Cristina Nanni • Valentina Ambrosini • Lucia Zanoni • Monica Celli • Paolo Castellucci • Stefano Fanti
Because its introduction in the clinical practice, PET/CT has been associated with the use of fluorodeoxyglucose (FDG). FDG, a glucose analog, that is vigorously accumulated in more than 90% of malignant tumor cells. FDG PET/CT is clinically very useful for staging, restaging, assessing therapy response, and during the follow-up of most malignancies. Furthermore, FDG can be employed to identify several types of dementia, in advance of a clinical diagnosis, and for the evaluation of cardiac viability.
Despite the spectrum of applications, FDG is relatively insensitive for the detection of some malignant well-differentiated tumors (such as prostate cancer, neuroendocrine tumors [NETs], hepatic tumors, and others), that are not characterized by an overexpression of the glycolytic pathway generally because of a relatively slow growth. Furthermore, FDG is not useful to evaluate malignant masses located in tissues (such as the central nervous system [CNS]) with high glucose utilization. Finally, a high incidence of false-positive (FP) results occurs in case of inflammation, complicating the differential diagnosis between benign and malignant processes.
In addition to FDG, other positron emission tomography (PET) tracers have been introduced into the diagnostic routine to cover the niches where FDG is noninformative. The most important tracers are choline and acetate (labeled either with 18Fluoride or 11carbon), 11C-methionine (11C-METH) and 18F-fluoroethyltyrosine (18F-FET) (and the family of labeled amino acids), 18F-DOPA, 68Ga-DOTA-NOC (and other somatostatine analogs), 18F-FLT, and 18F-fluoride (Tables 31.1 and 31.2). In this chapter, the most important (in terms of clinical impact on patient management) non-FDG tracers used in oncology will be described.
11C-Methionine PET/CT and Other Labeled Amino Acids
11C-METH is an amino acid PET tracer whose main use involves CNS tumors. Methionine is, in fact, one of the five essential amino acids involved in metabolic pathways at a cellular level. The main destiny of this molecule is the protein synthesis within ribosomes, but it can also enter the citric acid cycle (to produce energy) and serves as a cofactor to transfer monocarbon units.
The uptake mechanism of 11C-METH within malignant cells is still uncertain but the best corroborated theories combine passive diffusion throughout the damaged brain–blood barrier (BBB) and active tumor uptake mediated by a membrane-specific carrier, overexpressed as a response to the increased protein synthesis (related to the active proliferation).1
One of the great advantages of using 11C-METH for the diagnosis of brain tumors is the very low background that is found in healthy brain. Normal brain tissue, in fact, recognizes glucose as the sole metabolic substrate and, therefore, does not accumulate 11C-METH to a significant degree. On the other hand, brain tumors have increased protein synthesis and tracer uptake. Therefore, the tumor-to-background ratio is favorable, making the 11C-METH PET images relatively easy to read and interpret.
From a practical point of view, a 11C-METH PET/CT is easy and fast to perform. Usually 370 to 740 MBq of tracer are injected intravenously. Because of the short half-life of 11C-labeled tracers (20 minutes), the uptake time is only 20 to 30 minutes. At that time, a segmental static image acquisition is performed on brain for 10 to 15 minutes. No fasting is required and no collateral effects have ever been described.
The role of 11C-METH PET/CT in clinical practice is limited to those patients with brain tumors and inconclusive MRI (or CT) after radical treatment. It is well known, in fact, that after surgery, chemotherapy or radiotherapy paraphysiologic phenomena such as fibrosis, necrosis, or edema may occur as a response to therapy. Those phenomena may present with morphologic imaging characteristics resembling a disease relapse, rendering the interpretation of conventional imaging equivocal. However, the early identification of disease relapse is important in the clinical course of these patients, because second-line therapy can improve their survival (Fig. 31.1).2
Because fibrosis, necrosis, and edema are acellular processes, they do not present any significant increase in the tracer uptake. A differential diagnosis between disease relapse (positive 11C-METH PET) and benign response to therapy (negative 11C-METH PET) can be proposed with very good accuracy.
This accuracy is a consequence of the fact that no tracer uptake is detectable in necrosis; tracer distribution is not influenced by corticosteroid therapy (usually administrated as an antiedema therapy), its uptake is proportional to the grade of the disease (even low-grade brain tumos are positive) and FP results (quite rare) are caused by very well recognizable clinical events (very recent biopsy, acute inflammations, hematoma, acute stroke with reperfusion).3
UPTAKE MECHANISM OF PET TRACERS
The main limitations of this technique are the spatial resolution (approximately 5 mm) and the limited availability of the tracer. In fact, the short half-life of 11C prevents it from being distributed. A cyclotron-based PET center with experience synthesizing 11C-based molecules is required.
Despite the main clinical application of 11C-METH PET/CT to identify disease relapse of low- and high-grade tumors, this examination has been used also for other purposes. 11C-METH PET/CT can be successfully used to guide tumor biopsy by indicating the most active area inside the mass4 and can be used as a prognostic index, because its uptake is proportional to the malignancy of the tumor and its cellular proliferative index. In fact, the proportionality between tracer uptake and tumor grade may be used to show a change in tumor grade early without any invasive procedure. This is not always possible with magnetic resonance imaging (MRI) as many high-grade tumors do not have any significant enhancement.5 The tumor grade serves as a prognostic index, and therefore a correlation between 11C-METH uptake and survival was found. Patients with hypermetabolic tumors in terms of protein synthesis have a significantly worse prognosis, whereas patients with hypo- or isometabolic tumors have a better life expectancy.6
Furthermore, 11C-METH PET/CT may also be used to detect a response to both chemotherapy and radiation therapy.7 Although other PET tracers have been used to evaluate brain tumors (18F-FLT, 18F-FDG, 11C-choline), 11C-METH is the most accurate and therefore it is recognized as the PET tracer of choice.
Besides brain tumors, other minor applications of 11C-METH PET/CT have been reported. A possible role of this tracer was found in the diagnosis of hyperparathyroidism. The principle of 11C-METH uptake in hyperfunctioning parathyroids relies on the fact that this tracer is an amino acid that can be incorporated into parathyroid hormone, which is produced in these glands. The advantages over MIBI-SPECT are, of course, the lower radiation dose delivered to the patient, the much higher anatomical detail provided by the computed tomography (CT) (which is important for localization prior to surgical removal), the higher spatial resolution compared to conventional scintigraphy and the much shorter time needed to complete the entire procedure. It has been demonstrated that 11C-METH PET/CT is accurate both for primary and secondary hyperparathyroidism, with a true-positive rate of approximately 85%.8 Further studies are needed.
Besides 11C-METH, labeled amino acids are available for PET studies of CNS malignancies. 18F-FET (fluoro-ethyl-tyrosine) demonstrated results comparable to 11C-METH in identifying primary brain tumors. The uptake mechanism is similar to 11C-METH but the half-life of the tracer is much longer (110 minutes). While a disadvantage in terms of dose delivered to the patients, production and distribution of the compound allows potentially wider use.9
The same considerations are applicable to 18F-DOPA ([18F]-L-dihydroxyphenylalanine), whose diagnostic potential is high for NETs as well as primary and metastatic malignant brain lesions.10
FIGURE 31.1. Patient operated for astrocytoma. Diagnostic CT (A) showing postsurgical changes (red cross). 11C-Methionine PET/CT (B: PET; C: fused images) showing a disease relapse (red cross, hot area).
68Ga-DOTA-Peptides and 18F-DOPA PET/CT Imaging
68Ga-DOTA-peptides and 18F-DOPA are most frequently used to study NETs. NETs are heterogeneous group of slow-growing tumors characterized by their endocrine metabolism and histology pattern, occurring in 1 to 4/100,000 people per year.11 NETs derive from neuroendocrine cells which are widely dispersed in the human body (bronchopulmonary endocrine cells, thyroid C cells, paraganglia, gastro-entero-pancreatic neuroendocrine cells, adrenal medulla, glial cells, leptomeninx, anterior pituitary gland, Merkel skin cells, and neuroendocrine cells in miscellaneous sites including ovary, endometrium, breast, kidney, larynx). The most common sites of NET occurrence are the gastro-entero-pancreatic tract followed by the bronchus and lungs. Less frequent sites of localization include the skin, adrenal glands, thyroid, and genital tract.11,12
For decades, somatostatin receptor scintigraphy (SRS) has represented the imaging method of choice to study NET. Several studies reported on the utility of SRS was the diagnosis of somatostatin receptor (SSR) positive tumors13,14 with an overall detection rate of 80% to 100%. The radiopharmaceuticals used by SRS allow tumor visualization as well as confirm the presence of SSR receptors which allows selection of patients for treatment with either hot or cold somatostatin analogs. Currently, the most frequently used compound for SRS imaging is 111-Indium DTPA-octreotide, commercially available as Octreoscan (Covidien, Petten, Netherlands). Octreoscan specifically binds to SSR which are over-expressed on NET cells, with particular affinity to subtypes 2 and 5. However, SRS has some limitations including imaging organs with higher physiologic uptake (e.g., liver), limited detection of small lesions (because of suboptimal physical resolution of 111Indium SPECT imaging),15,16 the relatively higher costs (as compared to PET imaging), and the longer image acquisition time.
New positron-emitting radiopharmaceuticals have been introduced in clinical practice with PET/CT used for the diagnostic assessment of NET. PET/CT has several advantages over SRS including better spatial resolution equipment that is superior to the γ-camera, the possibility to integrate PET functional data on CT anatomical information, and the opportunity to better characterize the disease by using both metabolic (18F-DOPA) and receptor-based tracers (68Ga-DOTA-peptides). In fact neuroendocrine cells present peculiar biologic characteristics that have been exploited to design specific PET tracers; neuroendocrine cells contain secretory granules, produce biogenic amines and polypeptides, have the ability to take up and decarboxylate amine precursors and express in high quantities several different peptide receptors on cell surface.
68Ga-DOTA-Peptides and PET/CT Imaging
Somatostatin receptors (SSR), over-expressed on NET cell surfaces, serve as the target of a novel group of PET radiopharmaceuticals named 68Ga-DOTA-peptides. Of the five different SSR, most NET express SSR2 predominantly with lower percentages of SSR1 and SSR5.17 The 68Ga-DOTA-peptides (DOTA-TOC, DOTA-NOC, DOTA-TATE) all bind SSR with different affinity; although all bind SSR2 and SSR5, only 68Ga-DOTA-NOC has high affinity for SSR3.17–19 The first compound used for NET PET imaging was DOTA-TOC, however both DOTA-NOC and DOTA-TATE have been used more often in the past few years at specialized centers in Europe. At present, there is insufficient evidence to support the preferential use of one analog over the other.20 Factors supporting the use of DOTA-NOC include the wider spectrum of SSR affinity and lower dosimetry, whereas on the other side both DOTA-TOC and DOTA-TATE have the advantage that they can be used for diagnosis (labeled with 68Ga) and subsequent treatment (when labeled with 177lutetium or 90yttrium).
From a technical point of view, 68Ga-peptides present several advantages for PET/CT imaging of NET. The synthesis and labeling process is quite easy and economic: Gallium-68 can be easily eluted from a commercially available Ge-68/Ga-68 generator, not requiring an on-site cyclotron. 68Ga (t1/2 = 68 minutes) has an 89% positron emission and negligible γ-emission (1,077 keV) of 3.2%. The long half-life of the parent radionuclide 68Ge (270.8 days) makes it possible to use the generator for approximately 9 to 12 months depending upon the requirement, rendering the whole procedure relatively economic.21 Moreover directly binding to SSR, 68Ga-peptides provide an indirect measure of tumor cell differentiation, offering data not only on disease extension but also on tumor cell receptor expression status, particularly relevant before initiating targeted nuclide therapy. Finally, compared to SRS, PET/CT is a single-day examination.
