonstrated to play a role in selected 18FDG-negative melanoma metastases.84 18F-DOPA reflects the activity of L-DOPA decarboxylase that may be increased in malignant melanoma. Ishiwata et al.85 showed a high uptake of 18F-DOPA in experimental studies using rats with transplanted B18 melanoma cells. Van Langevelde et al.86 demonstrated 11C-L-DOPA localization in a malignant melanoma. PET study obtained 40 to 80 minutes after injection of the [1-11C] labelled DOPA confirmed that melanoma detection with D-DOPA was promising, producing better image than L-DOPA. D-DOPA showed a high uptake in tumor tissue, with relatively low uptake in bone, skin, and eye resulting in high tumor/nontumor ratio.
Kubota et al.87 showed a preferential accumulation of 18F-DOPA in cells of the S-phase in rats with transplanted B16 melanoma cells. Further clinical trials are needed to clarify the role of 18F-DOPA in melanoma patients.
PRIMARY MUCOSAL MELANOMA
Primary mucosal melanomas represent a rare and aggressive malignancy, accounting for 1.3% to 1.4% of all melanomas.88 The median age at presentation is the seventh decade. It origins from melanocytes in noncutaneous tissues, such as uvea, mucosa of the gastrointestinal, respiratory, and genitourinary tracts, differing from cutaneous melanoma in terms of risk factor and biological behavior.89 The most common site of presentation is the head and neck region (50% of cases), followed by female genital tract, anal/rectal, and urinary tract. The biological course is characterized by multiple local recurrences, development of distant metastases, and poor prognosis.90 Local excision does not usually change the 5-year overall survival which is only 25%. In loco-regionally advanced disease, adjuvant RT may be useful if combined with surgical approach, especially in head and neck region or genital tract. New biological therapies may play a role in the management of this cancer.91,92 Uveal melanoma differs from other mucosal melanomas in terms of etiopathogenesis, treatment, and prognosis and it should be considered as a different biological entity.
Role of Nuclear Medicine in Mucosal Melanoma
The diagnosis of mucosal melanomas is often delayed because of the absence of symptoms and anatomical localization. CT, magnetic resonance, and 18FDG PET are fundamental diagnostic methods to evaluate the extent of disease even though staging system has not been well established. 18FDG PET/CT detects the presence of loco-regional disease and distant metastases in patients with uveal melanoma with high sensitivity and specificity, improving the staging when combined with conventional diagnostic methods.93,94 18FDG PET has not been found to be superior to MRI for the detection of small liver metastases. It may be useful, however, for the evaluation of early response to treatment of hepatic lesions, based on the change of standardized uptake value (SUV). In general, liver metastases from uveal melanoma show significantly lower SUV(max) compared to liver metastases from cutaneous melanoma.95Furthermore, 18FDG PET/CT in uveal melanoma is an important tool after ophthalmic plaque radiation therapy to verify the efficacy of the treatment.
SPECT with N-isopropyl-p-[123I]iodoamphetamine (123I-IMP), a radiotracer to assess cerebral blood flow, is a sensitive and specific technique for the diagnosis of uveal melanoma, especially in patients in whom conventional diagnostic techniques are inconclusive.96,97
RADIONUCLIDE THERAPY IN MALIGNANT MELANOMA
Several years ago, the feasibility of targeting melanin, an intracellular melanocyte pigment, to deliver cytotoxic radiation to human melanoma cells in vivo was demonstrated using a fungal melanin-binding monoclonal antibody (mAb 6D2) with promising therapeutic results.98
A recent study showed that 188Rhenium-labeled melanin-binding deca- or heptapeptides is a promising therapy in experimental melanoma.99 The melanin-binding decapeptide 4B4 was radiolabeled with 177Lu, 188Ho, and 153Sm via a DO3A chelate.