Indications to perform 68Ga-DOTA-peptide PET/CT imaging of NET patients include disease staging/restaging, follow-up, detection of unknown primary tumor sites, and evaluation before and after therapy. Clinical literature supports the superiority of 68Ga-DOTA-peptide PET/CT imaging over SRS for both the detection of primary and secondary lesions (Fig. 31.2). One of the largest studies (84 patients)22 demonstrated that 18Ga-DOTA-TOC PET/CT accuracy (96%) was significantly higher than that of CT (75%) and In-111 SRS-SPECT (58%). The sites in which PET/CT was superior to CT alone or SPECT were lymph nodes, liver, and bone. Overall, PET/CT-derived data changed clinical management in 14% of the cases compared to SPECT and in 21% compared to CT.
68Ga-DOTA-peptides influenced the clinical management of NET patients.23 In a population of 90 cases with biopsy-proven NET, 68Ga-DOTA-NOC PET/CT findings affected either stage classification or therapy modifications in half the patients. Moreover, PET/CT with 68Ga-DOTA-NOC provided prognostic information.24 In fact, 68Ga-DOTA-peptide positive lesions have a higher differentiation grade, are associated with a better prognosis, and are more likely to positively respond to treatment with either hot or cold somatostatin analogs. Semiquantitative and visual interpretation of the uptake of 68Ga-DOTA-peptides measured by PET/CT is used to guide the quantity of radiation and the timing of targeted radionuclide therapy, using 177Lu- or 90Y-DOTA-TOC. In this setting, 68Ga-DOTA-peptide PET/CT represents an indispensable procedure to plan targeted treatment.
Another clinical setting in which PET/CT has been increasingly used to detect primary tumor site (Cancer of unknown primary, CUP) in patients with biopsy-proven secondary lesions and negative physical examination, laboratory tests, and conventional imaging procedures (including chest x-ray, abdominal and pelvic CT, mammography in women, etc.). Conventional imaging, in fact, fails to identify the primary lesion in approximately 20% to 27% of cases. From a clinical perspective, the identification of the primary tumor is of crucial importance to choose the most appropriate intervention. The single published study addressing this issue reports high accuracy of 68Ga-DOTA-NOC to identify the primary tumor site.25
There are only a few studies directly comparing 68Ga-DOTA-peptides and 18F-DOPA imaging. 68Ga-DOTA-peptides are superior in well-differentiated NET for detection of both the primary tumor and metastatic sites.26,27 68Ga-DOTA-peptides are easy to prepare and are economical.28,29 They provide the possibility to study SSR expression prior to treatment. Hence, they are more frequently used than the 18F-DOPA in well-differentiated NET. By contrast, 18F-DOPA has advantages for the detection of tumors with a low or absent expression of SSR (such as medullar thyroid carcinoma, neuroblastoma, and undifferentiated NET) and to study diseases at sites of known physiologic 68Ga-DOTA-peptide uptake (e.g., adrenals).
FIGURE 31.2. Patient affected by metastatic neuroendocrine tumor. 68Ga-DOTA-NOC PET/CT shows increased uptake also in small lymph node metastatic lesions (A: PET MIP; B: CT axial; C: fused images). The arrow indicates a laterocervical lymph nodal metastasis.
PET/CT Imaging Using 68Ga-DOTA-Peptides: Technical Aspects
Recently the European Association of Nuclear Medicine (EANM) published guidelines for 68Ga-DOTA-peptide PET/CT image acquisition.20 68Ga may be eluted from a commercially available 68Ge/68Ga generator and the labeling of the DOTA-peptide with 68Ga is performed following standard procedures using semiautomated or fully automated systems. Prepurification and concentration of the generator eluate using an anion-exchange28,29 or cation-exchange technique,30,31 can be used as just a fraction of the generator eluate directly for radiolabeling.32,33 Radiolabeling is performed with a suitable buffer at an elevated temperature followed by chromatographic purification of the radiolabeling solution using a C-18 cartridge and an appropriate aseptic formulation. The method employed ensures the level of 68Ge in the final preparation is less than 0.001% of the 68Ga radioactivity. Quality control protocols include tests for radionuclide purity, radiochemical purity (HPLC, TLC), chemical purity (buffer, solvents), and sterility and endotoxin testing using validated methods.20
PET/CT imaging is performed following the intravenous administration of approximately 100 MBq (75 to 250 MBq) of the radiolabeled peptide (68Ga-DOTA-NOC, DOTA-TOC, DOTA-TATE). Images are generally acquired after an uptake time of 60 minutes (45 to 90 minutes).
68Ga-DOTA-peptides have physiologic uptake in the pituitary gland, spleen, liver, adrenal glands, the head of the pancreas, thyroid (very mild), and the urinary tract (kidneys and urinary bladder). No fasting or discontinuation of somatostatin analog treatment are required before imaging.20,34 Patients are encouraged to void before image acquisition to reduce the background noise as well as the radiation dose to kidneys and bladder.
Pitfalls in image interpretation include the presence of accessory spleens, inflammation (because of the presence of SSR on activated lymphocytes), and the presence of increased tracer uptake at the head of the pancreas. In fact, increased 68Ga-DOTA-peptide uptake in the exocrine pancreas (head) has been reported to be a relatively frequent finding (Gabriel et al.: DOTA-TOC, 67.8%; Castellucci et al.: DOTA-NOC, 31%) not necessarily associated with the presence of disease.22,35 Small lesion dimension (<5 mm) and variable or absent expression of SSR account for false-negative (FN) reporting.
18F-DOPA PET/CT Imaging
NET cells belong to the APUD (amine precuros and decarboxylation) cell system and are avid of 18F-DOPA, an aromatic amino-acid labeled with 18Fluorine. At the central and peripheral nervous system level, 18F-DOPA is transformed by the catechol-O-methyl-transferase (COMT) in 3-O-methyl-fluoro-L-DOPA (3-OMFD) and by aromatic amino acid decarboxylase (AAAD) in 6-fluorodopamine (FDA) which, in turn, is stored in secretory granules. Accordingly, 18F-DOPA can be used as a marker of NET metabolism.
Clinical indications to perform PET/CT with 18F-DOPA in NET include staging/restaging, assessment of tumor response to treatment, and detection of the unknown primary tumor. Several studies report that 18F-DOPA was useful to assess NETs and performed better than conventional morphologic procedures (US, CT, MRI) and SRS, with reported sensitivities ranging from 65% to 100%.36,37 Moreover, 18F-DOPA PET was reported to influence the clinical management in a limited population of biopsy-proven NET38 patients with an unclear clinical presentation or with inconclusive findings on other imaging modalities (US, CT, SRS, MRI).
The difficulties in 18F-DOPA synthesis process and the relatively high costs have limited its widespread clinical use. Conditions in which 18F-DOPA may still offer clear advantages over 68Ga-DOTA-peptides include cases with NET showing a low or variable SSR expression (e.g., medullary thyroid carcinoma, neuroblastoma). In fact, the 2009 American Thyroid Association MTC management guidelines39 suggest that 18F-DOPA PET/CT should be performed in addition to cervical ultrasonography in any patient with a postsurgical basal calcitonin level of ≥150 pg/mL. The clinical utility of 18F-DOPA over 18F-FDG in medullary thyroid carcinoma is still somewhat controversial although most studies show clear superiority of 18F-DOPA PET/CT over 18F-FDG PET/CT.40,41 It is assumed that the combination of the two tracers would probably provide the highest diagnostic sensitivity and specificity.42 18F-DOPA has been reported to be superior to 68Ga-DOTA-peptides to detect recurrent medullary thyroid carcinoma lesions in patients with elevated serum calcitonin levels.43
Successful 18F-DOPA PET/CT utilization is reported in neuroblastoma,44 another condition with variable expression of SSR.45 In a recent report in a small patient population with stage 3 and 4 neuroblastoma, 18F-DOPA PET/CT showed higher sensitivity and accuracy (95% and 96%) than 123I-metaiodobenzylguanidine (MIBG) scintigraphy (68% and 64%, respectively).
Finally, to assess the response to treatment, 18F-DOPA PET/CT may be more useful than 68Ga-DOTA-peptides. In fact, although the detection of a lower posttreatment 18F-DOPA uptake directly indicates efficacy, a reduced 68Ga-DOTA-peptide uptake merely reflects a lower receptor expression that may be explained also by the presence of less differentiated cells.
PET/CT Imaging Using 18F-DOPA: Technical Aspects
PET/CT image acquisition starts 60 to 90 minutes after the intravenous injection of 5 to 6 MBq/kg of 18F-DOPA. Oral premedication with carbidopa, a peripheral aromatic amino acid decarboxylase inhibitor, has been reported to enhance sensitivity by increasing the tumor-to-background ratio of tracer uptake. Carbidopa administration may therefore be particularly useful for the evaluation of lesions at sites of increased 18F-DOPA physiologic uptake such as the pancreas. Physiologic 18F-DOPA uptake has been seen because of its excretion in the bile ducts, gallbladder, and digestive and urinary tracts. Additionally mild, nonfocal uptake in the striatum and pancreas has been reported.
Because activated macrophages and other inflammatory cells present increased glucose metabolism as well as cancer cells, 18FDG uptake is not tumor specific and it has been reported in aseptic inflammations (healing, burns) or septic bacterial, viral, or mycosis infection providing relatively frequent FP results. 18F-fluoro-3-deoxy-3-L-fluorothymidine has been developed as a PET proliferation tracer. As a key component of tumor development and growth, cell proliferation is an attractive biologic target in cancer imaging. 18FLT uptake is independent from glucose metabolism. Thymidine is a native nucleoside that is utilized by proliferating cells for DNA replication during the S-phase of the cell cycle. 18FLT is handled by cytosol thymidine kinase 1 (TK1), the salvage pathway key enzyme, whose activity is cell cycle dependent and three to four times higher in malignant cells than in benign cells. Indeed, in numerous tumor cell lines, excellent correlations have been demonstrated between FLT uptake and the proportion of cells in S-phase, TK1 over-expression, or Ki-67 rates. At present cellular proliferation can be assessed only by a number of in vitro assays that require tissue from biopsies. However, biopsy results may not be representative of the proliferating activity of the whole tumor because of tumor heterogeneity and they may be difficult to obtain. Furthermore, excision biopsy presents various grades of risk, particularly in mediastinal, abdominal, or radio-treated lesions.
18FLT has already been successfully used to study solid malignancies, and preliminary results confirmed its capability to discriminate cancer from inflammation. Excellent correlations were obtained between 18FLT standardized uptake value (SUV) and cancer proliferative activity measured by Ki-67 or PCNA immunostaining. The more recent literature on 18FLT is focused on monitoring tumor treatment response and exploring the effects of new anticancer agents on tumor cells.46
18FLT was originally developed as an antiviral compound for the treatment of patients with HIV and was subsequently labeled with 18F. A reliable radiosynthesis is based on [18F] fluoride displacement of a protected nosylate precursor. A fully automated method has been developed with a yield of 50% radiochemical yield by modifying a commercial FDG synthesizer and its disposable fluid pathway, however FLT preparation is still in the process of optimization.
After intravenous administration, capillary transport of fluorinated pyrimidine analogs occurs rapidly by passive diffusion in most tissue and organs (the brain being an exception). Cell transport is related to rapid, equilibrate, facilitated diffusion by an Na+-dependent carrier-mediated mechanism. Once inside the cell, 18FLT is accepted as a substrate by TK1, phosphorylated and thereafter trapped. 18FLT-monophosphate (MP) is further phosphorylated to diphosphate and triphosphate which is not significantly incorporated into DNA and persists in the cytosol (unlike 11C-thymidine and some other analogs). One major advantage of 18FLT over thymidine is that it is resistant to degradation by thymidine-phosphorylase (TP). The placement of fluorine in deoxy-ribose sugar at the 3′ position in 18FLT, stabilizes the glycosolic bond and prevents degradation. However, glucuronidation of 18FLT can still occur. This characteristic results in higher levels of FLT circulating blood concentration after injection. In addition 18FLT is not a substrate for thymidine-kinase 2 (TK2), implied in mitochondrial DNA replication and repair, resulting in an exclusive linkage to the thymidine salvage pathway in relation to nuclear DNA synthesis.
The limiting step of 18FLT accumulation in proliferating cells is TK1 phosphorylation but not direct incorporation into DNA, excluding 18FLT from the list of true tracer of proliferation. However, TK1 is selectively upgraded before and during S-phase and virtually absent in quiescent cell. It has been demonstrated that TK1 activity is three to four times higher in malignant cells than in benign cells and 18FLT uptake, as a measurement of TK-1 activity, correlates strongly with markers of cellular proliferation as PCNA and Ki-67 score.
18FLT provides high contrast images of proliferating tissues but research studies have shown interspecies differences of biodistribution. Human studies revealed high bone marrow uptake, and significant liver and urinary tract distribution (kidneys and bladder). Unlike 18FDG, 18FLT is not taken up by normal brain cortex, which is clearly an advantage in assessing brain lesions and presumably reflects the known lack of nucleoside transportation across the blood brain barrier (BBB). All the studies comparing 18FLT to 18FDG uptake revealed that 18FDG SUV was almost twice than that of 18FLT. Despite this limitation, 18FLT uptake in the surrounding tissue was proven to be low; therefore tumor/normal tissue ratios of 18FLT are similar to that of 18FDG, and result in good visualization of 18FLT-positive lesions.
In all species, physiologic accumulation was found in kidneys and bladder, related to the clearance of FLT, primarily undegraded, in the urine.
In terms of radiation exposure, 18F-FLT has similar whole-body and bladder wall absorbed radiation doses as 18FDG. In fact, brain and heart receive lower whereas liver and bone marrow receive higher doses compared to 18FDG. Unlike 18FDG, no dietary restriction or fasting is required before 18FLT imaging. This is an advantage in diabetic patients. Also, 18FDG biodistribution is not influenced by movement during the interval between injection and imaging.47
Tumor Proliferation Imaging (Differential Diagnosis of Tumor and Inflammation)
Preclinical 18F-FLT studies with tumor cell line studies and animal studies have included pancreatic cancer cell lines in vitro, human lung adenocarcinoma cells in vitro, B-cell lymphoma (both murine model and human disease), murine model bearing the radiation-induced fibrosarcoma 1 tumor (treated with 5-fluorouracil [5-FU]), rodent model with both tumor and sterile muscular inflammation, and dogs with spontaneous non-Hodgkin lymphoma (NHL) and soft tissue sarcoma.48
18FLT has shown strong correlation with Ki-67 immunohistochemical marker expression which is the gold standard to assess tumor proliferation status in clinical practice. However, it was not expected to have the same sensitivity as 18FDG because of a lower overall uptake in tumors and a higher background activity in liver and bone marrow, resulting in more FN findings and therefore, it is not regarded as a staging tool for cancer. However, the specificity of 18FLT was expected to be significantly higher than that of 18FDG PET, giving fewer FP findings because of negligible accumulation in granulocytes.49
Though most of initial reports were optimistic, new adverse data are emerging. Zhang et al.,50 validating FLT as a proliferation tracer in the preclinical setting in a panel of proliferating xenografts, concluded that FLT avidity failed to reflect the tumor growth rate across different tumor types, despite the high expressions of Ki-67 and TK1.
The human oncologic setting has been largely explored but the clinical relevance of these studies is limited, in part, because of the small cohort of patients and the methodologic heterogeneity.51,52
In several clinical studies FLT PET has been directly compared with 18FDG PET. In many series, 18FLT SUV has been shown to have good correlation with Ki-67 (breast cancer, hepatocarcinoma [HCC], thoracic tumors, ovarian cancer, lymphoma, brain cancer, colorectal cancer, sarcoma of the extremities).
In lymphoma patients, a clear dichotomy between aggressive and indolent forms was noted (Fig. 31.3); conversely, considering the high normal bone marrow uptake, FLT presented several limitations for staging.
FIGURE 31.3. Suspected bronchial NHL relapse detected by 18F-FDG PET/CT and confirmed by 18F-FLT. A 66-year-old underwent surveillance imaging for a marginal splenic non-Hodgkin’s lymphoma treated with splenectomy. 18F-FDG PET/CT (Left: A, MIP; B, transaxial CT; C, transaxial PET; D, transaxial fused images) presented intense uptake of the glycomimetic tracer in the right inferior bronchus (arrow, SUVmax = 8.6) and in the sternum (SUVmax = 3.8). 18F-FLT PET/CT (Right: E, MIP; F, transaxial CT; G, transaxial PET; H, transaxial fused images) showed intense uptake of the proliferative tracer in the same bronchus (arrow, SUVmax = 5.8). It was not possible to assess the sternum because of the high physiologic FLT uptake in the bone marrow. Finally, both lung disease and bone marrow involvement were biopsy proven. In addition, PET helped depict a histologic transformation into a more aggressive diffuse large B-cell lymphoma.
Unlike 18FDG, 18FLT has shown a very low background uptake in the brain related both to the low proliferation in normal brain and to the low ability of the tracer to cross the BBB. 18FLT is only retained in lesions where either the BBB is disrupted or significantly impaired or in high-grade gliomas, whereas a variable sensitivity of 79% to 82% suggests the possibility of FN results in case of low-grade gliomas. At the same time nontumorous lesions disrupting the BBB may represent FP results, in particular subacute infarction, multiple sclerosis, and focus of radiation necrosis which have already been depicted; furthermore, FLT PET may represent a potential outstanding indicator of prognosis.
With regard to colorectal cancer, 18FLT would be useful to evaluate treatment response to induction chemo–radiotherapy but it has no role for staging because of high-background activity in the liver.
Finally, few benign FP findings should be mentioned in patients with sarcoma of the extremities. Indeed, 18FLT was able to differentiate between low- and high-grade lesions but not between low-grade and benign lesions.
A number of studies report a lack of correlation between 18FLT uptake and Ki-67 expression, in particular as follows:
• Breast cancer: The tumor-to-background contrast of 18FLT is equivalent to 18FDG, in clinical practice, assessing the response to therapy in locally advanced breast cancer treated with neoadjuvant chemotherapy. 18FLT has been helpful to assess nodal involvement of both breast cancer and melanoma patients. The sentinel node procedure remains superior because of the limited spatial resolution of PET cameras.
• Esophageal cancer: Despite negative results, assessing regional lymph node in esophageal squamous cell carcinoma shows fewer FP results with 18FLT than with standard 18FDG.
• Aggressive NHL (treated with R-CHOP).
• Locally advanced gastric cancer (treated with cisplatin– leucovorin–5FU).
• Thoracic tumors.
• Metastatic germ cell tumors (treated with cisplatin).
• Colorectal cancer.
• Head and neck cancer: 18FLT has been valid to assess primary tumors but not for lymph nodes because of the high proliferation rate of B-lymphocytes in the germinal center of reactive nodes with abundant Ki-67 scores that cause a high rate of FP findings and very low specificity, 16.7%53; eventually there can be an advantage of 18FLT over 18FDG in differentiating postradiotherapy changes from recurrent or residual tumor.
A systematic review and meta-analysis of 27 different studies (from 1998 to 2011) with a total of 509 patients have been recently performed by the London group of Chalkidou and coworkers. It showed that, given an appropriate study design, the FLT/Ki-67 correlation is significant and independent of cancer type with either the use of Ki-67 average measurements regardless of nature of sample, or whole surgical samples when measuring Ki-67 maximum expression. In particular, there is already sufficient data to support a strong correlation for brain, lung, and breast cancer.54
The study by Brockenbrough et al. evaluating the relationship between preoperative FLT static lung and dynamic uptake measured by PET and TK1 protein expression and enzymatic activity in a series of 25 lung lesions, documented the absence of observable correlation of the imaging parameters with TK1 activity. Possible explanation are because of either sampling errors in the highly heterogeneous group of non–small cell lung cancer (NSCLC) or in vitro TK1 assay conditions which did not reflect the full complexity of the in vivo relationship.55
As stated above, many groups aimed to directly compare 18FLT with the standard tracer 18FDG. For example, in detecting primary lung cancer the sensitivity of 18FLT compared to FDG was 79% to 100% and 89% to 100%, and the specificity was 86% to 100% and 57% to 73%, respectively.
Despite the encouraging literature available, it is important to recognize that 18FLT may accumulate in lesions with a higher turnover and brisk proliferation of inflammatory cells. High tracer uptake in nonmetastatic lymph nodes has been proven to be related to reactive B-lymphocyte proliferation in the germinative center. FP results with FLT have been further detected in nonspecific interstitial pneumonia, squalene-induced lipoid pneumonia, and groin lymph nodes. In addition, increased perfusion and vascular permeability, aside from the proliferation of inflammatory cells, results in nonspecific 18FLT uptake in benign lesions.
Monitoring of Tumor Response to Therapy
Measuring tumor proliferative activity by 18FLT PET offers great potential for assessing the viability of tumor cells during or early after treatment. Most of the studies in a wide range of cancers registered significant early changes between baseline and posttreatment scan. To date, there has been little consistency in the clinical studies (scans performed 1 day to 2 weeks from the initiation of treatment administration). However, the early assessment of FLT uptake suppression occurs long before treatment-induced change in tumor size became measurable.
For example, 18FLT is a useful probe to monitor the efficacy of:
• the cytostatic drug cisplatin.
• docetaxel, an anticancer agent that induces G2/M block, in breast cancer; a recent study identified 18FLT as a sensitive negative predictor of lesion response.
• combined 5-FU-epirubicin, cyclophosphamide, in breast cancer, during the first week.
• R-CHOP/CHOP in non-Hodgkin lymphoma within 1 week after the first cycle; interestingly there was no reduction after Rituximab alone indicating no early antiproliferative effect of immunotherapy.
• chemoradiotherapy in head and neck squamous cell carcinomas during radiotherapy (4 weeks after initiation) FLT uptake decreased more significantly than 18FDG; higher specificity and overall accuracy was registered in particular for primary lesions.
These changes in 18FLT avidity do not always predict clinical response and improvement in survival rates. Highly effective and potentially curative treatments usually inhibit 18FLT uptake in most tumors but a complete response is achieved in only a subset of patients.
Moreover, 18FLT does not always reflect cellular proliferation and the use of 18FLT in the evaluation of chemotherapy is complicated by the fact that many antitumor drugs interfere with cellular nucleotide metabolism. It should be recalled that some chemotherapeutic agents such as 5-FU and methotrexate cause the cell arrest in S phase and an inhibition of the de novo synthesis of nucleotides leading to a compensatory up-regulation of the salvage pathway DNA synthesis and thus a transient induction of TK1 activity with a consequent increase in 18FLT uptake, whereas tumor cell proliferation remains impaired. Similar results were reported in an in vitro model involving the alkylating agent ACNU. The mechanism behind underpinning this 18FLT “flare effect” seems related in part to the redistribution of the human equilibrative nucleoside transporter type 1 (hENT1) to the outer cell membrane. In addition complex relation between Ki-67 index and pKi-67 mRNA have been observed in colorectal cancer, whereas in lung adenocarcinoma cells, functional p53 signaling is needed to maintain TK1 activity and S-phase percentage following radiation treatment. How this relationship is affected by cytostatic treatments remains to be fully understood.
Drugs inhibiting the de novo pathway of DNA are thus expected to induce an 18FLT “flare response” which has been documented, for example, at 1 hour after administration of capecitabine in human breast cancer and 24 hours after cytotoxic drugs (5-FU, methotrexate, and gemcitabine) in esophageal squamous cell carcinoma.