The melanocortin-1 receptor (MC1-R) is a family of G-protein linker receptors, which is primarily involved in the regulation of skin pigmentation. α-MSH peptides bind the MC1-R selectively with nanomolar to subnanomolar affinities,100 made it attractive molecule for the development of α-MSH peptide-based imaging and therapeutic agents because of the overexpression of MC1-R in malignant melanoma. Numerous α-MSH analogs have been developed with high affinities and specificities for α-MSH receptors.101 The preclinical studies demonstrate that α-MSH analogs, radiolabeled with SPECT and PET radionuclides, are able to image primary and metastatic melanoma tumors in mouse animal models. The therapeutic efficacies of α-MSH analogs labeled with α- and β-emitting radionuclide have been demonstrated in mouse melanoma models.
Receptor-mediated binding was only observed in tumor tissue, resulting in the selective deposition of imaging and therapeutic radionuclides in melanoma. The first radiolabeled peptide therapy study targeting the MC1-R was performed with 188Re-(Arg11)CCMSH and the therapy studies were performed in C57 mice-bearing B16/F1 syngeneic murine melanoma tumors.102 188Re-(Arg11)CCMSH administration resulted in tumor growth rate reduction, with no significant increase in the mean survival times of the treatment group compared to the control mice group. The biodistribution and therapeutic efficacy of 177Lu-DOTA-Re(Arg11)CCMSH were examined in B16/F1 tumor-bearing mice,102 showing encouraging results in terms of toxicity and efficacy. The therapeutic efficacy of 212Pb-DOTA-Re(Arg11)CCMSH has been proven with moderate kidney damage.103
Recently, the αvβ3 integrin avid peptide sequence Arg-Gly-Asp (RGD) was conjugated to the (Arg11)CCMSH α-MSH analog via a lysine spacer.104 The RGD peptide could induce cell apoptosis by activating procaspase-3 directly upon entering the cell. 99mTc-RGD-Lys-(Arg11)CCMSH exhibits rapid high melanoma uptake and prolonged tumor retention in B16/F1 melanoma mouse model.
The major challenges to successful clinical translation of radiolabeled MC1-R avid peptides for melanoma imaging and therapy are high nonspecific kidney uptake and low MC1-R receptor number. Radiolabeled benzamides are attractive candidates for targeted RT of metastatic melanoma as they bind melanin and exhibit high tumor uptake and retention per tumor cell. The role of radiolabeled benzamides for imaging melanoma was first realized in 1986 when iodine-labeled compounds under investigation for brain imaging were shown to localize to the pigmented eyes of C57BL6 mice but not the unpigmented eyes of Wistar albino rats.105
Subsequently, efforts have been focused on the development of melanin-localizing benzamides that exhibit rapid washout from nontarget tissues and prolonged tumor retention.
131I-MIP-1145 exhibited melanin-specific binding, rapid washout from nontarget tissues, and prolonged tumor retention, making it an ideal candidate for systemic RT. 131I-MIP-1145 inhibits the SK-MEL-3 tumor growth, resulting in tumor regression and a durable response with prolonged survival.
An accurate staging of patients with primary skin melanoma is fundamental to plan the treatment strategy. SN biopsy is the most important tool to correctly identify loco-regional staging in stage I to II melanoma. US evaluation with FNA or clinical diagnosis of lymph node involvement does not impact significantly on patient outcome because of the failure to identify micrometastases. 18F-FDG PET plays an important role in the evaluation of regional and distant metastases with higher sensitivity and specificity than other conventional diagnostic modalities. The role of other PET radiotracers such as 18F-DOPA or 11C-methionine is to be established. Radionuclide therapy in melanoma is still at its initial experimental stage and currently has no clinical application yet.
The authors would like to thank Dr. Emilio Bombardieri (Chief of Department of Diagnostic Imaging and Radiotherapy, Fondazione IRCSS, Istituto Nazionale dei Tumori, Milan, Italy) and Dr. Mario Santinami (Chief of Surgical Oncology, Fondazione IRCSS, Istituto Nazionale dei Tumori, Milan, Italy) for reviewing the manuscript and for their scientific contribution.
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