Apparently in the interpretation of images, a paradoxical increase of cellular 18FLT uptake after treatment must be considered. Therefore, it may be important to examine the mechanism of action of antitumor drugs and to select an appropriate time point for 18FLT PET imaging after treatment (the optimal time might differ for different drugs).56
It was concluded that 18FLT uptake and retention within the tumor cells is directed by a variety of undetermined factors. To review, the most probable factors that influence 18FLT uptake are as follows:
• Loss of cell cycle–specific regulation of TK1.
• Cell ATP levels.
• Contribution of the de novo and salvage pathways to DNA synthesis.
• Diversity of tumor entities in a specific cohort.
• Difference in the phosphorylation rate between FLT and thymidine.
• p53 regulation of TK1 activity: p53 function may be compromised in 50% of all human cancers inducing a state of differential cell cycle control in which TK1 activity is uncoupled from tumor proliferation and more closely associated with tumor DNA repair.
• Cell cycle–dependent rearrangement of hENT1.
• A multitude of transport mechanism.
• “Flare” 18FLT uptake enhancement.
• Indirect linkage with proliferation rate because FLT is not incorporated into DNA.
Hence, additional data are needed to validate the use of 18FLT PET to assess treatment.
Recommended Scan Acquisition and Interpretation Methods
18FLT uptake tends to be modest to low. Tumor localization should be based or confirmed with coregistered anatomic and 18F-FDG scans. Pitfalls may come from the following aspects:
• Region of interest definition (manual or automatic).
• Location and number of core biopsies used for correlation with proliferation markers.
• SUV analysis at later time points (beyond 1 hour) not fully accounting for contribution of labeled metabolites.
• SUV analysis, to evaluate the effects of novel drugs, affected by blood flow underestimating 18FLT metabolic flux because of reduced tracer delivery to the tumor.
Different methods have already been proposed for a better standardization.
• Method 1: dynamic PET at 0 to 90 minutes with a four-parameter model. The arterial input function can be estimated during the first several minutes from PET images of the heart or aorta and thereafter from a modest number of peripheral venous samples; the 18F-FLT component of the time–total activity curve for blood can be estimated with a chromatographic analysis of one sample taken near the end of the scanning sequence. This method is proposed in initial studies for a specific type of cancer or to determine the effect of novel therapies on tumor cell proliferation.
• Method 2: dynamic PET at 0 to 60 minutes with a three-parameter model. In both methods 1 and 2, voxel-by-voxel modeling can produce parametric images of the 18FLT flux (i.e., K 18FLT).
• Method 3: static PET at 40 to 60 minutes, with the tumor activity concentration expressed in terms of SUV.
To assess treatment responses, changes in tumor SUV more than 20% are considered significant. Recently a novel approach was elaborated by the Society of Nuclear Medicine for 18FDG PET and the same approach could be used for 18FLT: PET Response Criteria in Solid Tumours (PERCIST). The principal components of these criteria include assessing normal reference tissue values in a 3-cm diameter region of interest (liver for FDG) using a consistent PET protocol, a fixed small region of interest 1 cm3 in volume in the most active region metabolically active tumors to minimize statistical variability, assessing tumor size, treating SUV measurements in the 1 up to 5 most metabolically active tumor focus as a continuous variable, requiring a 30% decline in SUV for response, and deferring to RECIST 1.1 criteria in nonavid or technically unsuitable cases.
The recommended timing and duration is considered optimal for lung and high-grade brain tumors but may be slightly different for other tumor types.57
Other Thymidine Analogs
18F-FLT has undergone the most widespread testing and is considered to be an attractive tracer.
Thymidine is the only nucleoside that is DNA specific, incorporated in DNA but not in RNA.
The halogen substitution in the in the 3′position of 18F-FLT results in a decreased affinity for the pyrimidine transporter compared with thymidine. Moreover, 18FLT affinity for TK1 is reported to be 30% lower than the affinity of thymidine (explaining low uptake in tumor cells).
In mice and rats, the tracer 11C-Thymidine shows prolonged retention in rapid proliferating tissue such as thymus, spleen, and duodenum, whereas in dogs and humans, it is converted to a spectrum of blood metabolites thus hampering PET image interpretation. In addition to this rapid catabolism, the short half-life of 11C and the need for an on-site cyclotron make it less suitable for clinical practice.
Other pyrimidine analogs such as 2′-fluoro-5-methyl-1-β-D-arabinofuranosyluracil (FMAU) and 2′-deoxy-2′-fluoro-5-bromo-1-β-D-arabinofuranosyluracil (FBAU) offer the potential advantage over FLT with retention in DNA, reflecting DNA synthesis directly. However, being also substrates for TK1, some of the same limitations as 18FLT should be encountered.
18F-FMAU rapid liver uptake appears to decrease clearance into kidneys and bladder, allowing improved imaging in the pelvis compared to 18FLT and 18FDG. Therefore, it can be used to image primary tumors within the prostate. Moreover, less retention is seen in the bone marrow leading to the ability to detect bone metastases (BMs). It is a better substrate for TK2, and it is therefore more relevant to measure oxidative stress rather than proliferation.58
18F-Fluoride PET/CT in the Assessment of Bone Metastases from Solid Primary Tumors
Bone metastases (BMs) represent the most common bone malignancy as they affect 30% to 70% of the whole oncologic population. In women, the leading cause for BMs is breast cancer whereas in men, it is prostate cancer, followed by lung neoplasms in both sexes.
When confined exclusively to bone, metastases often translate into a prolonged clinical course compared to visceral involvement. However, if untreated, BMs may eventually give rise to dreadful complications, known as “skeletal-related events” (SREs), including severe refractory pain, pathologic fractures, cord compression, and hypercalcemia impairing patient’s quality of life and survival and creating considerable health care costs.
Early detection of BMs and accurate assessment of their true extent is pivotal to guide the most appropriate therapy and for prognostic stratification. For decades whole-body assessment of bone metastatic spread has been provided by 99mTc-diphosphonate scintigraphy (BS) as a consequence of its high sensitivity, wide availability, and low cost, although burdened by a relatively low specificity, high incidence of equivocal findings, and lack of morphologic correlation which may eventually delay diagnosis.59 Complementary single photon emission tomography (SPET) and hybrid single photon emission tomography/computed tomography (SPET/CT) acquisitions may obviate these drawbacks and improve BS diagnostic accuracy, especially in the spine. This approach, however, may not be routinely feasible.60,61 Moreover, published studies comparing the diagnostic accuracy of 99mTc-diphosphonate bone imaging and tumor-specific PET radiopharmaceuticals which directly visualize tumor cells (e.g. 18FDG), have highlighted the superiority of the latter to detect early bone marrow–based and osteolytic lesions as well as providing extra-osseous disease assessment and a tool for therapy monitoring. In such a composite scenario, encouraged by the recent worldwide 99Mo/99mTc supply shortage, 18F-fluoride has emerged as an extremely sensitive bone-specific PET agent, able to image areas of abnormal osteogenic activity in a shorter time than BS and with high accuracy for both lytic and sclerotic lesions, leading to the suggestion that 18F-fluoride might be a complementary study to tumor-specific PET imaging.
Physiopathology of Bone Metastases
Bone architecture consists of a structural compartment (cortex and trabeculae) that houses the bone marrow. Structural bone is a dynamic tissue consisting of osteoblasts that deposit osteoid matrix and promote its mineralization with hydroxyapatite crystals (bone formation) and osteoclasts that are bone-resorbing cells. When injured, the structural bone reacts by increasing its bone mineral turnover in the attempt to self-repair.
The role of red bone marrow is hematopoiesis and its microenvironment is characterized by a rich blood supply (e.g., Batson vertebral venous plexus) and an abundance of growth factors that generally make it the first and most suitable “soil” for hematogenous metastatic “seeds” from any osteophilic solid tumors.
Once in the red bone marrow, metastatic cells may remain dormant for a variable time but eventually activate, establishing complex, bidirectional biochemical interactions with the surrounding microenvironment leading to neoangiogenesis, metastatic growth, and local uncoupling of bone deposition and resorption, generally in favor of the latter.
More aggressive tumor histologies are generally thought to be responsible for predominantly lytic BMs, whereas predominantly sclerotic lesions represent a slow tumor growth rate, as sclerosis is caused by the reparative reaction of surrounding bone.
18F-Fluoride is a short-lived β+ emitter isotope (β+ decay = 97%; Eβ+ = 0.635 MeV; t1/2: 109.8 minutes) and a hydroxyl ion analog which is highly sensitive to detect areas of abnormally increased osteogenic activity. First introduced by Blau et al.62 in 1962 and despite its ideal bone pharmacokinetics, 18F-Fluoride was soon abandoned in favor of 99mTc-diphosphonate as the high-energy photons were unsuitable for the thin crystals used for low-energy γ-imaging. Nevertheless, the advent of PET imaging in the clinical practice has revived interest in 18F-fluoride as the ideal bone-seeking PET agent.
Similar to the 99mTc-diphosphonate compounds, 18F-Fluoride accumulates as a consequence of both local blood flow and osteoblastic activity. Soon after intravenous injection, 18F-Fluoride first-pass distribution depicts blood supply that varies among different bones.63 18F-Fluoride rapidly clears from plasma (two-fold faster than 99mTc-diphosphonate) being freely diffusible across membranes and not binding to plasmatic proteins.64 Once diffused through bone capillaries virtually all 18F-Fluoride ions reach the extracellular fluid and are chemisorbed onto actively remodeling bone surfaces by replacing a hydroxyl group in hydroxyapatite crystals to form fluoroapatite.65,66
18F-Fluoride uptake in BMs is fast and it is 3- to 10-fold higher than that of normal bone resulting in optimal disease to normal bone ratios with short uptake time (approximately 45 minutes). Blastic and mixed lesions appear diffusely 18F-Fluoride avid, whereas osteolytic metastases may appear as relatively “cold” lesions with a “hot” rim, representing the reparative osteoblastic reaction. Lytic lesions are the most common expression of bone metastatic growth (60%). Purely lytic metastases are quite uncommon as the majority of metastases prompt some degree of osteoblastic reaction, albeit not detectable on CT. Some tumor histologies such as thyroid cancer, multiple myeloma, neuroblastoma may present with purely lytic metastases and 18F-Fluoride PET, like 99mTc-diphosphonate, but with higher contrast, would identify them as photopenic lesions compared to normal bone background. The high resolution of PET images with morphologic and anatomical correlation of low-dose CT in modern hybrid PET/CT systems allows for detection of even small metastatic foci.
18F-Fluoride is not tumor-specific. Semiquantitative analysis of its uptake by means of SUVmax does not discriminate between malignancy and benign causes for bone remodeling (i.e., trauma, degenerative changes, benign tumors). However, uptake pattern and morphologic correlation with low-dose CT of hybrid PET/CT systems permit easy discrimination between benign and malignant findings.
Synthesis and Safety
A cyclotron-derived radioisotope, 18F-Fluoride results from direct 11 MeV proton irradiation of 18O-enriched water. 18F-Fluoride is then separated by adsorption on anion exchange resins and eluted with sterile isotonic saline yielding 18F-Fluoride in physiologic solution (Na18F).67 18F-Fluoride synthesis is simple and with high yields that make it relatively inexpensive and, although cyclotron produced, its half-life (t1/2 = 109.8 minutes) allows for distribution to peripheral PET facilities.
18F-Fluoride has an established monograph in the U.S. Pharmacopoeia68 and an FDA-approved NDA for skeletal PET. In the E.U., 18F-Fluoride quality controls such as standards for production, radioisotopic and radiochemical purity are described in the European Pharmacopoeia (Ph. Eur., 7th ed., 2011).69 Its clinical use, however, is also subject to variable national regulatory authorities and it is not extensively accepted. All U.S. and E.U. suppliers must comply with national directives and GMP guidelines. To date, no safety issues concerning clinical use of 18F-Fluoride have been reported.
No specific preparation is required for patients undergoing a 18F-Fluoride PET/CT except good hydration on the day of examination (possibly two glasses of water before 18F-Fluoride administration and two more glasses during the uptake time). Patients can eat normally and take their usual medications. It should be noted that the impact of ongoing treatments such as bisphosphonates, antihormonal therapy, chemotherapy, and radiotherapy on 18F-Fluoride uptake has not been determined. History of recent traumas, orthopedic surgery, and bone metabolic disorders should be obtained.
Recommended 18F-Fluoride activities for adults range from 185 to 370 MBq. Higher doses (444 MBq) are reserved for obese patients. Pediatric activity is weight based (2.1 MBq/kg), ranging from 19 to 148 MBq. For intravenous administration an indwelling catheter should be used to avoid accidental tracer extravasation. Images can be acquired 45 minutes after 18F-Fluoride administration in patients with normal renal function. Patients should be invited to void before scanning and even after the study has been completed to reduce the effective dose to the bladder and pelvic organs. 3D-mode PET acquisition is preferable to limit the scanning time of a large area. A total-body scan (from vertex of skull to toes) is generally preferable, however whole-body studies from vertex to mid-tibiae have also been suggested for cancers that predominantly metastasize via venous retrograde flow (e.g., prostate and breast cancers) and with no clinical suspicion of peripheral involvement. This would reduce effective dose and acquisition time. Conversely, neoplasms likely to spread via arterial embolization (e.g., lung and kidney carcinomas) as well as clinical suspicion of peripheral skeletal involvement prompt a total-body scan.70
The effective dose for an adult receiving 370 MBq of 18F-Fluoride is 8.9 mSv (effective dose equivalent = 0.024 mSv/MBq). If a whole-body low-dose CT is performed (hybrid PET/CT systems), additional 7 mSv should be considered so that the cumulative dose for an 18F-Fluoride PET/CT is approximately 16 mSv 15.9 mSv. Compared to a typical 99mTc-methylene diphosphonate [MDP] BS (740 MBq, Effective dose = 4.2 mSv), 18F-Fluoride PET/CT radiation dose to patients is approximately 3- to 4-fold higher.
The highest absorbed doses extrapolated to patients are in bone surface, bone red marrow, and bladder walls for both modalities.71,72
99mTc-MDP BS is still the most widely used whole-body imaging technique to assess bone metastatic disease, its use being traditionally advocated in staging high-risk patients and in restaging symptomatic patients and/or patients with biochemical failure or radiologic evidence of bone relapse. If BMs are known, 99mTc-MDP BS clinical indication should imply a possible change in clinical management. Recently PET imaging has also demonstrated high diagnostic accuracy in assessing skeletal disease in similar diagnostic scenarios by means of several tumor-specific tracers (i.e., 18FDG, 18F-Choline, 68Ga-DOTA-TOC) which directly image tumor tissue.
Comparative studies have demonstrated the superiority of oncotropic PET/CT compared to BS in detecting predominantly lytic and bone marrow–based BMs. In spite of this superiority, 18FDG PET may still fail to image sclerotic BMs with scarce, slow-proliferating cellularity, not prone to hypoxia and non–18FDG-avid BMs.73–75 A highly sensitive bone-seeking PET agent, 18F-Fluoride can detect early and accurately BMs regardless of their prevalent radiologic appearance (lytic, blastic, and mixed) proposing as a complementary study to tumor-specific PET/CT imaging in staging and restaging contexts (Fig. 31.4).
Comparative studies have highlighted the superior diagnostic accuracy of 18F-Fluoride PET compared to 99mTc-MDP BS and SPET in a wide range of solid malignancies. 18F-Fluoride PET has proven significantly more accurate in detecting both blastic and lytic lesions, regardless of their anatomical localization and easily differentiates benign from malignant lesions.76–79 The use of low-dose CT (hybrid PET/CT systems) improves specificity by reducing the number of equivocal findings and helps lesion localization.80–82 Although 18F-Fluoride PET approximately changed clinical management in approximately 10% of selected populations, its cost-effectiveness compared to 99mTc-MDP BS and SPET has not been proved yet. When compared to tumor-specific PET, 18F-Fluoride PET confirms its highest diagnostic accuracy in detecting both lytic and blastic lesions but may fail to detect early bone marrow–based metastases which have not evoked bone remodeling.83,84
FIGURE 31.4. Staging [18F-]fluoride PET/CT in lung adenocarcinoma. A: There are three [18F-]fluoride avid lesions: One small, sclerotic lesion at the left posterior aspect of T10; second lesion at the same level, right-sided, shows very mild and diffuse sclerosis and it is hardly detectable on CT. Third lesion is sclerotic and involves the seventh left rib. B: Median lytic lesion in L2 vertebral body, posteriorly. Patchy [18F-]fluoride uptake observed along a very thin sclerotic rim.
In prostate cancer, 18F-Fluoride PET/CT could play a complementary role to conventional radiology and tumor-specific 18F-Choline PET imaging to stage high-risk patients (i.e., PSA > 10 ng/mL, T3 or T4, Gleason score (Gs) > 7) or symptomatic patients and/or patients with equivocal findings (e.g., non–18F-Choline-avid sclerotic lesions). 18F-Fluoride PET could be equally advocated for restaging patients with bony pain and/or biochemical failure (i.e., PSA > 10 ng/mL, PSA doubling time < 6 months) or evidence of skeletal recurrence on conventional imaging (XR, CT, MRI) if whole-body assessment is thought to potentially influence therapeutic management.
Assessing response to hormonal/systemic treatments by means of 18F-Fluoride PET could be insidious. Indeed an effective response to treatment can foster bone healing at BM-involved sites, resulting in a “flare” pattern on 18F-Fluoride PET.84
In breast cancer BMs are predominantly lytic (60%) and are found in up to 70% of patients in advanced stages. Skeleton is the first site of relapse in approximately 25% of patients. Risk factors for metastatic involvement include primary tumor size (T2 or more), lymph nodal involvement (>3 metastatic nodes), and a positive estrogen receptor status. In patients with a locally advanced breast tumor 18F-Fluoride PET/CT could be useful to complete the staging work-up, when conventional imaging is negative but concerns remain about skeletal disease. During follow-up 18F-Fluoride PET/CT can also be indicated if patients become symptomatic or there is a rise in tumor or bone markers (i.e., CA 15.3, alkaline phosphatase [ALP]).
In small cell lung cancer (SCLC) and locally advanced NSCLC total-body 18F-Fluoride PET/CT may assist 18FDG imaging in preoperative staging if no BMs have been detected but the risk of skeletal spread is deemed high.
CHOLINE PET/CT FOR PROSTATE CANCER: MAIN CLINICAL APPLICATIONS
The most widely used radiopharmaceutical used in functional imaging of prostate cancer is choline, labeled with 11C or 18F.
Choline is a substrate for the synthesis of phosphatidylcholine that is the major phospholipids in the cell membrane.85 Choline kinase activity is substantially upregulated in tumor cells.86 A recent in vitro study by Müller et al.87showed that the uptake of choline is mediated by a selective choline transporter.
Images of good diagnostic quality are usually obtained 3 to 5 minutes after tracer injection with 11C-choline and with dual image acquisition (after 10 and 60 minutes from injection) with 18F-Choline. The physiologic uptake of 11C-choline includes salivary glands, liver, kidneys parenchyma, and pancreas and faint uptake in spleen, bone marrow, and muscles. Bowel and bladder activity can occasionally be observed. 11C has a half-life of 20 minutes, so it must be used rapidly after production. This is the main drawback of 11C agents which require the presence of an on-site cyclotron. Consequently their distribution to distant PET centers is not possible. To overcome the drawback of short half-life of 11C, an 18F-labeled choline tracer (18F-fluorocholine or FCH) was developed as an alternative. The main difference between 11C-choline and 18F-choline is the earlier urinary appearance of the 18F probably because of incomplete tubular reabsorption.88
11C or 18F-Choline PET/CT imaging has been proposed to detect primary prostate cancer, to stage the tumor, to identify nodal involvement, and finally for the detection of tumor recurrence in case of biochemical relapse. In a recent publication, Soyka et al.89 showed that the use of choline PET/CT in 156 patients resulted in major changes in clinical management in 31%. In 21%, the approach changed from palliative to curative and in 10% from curative to palliative.
Accurate staging should exactly define the extent of local disease (T stage) and assess the presence of locoregional nodal and distant metastasis (stage N and M). In case of localized disease radical prostatectomy (RP), with or without pelvic lymphadenectomy (PLND), or radiation therapy (RT) are currently the only therapeutic options. In case of distant metastasis a less invasive approach is suggested (RT and/or systemic therapy).
At the present time, PET/CT with choline has limited value in assessing T status, and it has probably no role in assessing extracapsular extension of the disease.
Metastatic lymph nodal spread is relatively low; it occurs in about 20% to 35% of patients with high-risk histology according to nomograms,90 based on PSA levels, age, Gs, T status. The 5-year disease-free survival rate decreases from 85% for pN0 patients to 50% for pN1 patients.91 European guidelines on prostate cancers92 do not suggest any imaging method to stage the disease because of the lack of accuracy to date that either CT or MRI have shown in the detection of lymph nodal metastasis 93 In low-risk patients (because the probability to have lymph nodal metastasis is lower than 10%), European Guidelines do not suggest any imaging procedure, whereas in high-risk patients the only recommended procedure is the extended pelvic lymph node dissection (ePLND).
The three largest studies of PET/CT either labeled with 18F or 11C-choline for nodal staging, gave similar results that PET/CT has a high specificity and a low sensitivity. In 2008, Schiavina et al.94 studied 11C-choline PET/CT in 57 patients with intermediate- and high-risk PC using histology as reference after ePLND. On a patient basis, sensitivity, specificity, PPV, NPV, and accuracy were 60%, 97%, 90%, 87%, and 87% respectively; on a lymph node basis these values were 41%, 99%, 94%, 97%, and 97% respectively. Beheshti et al.95 evaluated 18F-choline PET/CT in 111 patients, who subsequently underwent RP with ePLND. On a patient basis, PET/CT showed a sensitivity, specificity, PPV, and NPV of 66%, 96%, 82%, and 92%, respectively; on a lymph-node basis PET/CT showed a sensitivity, specificity, PPV and NPV of 45%, 96%, 82%, and 83%, respectively. Moreover, early BM were detected in two patients exclusively by PET/CT. Overall PET/CT led to a change in therapy in 15% of all patients and 20% of high-risk patients.
Poulsen et al.96 confirmed these data in a study performed on 210 patients with intermediate- and high-risk prostate cancer. A sensitivity of 73% and 56% was observed on a patient, and a lymph node basis respectively with a specificity of 87% and 94%. The main drawback of this study is that “only” 1,093 lymph nodes were removed in the 210 patients (a mean of 5 lymph nodes per patient) and only 73/1,093 were positives.
In summary, the use of choline-PET/CT, labeled with either 11C or 18F, for preoperative lymph nodal staging showed good specificity and poor sensitivity. This could lead to the use of PET/CT with choline as a whole-body staging procedure in patients who are high risk for lymph nodal positive status (according to nomograms), to reduce the number of negative or inconclusive choline PET/CT performed (Fig. 31.5). More prospective studies are needed to determine cost-effectiveness and the real impact of choline PET/CT.
FIGURE 31.5. A 63-year-old patient with prostate cancer staged wit 11C-choline PET/CT before primary treatment. PSA 23 ng/mL; Gs 4+4; T3a. A: MIP projection; B: left subclavicle positive lymph node; C and D: para-aortic and right common iliac positive lymph nodes; E: primary prostate cancer.
Monitoring PSA serum level is the best way to detect early recurrence of the disease after primary treatment. In the event of biochemical relapse (PSA > 0.2 ng/mL), the role of imaging is to find the site of recurrence to establish the most appropriate therapy (“salvage” radiotherapy in the prostatic fossa; “palliative” treatment with hormonal deprivation; chemotherapy, etc.). The probability of the site of recurrence, local versus distal, is often guided by the PSA value and by its kinetics (rate of increase).97 As for staging, there are no precise guidelines about the role of imaging in the restaging of PC patients after biochemical relapse. The European Guidelines on prostate cancer92 do not suggest any imaging procedure if PSA is lower than 20 ng/mL, because the accuracy of TRUS, CT, MRI, and BS is limited,98 particularly in patients with low PSA values.
The first large prospective study of 11C-choline was performed by Picchio et al.99 comparing 11C-choline PET to 18F-FDG PET results in 100 PC patients with biochemical recurrence (mean PSA value: 6.57 ng/mL). 11C-choline PET detected areas of abnormal uptake in 47% of patients, 27% with 18F-FDG PET. None of FDG PET-positive patients were choline PET-negative.
Krause et al.100 evaluated 63 PC patients with biochemical relapse (mean PSA: 5.9 ng/mL) after primary treatment by 11C-choline PET/CT. They demonstrated a significant and strict correlation between PET/CT detection rate and PSA serum levels and showed an overall detection rate of 59% for 11C-choline PET/CT. In 358 PC patients (mean PSA: 3.7 ng/mL), Giovacchini et al.101 found that on multivariate analysis, PSA levels as well as advanced pathologic stage, previous biochemical failure, and older age (>65 years) were significantly (p < 0.05) associated with an increased the likelihood of positive 11C-choline PET/CT findings.
Castellucci et al.102 investigated the relationship between 11C-choline PET/CT detection rate and PSA kinetics (PSA velocity and PSA doubling time). They enrolled a total of 190 patients after RP who showed an increase in PSA (mean: 4.2 ng/mL; median: 2.1 ng/mL; range: 0.2 to 25.4 ng/mL). Authors found that PSAdt and PSAvel values were statistically different between patients with PET-positive and PET-negative findings. Authors concluded that PSA kinetics should always be taken into consideration before performing 11C-choline PET/CT in patients with biochemical failure because it is the most relevant factor for a positive result.
Another study in which the influence of PSA kinetics has been proved,103 evaluated 102 patients previously treated by RP who presented only a mild increase of PSA serum levels <1.5 ng/mL (mean: 0.86 ± 0.40 ng/mL). Overall, 11C-choline PET/CT showed positive findings in 29/102 patients (28%). Using a retrospective cutoff value for PSAdt of 7.25 months, there were only 2/56 (only 3%) positive 11C-choline PET/CT in patients with a PSAdt slower than the cutoff, whereas there were 27/46 (58%) positive 11C-choline PET/CT in patients with a faster PSAdt value (p < 0.001).
In summary, choline PET/CT could represent an important imaging modality in the detection of lymph nodal and distant recurrence in PC patients with biochemical recurrence. In particular, it could play a crucial role as the first diagnostic procedure in patients who demonstrate a fast PSA kinetics and low PSA values. In patients with slow PSA kinetics, the sensitivity of 11C-choline PET/CT is limited.
Molecular imaging includes a range of techniques meant to visualize molecular events at the cellular level in living organisms in a noninvasive way. In the preclinical setting, the most interesting molecular imaging techniques are PET104 and MRI with molecular contrast agents that allow in vivo accurate quantitation or semiquantitation of many molecular phenomena. Another important technique is the CT with or without vascular or liver contrast agents. CT does not provide “molecular” information, but is very useful for observing the morphology of tissues and lesions (e.g., to accurately measure a tumoral mass over time) because it is very fast and complements the data obtained by PET and MRI.
It is now possible to utilize small animal PET, MRI, and CT scanners and the future likely includes hybrid scanners. At present, small animal PET/CT scanners are already available, whereas PET/MRI scanners are still emerging.
PET is used to provide information about tumoral metabolic activity105,106 and allows for the exploration of different metabolic pathways in physiologic and pathologic tissues. The main advantage of small animal PET and MRI, over the standard methods of preclinical experimentation requiring ex vivo examination, is the possibility to analyze the same animal more than once over time, allowing one to observe, for example, the response of a disease condition to a new therapeutic agent or the development of disease, thereby significantly reducing the number of animals employed and increasing the reliability of the results.
Furthermore, the use of small animal PET technology allows for the detection of very low (picomolar) concentrations of radiotracers with great sensitivity to detect very small variations in uptake.104 MRI, although less sensitive than PET, produces very high resolution imaging.
Another very interesting characteristic of preclinical molecular imaging is summarized by the word “translational.” By employing the same technology (PET, MRI, or CT) in the experimental setting and in clinical practice, the step between preclinical science and clinical applications in human patients is shortened, reducing the overall time required to effectively verify the clinical utility of a new approach. One significant example in this field is the in vivo testing of new radiolabeled compounds designed to increase the specificity of PET imaging for a specific disease. The creation of animal models of human disease for in vivo testing of new compounds avoids evaluating compounds that are neither sensitive nor specific for the disease process of interest.
The translational applicability of these techniques, the possibility for accurate quantitation106 and the high spatial resolution (1 mm for PET and <1 mm for MRI and CT) are all very important to study small animals like rodents in vivo. The high sensitivity and the possibility to use targeted probes to increase specificity of disease characterization are features that make these procedures very desirable in the preclinical scenario, despite their relatively high cost compared with that of standard ex vivo studies.
Applications of Small Animal PET in Oncology
The vast majority of studies employing small animal PET scanners are not based on the comparison between PET results and other preclinical imaging procedures but instead take into consideration histochemical analyses or autoradiography to verify imaging results. This is mainly because of the high costs of the scanners, which makes it very difficult to access multimodality technology. In the future, it is likely that this approach will change, and more and more complementary imaging techniques will be employed for evaluation of the same animal models.
Small animal PET allows one to noninvasively measure a range of tumor-relevant parameters at both the cellular and the molecular level, which can be observed longitudinally over time. Studies to evaluate tumor response to a therapeutic intervention can achieve statistical significance using smaller groups of animals, as tumor cell physiology and tumor burden can be accurately determined before and after therapeutic intervention.
The most widely employed PET imaging probe is [18F]-2-fluoro-2-deoxy-D-glucose, which achieves tumor-specific accumulation because tumor cells have a higher rate of glucose uptake and metabolism (glycolysis) than normal tissues. FDG is generally used in oncology to predict cancer cell engraftment107 and to measure the response to therapy. [18F]-3′-fluoro-3′-deoxy-L-thymidine (FLT) and its analogs (e.g., [18F]-1-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) thymine are another family of compounds that are widely used in preclinical PET because they demonstrate the proliferative index of tumor masses with high accuracy, which is far higher for animal models of cancer than for human patients.108
Many other PET probes have either been developed or are under development to obtain tumor specificity via a variety of tumor-specific mechanisms. The development of targeted radiolabeled ligands has further enabled PET to image many aspects of in vivo tumor biology. Radiolabeled annexin-V, arginine-glycine-aspartic acid (RGD) peptide, vascular endothelial growth factor (VEGF), and αvβ3 integrin, for example, have successfully been tested in tumor models as well as in models of cardiac infarction. The pharmacokinetics and pharmacodynamics of radiolabeled anticancer therapeutics can, in principle, also be monitored by these methods, leading to rapid improvements in drug dose scheduling or design.
Finally, the effects of receptor therapies (e.g., inhibitors of androgen receptors, estrogen receptors, and of epithelial growth factor receptor [EGFR]) can theoretically be predicted thanks to the in vivo demonstration of the receptor after injection of a particular radiolabeled ligand.106
The literature includes a wide number of PET radiolabeled compounds for preclinical evaluation of specific molecular events, and it would be very difficult to provide a complete list of all proposed compounds for oncologic studies from the past decade.
Small animal CT can be used as a supporting modality for small animal PET for several reasons. First, it allows, as in the clinical setting, the correct anatomical localization of PET findings. This is of particular importance in the field of small animal imaging. Second, the CT image can be used as an attenuation correction map for PET images, as in the clinical setting for humans. Actually this is of less importance because the animal body is small and the highly energetic photons are subjected to negligible tissue attenuation, although it can be important to achieve accurate quantitation of PET radiotracer uptake when one is studying larger animals such as primates. Third, CT images are useful to integrate the metabolic results obtained by the PET. CT images can be used, for example, to exactly measure the size or volume of organs or tumors and these measurements can be noninvasively monitored over time. CT also provides an attenuation map which can be useful to detect the onset of lesional necrosis, small hepatic lesions, ascites, and so on. Small animal CT is especially useful for the evaluation of the osseous structures and the lungs, even in the absence of intravenous contrast material.109
Recently, a technologic development has enabled CT images with cardiac gating and/or respiratory gating. Although respiratory gating is used to improve the spatial resolution predominantly at the lung bases by compensating for respiratory motion artifacts, cardiac gating has much more scientific utility in the study of cardiac performance such as ejection fraction. Most of the published studies regarding the small animal PET/CT imaging are performed using two separate scanners with a multimodality gantry bed that is shifted from one scanner to the other. In this way, the change in the position of the experimental animal is kept to a minimum, and the image sets (usually in DICOM format) can be coregistered.
Labeled amino acids to image brain malignancies is now a reality, even though only a few patients have benefited to date. 68Ga-DOTA-peptides and 18F-DOPA are principally useful to evaluate patients with NET. Both 68Ga-DOTA-peptides and 18F-DOPA are valuable tracers. A consistent literature demonstrates their accuracy compared to conventional imaging (CT, MRI) and SRS. 68Ga-DOTA-peptides are likely to become the tracers of choice to study patients with well-differentiated NET. At present, however, their use is limited to specialized centers in Europe. The advantages of these tracers include not only to a better overall accuracy but also the possibility to obtain data regarding SSR expression on target lesions. These data noninvasively identify patients eligible for therapy with either hot or cold somatostatin analogs. From a technical point of view, it is also worth mentioning that the relatively easy and economic synthesis process of these tracers renders them suitable for use even in small centers without an on-site cyclotron. In view of both the relatively difficult and expensive synthesis process and the documented lower accuracy, 18F-DOPA role may be more relevant in the clinical settings in which 68Ga-DOTA-peptides show suboptimal performance. For example, 18F-DOPA PET/CT is a valuable to image NET cases with variable or absent SSR expression (e.g., medullary thyroid carcinoma, neuroblastoma) or to assess the response to treatment. Finally, two issues still need to be addressed. First of all, considering the wide spread clinical use of SRS, it is worth mentioning that SRS is still a valuable technique to study well-differentiated NET. Of course, as stated above, the use of PET/CT with 68Ga-DOTA-peptides, when available, offers several advantages. The employment of a more sensitive imaging procedure, however, may not always have a direct impact on clinical management. In fact, in most cases the detection of a higher number of metastatic lesions by PET/CT compared to morphologic imaging or SRS, is not followed by a change in the therapeutic approach. On the contrary, therapeutic management may be influenced by the detection of unsuspected metastatic spread or local relapse, by the identification of the site of the occult primary tumor or by the confirmation or exclusion of SSR expression on tumor cells. Therefore, the choice between performing PET/CT or SRS should be mainly driven by the local availability of the procedure itself. If SRS is performed, PET/CT should be recommended only in cases in which the detection of a more extensive disease would change the therapeutic approach (e.g., SRS-negative cases, cases in which SRS showed only the primary, equivocal SRS findings). Another relevant issue to be discussed is the potential role of 18F-FDG compared to 18F-DOPA and 68Ga-DOTA-peptides in NET lesion detection. Although a detailed analysis of the role of 18F-FDG is beyond the scope of this chapter, it is worth mentioning that it may provide valuable information in NET forms characterized by lower differentiation grade. In fact, although it is well known that 18F-FDG has a limited role in the assessment of well-differentiated NET, because of their slow glucose metabolic rate, it should also be reminded that NET may show variable degrees of differentiation both between patients and in different lesions within the same patient. From a prognostic point of view, the detection of FDG-avid lesions is associated with a worse prognosis, reflecting the presence of lesions with a lower differentiation grade.
18FLT is more tumor specific than 18FDG but its tumor uptake is lower resulting in lower sensitivity. It is not useful for tumors within the liver, bone, or pelvic region because of high accumulation of the radiotracer. Although cellular proliferation is a hallmark of malignancy, cell division can also occur in benign processes, including certain forms of infection and inflammation. Therefore, proliferation markers cannot replace 18FDG but rather should be used as complementary tools to provide additional diagnostic specificity and biologic information for therapy planning and monitoring. 18FLT PET could serve as a noninvasive tool to establish tumor grade, to visualize heterogeneity within the lesion, to help defining the optimum biopsy site, and to select patients who might benefit from adjuvant treatment.
Imaging BM often results in a complex and multimodality strategy that is primarily influenced by a patient’s underlying primary tumor, clinical situation, and expected change in clinical management. Nonetheless when a whole-body skeletal assessment is needed, 99mTc-DP BS is the modality of choice in a variety of solid primaries. Despite preliminary well-controlled reports of higher diagnostic accuracy compared to 99mTc-diphosphonate planar and tomographic imaging, [18F-]fluoride PET has found it hard to enter clinical practice. This could be ascribed to the parallel increasing availability of tumor-specific PET and PET/CT in staging and restaging scenarios which has partially obviated the need for bone-seeking radionuclide imaging. Indeed 18PET has been proven effective in assessing not only primary tumors and their visceral metastases but also early bone marrow–based and lytic disease with better accuracy than 99mTc-DPD BS. Similar results have been described for other metabolic and receptorial tumor-specific radiopharmaceuticals (i.e., 11C-choline in prostate cancer, 68Ga-DOTA-TOC in NETs84,110). Moreover, impaired diagnostic accuracy of tumor-specific PET in osteoblastic lesions could be partially obviated by low-dose CT from modern hybrid PET/CT systems. Although 18F-fluoride PET has been proven clearly to be more accurate than 99mTc-diphosponate imaging in detecting blastic, mixed, and lytic lesions even with poor and/or undetectable osteoblastic reaction on CT, its costs and relatively lower availability seem to represent a major disadvantage.
Clinical scenarios in which improved 18F-Fluoride PET diagnostic accuracy would significantly impact on patient’s therapeutic management skill needs to be determined. Because [18F-]fluoride PET imaging uniquely assessed the skeleton and may fail to detect bone marrow–based lesions which represent the early stage of metastatic growth, its role in diagnostic work-up is generally complementary to tumor-specific PET, and reserved to specific patient subsets. Undoubtedly 18F-fluoride PET studies would be of great value in assessing those primaries which cannot be effectively investigated by any oncotropic PET agent.
The role of functional imaging in the evaluation of prostate cancer patients is not well established. The use of choline PET/CT for staging is recommended only in patients with high-risk prostate. The main clinical application of choline PET/CT is the restaging of the disease in cases of biochemical relapse to detect LN and distant recurrence.
It is likely that there will be wider use of amino acid tracers in the future as the means of production and distribution improves. Considering the polymorphic nature of neuroendocrine cells (the wide range of both surface receptors and hormone products), further studies are needed to ascertain whether the use of alternative radiolabeled compounds or the definition of criteria for the combined use of available radiotracers may provide additional value for NET patient management.
Larger, prospective studies are warranted to better validate 18F-FLT. These should include:
• use of PET-guided biopsy particularly for highly heterogeneous tissues.
• use of mean and maximum measurement both for the SUV and Ki-67 scoring, and the use of SUVpeak (1 mL volume containing the highest SUV).
• development of guidelines with detailed report of methodology and execution of both FLT PET/CT and histopathology measurements.
The added value of 18F-fluoride PET/CT to assess BM should imply a significant change in patients’ therapeutic management compared to well-established diagnostic techniques. With this regard well-controlled trials are supported by Medicare and Medicaid Services (CMS) with the aim of providing evidence on the real cost-effectiveness of 18F-fluoride PET/CT, especially in assisting the primary therapeutic strategy by the detection and quantification of bone lesions in patients in whom metastases are strongly suspected, based on clinical symptoms or results of other diagnostic studies.
Better understanding of both malignant and benign causes for 18F-fluoride uptake may result from the introduction of hybrid PET/MRI systems in clinical settings. Indeed not only will tumor-specific and bone-specific PET imaging be implemented with high-detailed morphostructural information but further functional data will help better understand BM pathophysiology.
Also, 18F-fluoride shares a similar mechanism of uptake with 223Ra-chloride, an α-emitter recently introduced for systemic treatment of multimetastatic patients. Exploiting this similarity, a recent pilot study111 has proposed semiquantitative 18F-fluoride PET as a method to monitor BM treatment response at clinically relevant intervals.
Future efforts should aim to study the role of 11C-choline PET/CT imaging as the initial diagnostic imaging procedure in patients with prostate carcinoma with fast PSA kinetics.
1. Moulin-Romsée G, D’Hondt E, de Groot T, et al. Non-invasive grading of brain tumours using dynamic amino acid PET imaging: Does it work for 11C-methionine? Eur J Nucl Med Mol Imaging. 2007;34(12):2082–2087.
2. Price SJ. The role of advanced MR imaging in understanding brain tumour pathology. Br J Neurosurg. 2007;21(6):562–575.
3. Kracht LW, Friese M, Herholz K, et al. Methyl-[11C]-l-methionine uptake as measured by positron emission tomography correlates to microvessel density in patients with glioma. Eur J Nucl Med Mol Imaging. 2003;30(6):868–873.
4. Pirotte B, Goldman S, Massager N, et al. Comparison of 18F-FDG and 11C-methionine for PET-guided stereotactic brain biopsy of gliomas. J Nucl Med. 2004;45(8):1293–1298.
5. Scott JN, Brasher PMA, Sevick RJ, et al. How often are nonenhancing supratentorial gliomas malignant? A population study. Neurology. 2002;59:947–949.
6. Kim S, Chung JK, Im SH, et al. 11C-methionine PET as a prognostic marker in patients with glioma: comparison with 18F-FDG PET. Eur J Nucl Med Mol Imaging. 2005;32(1):52–59.
7. Galldiks N, Kracht LW, Burghaus L, et al. Use of 11C-methionine PET to monitor the effects of temozolomide chemotherapy in malignant gliomas. Eur J Nucl Med Mol Imaging. 2006;33(5):516–524.
8. Sundin A, Johansson C, Hellman P, et al. PET and parathyroid L-[carbon-11]methionine accumulation in hyperparathyroidism. J Nucl Med. 1996;37(11): 1766–1770.
9. Dunet V, Rossier C, Buck A, et al. Performance of 18F-fluoro-ethyl-tyrosine (18F-FET) PET for the differential diagnosis of primary brain tumor: A systematic review and metaanalysis. J Nucl Med. 2012;53(2):207–214.
10. Calabria F, Chiaravalloti A, Di Pietro B, et al. Molecular imaging of brain tumors with 18F-DOPA PET and PET/CT. Nucl Med Commun. 2012;33(6):563–570.
11. Modlin IM, Kidd M, Latich I, et al. Current status of gastrointestinal carcinoids. Gastroenterology. 2005;128(6):1717–1751.
12. Sundin A, Garske U, Orlefors H. Nuclear imaging of neuroendocrine tumours. Best Pract Res Clin Endocrinol Metab. 2007;21(1):69–85.
13. Krenning EP, Kwekkeboom DJ, Bakker WH, et al. Somatostatin receptor scintigraphy with [111In-DTPA-D-Phe1]-and [123I-Tyr3]-octreotide: The Rotterdam experience with more than 1000 patients. Eur J Nucl Med. 1993; 20:716–731.
14. Cimitan M, Buonadonna A, Cannizzaro R, et al. Somatostatin receptor scintigraphy versus chromogranin A assay in the management of patients with neuroendocrine tumors of different types: clinical role. Ann Oncol. 2003; 14(7):1135–1141.
15. Kowalski J, Henze M, Schuhmacher J, et al. Evaluation of positron emission tomography imaging using [68Ga]-DOTA-D-Phe1-Tyr3-octreotide in comparison to [111In]-DTPAOC SPECT. First results in patients with neuroendocrine tumors. Mol Imaging Biol. 2003;5:42–48.
16. Buchmann I, Henze M, Engelbrecht S, et al. Comparison of 68Ga-DOTATOC PET and 111In-DTPAOC (Octreoscan) SPECT in patients with neuroendocrine tumours. Eur J Nucl Med Mol Imaging. 2007;34(10):1617–1626.
17. Antunes P, Ginj M, Zhang H, et al. Are radiogallium-labelled DOTA-conjugated somatostatin analogues superior to those labelled with other radiometals? Eur J Nucl Med Mol Imaging. 2007;34(7):982–993.
18. Reubi JC. Peptide receptors as molecular targets for cancer diagnosis and therapy. Endocr Rev. 2003;24(4):389–427.
19. Reubi JC, Waser B. Concomitant expression of several peptide receptors in neuroendocrine tumors: molecular basis for in vivo multireceptor tumour targeting. Eur J Nucl Med Mol Imaging. 2003;30(5):781–793.
20. Virgolini I, Ambrosini V, Bomanji JB, et al. Procedure guidelines for PET/CT tumour imaging with 68Ga-DOTA-conjugated peptides: 68Ga-DOTA-TOC, 68Ga-DOTA-NOC, 68Ga-DOTA-TATE. Eur J Nucl Med Mol Imaging. 2010; 37(10):2004–2010.
21. Schreiter NF, Brenner W, Nogami M, et al. Cost comparison of 111In-DTPA-octreotide scintigraphy and 68Ga-DOTATOC PET/CT for staging enteropancreatic neuroendocrine tumours. Eur J Nucl Med Mol Imaging. 2012;39(1):72–82.
22. Gabriel M, Decristoforo C, Kendler D, et al. 68Ga-DOTA-Tyr3-octreotide PET in neuroendocrine tumors: Comparison with somatostatin receptor scintigraphy and CT. J Nucl Med. 2007;48(4):508–518.
23. Ambrosini V, Campana D, Bodei L, et al. 68Ga-DOTANOC PET/CT clinical impact in patients with neuroendocrine tumors. J Nucl Med. 2010;51(5):669–673.
24. Campana D, Ambrosini V, Pezzilli R, et al. Standardized uptake values of (68)Ga-DOTANOC PET: A promising prognostic tool in neuroendocrine tumors. J Nucl Med. 2010;51(3):353–359.
25. Prasad V, Ambrosini V, Hommann M, et al. Detection of unknown primary neuroendocrine tumours (CUP-NET) using (68)Ga-DOTA-NOC receptor PET/CT. Eur J Nucl Med Mol Imaging. 2010;37(1):67–77.
26. Ambrosini V, Tomassetti P, Castellucci P, et al. Comparison between 68Ga-DOTA-NOC and 18F-DOPA PET for the detection of gastro-entero-pancreatic and lung neuroendocrine tumours. Eur J Nucl Med Mol Imaging. 2008;35(8):1431–1438.
27. Haug A, Auernhammer CJ, Wängler B, et al. Intraindividual comparison of 68Ga-DOTA-TATE and 18F-DOPA PET in patients with well-differentiated metastatic neuroendocrine tumours. Eur J Nucl Med Mol Imaging. 2009;36(5):765–770.
28. Velikyan I, Beyer GJ, Långström B. Microwave-supported preparation of 68Ga bioconjugates with high specific radioactivity. Bioconjug Chem. 2004;15:554–560.
29. Meyer GJ, Maecke H, Schuhmacher J, et al. 68Ga-labelled DOTA-derivatised peptide ligands. Eur J Nucl Med Mol Imaging. 2004;31:1097–1104.
30. Zhernosekov KP, Filosofov DV, Baum RP, et al. Processing of generator-produced 68Ga for medical application. J Nucl Med. 2007;48:1741–1748.
31. Di Pierro D, Rizzello A, Cicoria G, et al. Radiolabelling, quality control and radiochemical purity assessment of the octreotide analogue 68Ga DOTA NOC. Appl Radiat Isot. 2008;66:1091–1096.
32. Breeman WA, de Jong M, de Blois E, et al. Radiolabelling DOTA-peptides with 68Ga. Eur J Nucl Med Mol Imaging. 2005;32:478–485.
33. Decristoforo C, Knopp R, von Guggenberg E, et al. A fully automated synthesis for the preparation of 68Ga-labelled peptides. Nucl Med Commun. 2007;28:870–875.
34. Haug AR, Rominger A, Mustafa M, et al. Treatment with octreotide does not reduce tumor uptake of (68)Ga-DOTATATE as measured by PET/CT in patients with neuroendocrine tumors. J Nucl Med. 2011;52(11):1679–1683.
35. Castellucci P, Pou Ucha J, Fuccio C, et al. Incidence of increased 68Ga-DOTANOC uptake in the pancreatic head in a large series of extrapancreatic NET patients studied with sequential PET/CT. J Nucl Med. 2011;52(6):886–890.
36. Hoegerle S, Altehoefer C, Ghanem N, et al. Whole-body 18F-DOPA PET for detection of gastrointestinal carcinoid tumors. Radiology. 2001;220(2):373–380.
37. Koopmans KP, de Vries EG, Kema IP, et al. Staging of carcinoid tumours with 18F-DOPA PET: A prospective, diagnostic accuracy study. Lancet Oncol. 2006;7(9):728–734.
38. Ambrosini V, Tomassetti P, Rubello D, et al. Role of 18F-dopa PET/CT imaging in the management of patients with 111In-pentetreotide negative GEP tumours. Nucl Med Commun. 2007;28(6):473–477.
39. Kloos RT, Eng C, Evans DB, et al. Medullary thyroid cancer: Management guidelines of the American Thyroid Association. Thyroid. 2009;19:565–612.
40. Beheshti M, Pocher S, Vali R, et al. The value of 18F-DOPA PET-CT in patients with medullary thyroid carcinoma: Comparison with 18F-FDG PET-CT. Eur Radiol. 2009;19:1425–1434.
41. Koopmans KP, de Groot JW, Plukker JT, et al. 18F-dihydroxyphenylalanine PET in patients with biochemical evidence of medullary thyroid cancer: Relation to tumor differentiation. J Nucl Med. 2008;49:524–531.
42. Kauhanen S, Schalin-Jantti C, Seppanen M, et al. Complementary roles of 18F-DOPA PET/CT and18F-FDG PET/CT in medullary thyroid cancer. J Nucl Med. 2011;52:1855–1863.
43. Treglia G, Castaldi P, Villani MF, et al. Comparison of 18F-DOPA, 18F-FDG and 68Ga-somatostatin analogue PET/CT in patients with recurrent medullary thyroid carcinoma. Eur J Nucl Med Mol Imaging. 2012;39(4):569–580.
44. Piccardo A, Lopci E, Conte M, et al. Comparison of 18F-dopa PET/CT and 123I-MIBG scintigraphy in stage 3 and 4 neuroblastoma: A pilot study. Eur J Nucl Med Mol Imaging. 2012;39(1):57–71.
45. Georgantzi K, Tsolakis AV, Stridsberg M, et al. Differentiated expression of somatostatin receptor subtypes in experimental models and clinical neuroblastoma. Pediatr Blood Cancer. 2011;56(4):584–589.
46. Krohn KA, Mankoff DA, Muzi M, et al. True tracers: Comparing FDG with glucose and FLT with thymidine. Nucl Med Biol. 2005;32:663–671.
47. Salskov A, Tammisetti VS, Grierson J, et al. FLT: Measuring tumour cell proliferation in vivo with positron emission tomography and 3′-deoxy-3′-[18F]fluorothymidine. Semin Nucl Med. 2007;37:429–439.
48. Been LB, Suurmeijer AJ, Cobben DC, et al. [18F]FLT-PET in oncology: Current status and opportunities. Eur J Nucl Med Mol Imaging. 2004;31:1659–1672.
49. van Waarde A, Elsinga PH. Proliferation markers for the differential diagnosis of tumour and inflammation. Curr Pharm Des. 2008;14:3326–3339.
50. Zhang CC, Yan Z, Li W, et al. [(18)F]FLT-PET imaging does not always “light up” proliferating tumour cells. Clin Cancer Res. 2012;18:1303–1312.
51. Mankoff DA, Shields AF, Krohn KA. PET imaging of cellular proliferation. Radiol Clin North Am. 2005;43:153–167.
52. Barwick T, Bencherif B, Mountz JM, et al. Molecular PET and PET/CT imaging of tumour cell proliferation using F-18 fluoro-L-thymidine: A comprehensive evaluation. Nucl Med Commun. 2009;30:908–917.
53. Troost EG, Vogel WV, Merkx MA, et al. 18F-FLT PET does not discriminate between reactive and metastatic lymph nodes in primary head and neck cancer patients. J Nucl Med. 2007;48:726–735.
54. Chalkidou A, Landau DB, Odell EW, et al. Correlation between Ki-67 immunohistochemistry and 18F-Fluorothymidine uptake in patients with cancer: A systematic review and meta-analysis. Eur J Cancer. 2012;48:3499–3513
55. Brockenbrough JS, Souquet T, Morihara JK, et al. Tumor 3′-deoxy-3′-(18) F-fluorothymidine ((18)F-FLT) uptake by PET correlates with thymidine kinase 1 expression: Static and kinetic analysis of (18)F-FLT PET studies in lung tumors. J Nucl Med. 2011;52:1181–1188.
56. Soloviev D, Lewis D, Honess D, et al. [(18)F]FLT: An imaging biomarker of tumour proliferation for assessment of tumour response to treatment. Eur J Cancer. 2012;48:416–424.
57. Bading JR, Shields AF. Imaging of cell proliferation: Status and prospects. J Nucl Med. 2008;49:64S–80S.
58. Shields AF. Positron emission tomography measurement of tumour metabolism and growth: Its expanding role in oncology. Mol Imaging Biol. 2006;8:141–150.
59. Hamaoka T, Madewell JE, Podoloff DA, et al. Bone imaging in metastatic breast cancer. J Clin Oncol. 2004;22(14):2942–2295.
60. Taoka T, Mayr N, Lee HJ, et al. Factors influencing visualization of vertebral metastases on MR imaging versus bone scintigraphy. AJR Am J Roentgenol. 2001;176:1525–1530.
61. Savelli G, Chiti A, Grasselli G, et al. The role of bone SPET study in diagnosis of single vertebral metastases. Anticancer Res. 2000;20(2B):1115–1120.
62. Blau M, Nagler W, Bender M. Fluorine-18: A new isotope for bone scanning. J Nucl Med. 1962;3:332–334.
63. Frost ML, Blake GM, Cook GJ, et al. Differences in regional bone perfusion and turnover between lumbar spine and distal humerus: 18F-fluoride PET study of treatment-naive and treated postmenopausal women. Bone. 2009;45:942–948.
64. Carlson CH, Armstrong WD, Singer L. Distribution, migration and binding of whole blood fluoride evaluated with radiofluoride. Am J Physiol. 1960;199:187–189.
65. Wootton R, Dore C. The single-passage extraction of 18F in rabbit bone. Clin Phys Physiol Meas. 1986;7:333–343.
66. Blake GM, Park-Holohan S, Cook GJR, et al. Quantitative studies of bone with the use of 18F-fluoride and 99mTc-methylene diphosphonate. Semin Nucl Med. 2001;31(1):28–49.
67. Satyamurthy N, Amarasekera B, Alvord C, et al. Tantalum [18O]water target for the production of [18F] fluoride with high reactivity for the preparation of 2-deoxy-2-[18F] fluoro-D-glucose. Mol Imaging Biol. 2002;4:65–70.
68. U. S. Pharmacopeia: Sodium Fluoride F 18 Injection. USP 32–NF 27, 2009.
70. Segall G, Delbeke D, Stabin MG, et al. SNM practice guideline for sodium 18F-fluoride PET/CT bone scans 1.0. J Nucl Med. 2010;51(11):1813–1820.
71. Publication 35: Radiation dose to patients from radiopharmaceuticals. Ann ICRP. 1987;17:74. Pergamon Press, Oxford
72. Publication 80: Radiation dose to patients from radiopharmaceuticals. Ann ICRP. 1998;28:3. Pergamon Press, Oxford
73. Nakai T, Okuyama C, Kubota T, et al. Pitfalls of FDG-PET for the diagnosis of osteoblastic bone metastases in patients with breast cancer. Eur J Nucl Med Mol Imaging. 2005;32:1253–1258.
74. Du Y, Cullum I, Illidge TM, et al. Fusion of metabolic function and morphology: Sequential [18F] Fluorodeoxyglucose positron-emission tomography/computed tomography studies yield new insights into the natural history of bone metastases in breast cancer. J Clin Oncol.2007;25:3440–3447.
75. Cook GJ, Houston S, Rubens R, et al. Detection of bone metastases in breast cancer by 18FDG PET: Differing metabolic activity in osteoblastic and osteolytic lesions. J Clin Oncol. 1998;16:3375–3379.
76. Schirrmeister H, Guhlmann A, Elsner K, et al. Sensitivity in detecting osseous lesions depends on anatomic localization: Planar bone scintigraphy versus 18F PET. J Nucl Med. 1999;40(10):1623–1629.
77. Schirrmeister H, Guhlmann A, Kotzerke J, et al. Early detection and accurate description of extent of metastatic bone disease in breast cancer with fluoride ion and positron emission tomography. J Clin Oncol. 1999;17:2381–2389.
78. Schirrmeister H, Glatting G, Hetzel J, et al. Prospective evaluation of the clinical value of planar bone scans, SPECT, and 18F-labeled NaF PET in newly diagnosed lung cancer. J Nucl Med. 2001;42(12):1800–1804.
79. Hetzel M, Arslandemir C, König HH, et al. 18NaF PET for detection of bone metastases in lung cancer: Accuracy, cost-effectiveness, and impact on patient management. J Bone Miner Res. 2003;18(12):2206–2214.
80. Even-Sapir E, Metser U, Mishani E, et al. The detection of bone metastases in patients with high-risk prostate cancer: 99mTc-MDP planar bone scintigraphy, single- and multi-field-of-view SPECT, 18F-fluoride PET, and 18F- fluoride PET/CT. J Nucl Med. 2006;47(2):287–297.
81. Even-Sapir E, Metser U, Flusser G, et al. Assessment of malignant skeletal disease: Initial experience with 18 F-fluoride PET/CT and comparison between 18 F-fluoride PET and 18 F-fluoride PET-CT. J Nucl Med. 2004; 45(2):272–278.
82. Withofs N, Grayet B, Tancredi T, et al. 18F-fluoride PET/CT for assessing bone involvement in prostate and breast cancers. Nucl Med Commun. 2011; 32(3):168–176.
83. Krüger S, Buck AK, Mottaghy FM, et al. Detection of bone metastases in patients with lung cancer: 99mTc-MDP planar bone scintigraphy, 18F-fluoride PET or 18F-FDG PET/CT. Eur J Nucl Med Mol Imaging. 2009;36:1807–1812.
84. Beheshti M, Vali R, Waldenberger P, et al. Detection of bone metastases in patients with prostate cancer by 18F fluorocholine and 18F fluoride PET–CT: A comparative study. Eur J Nucl Med Mol Imaging. 2008;35:1766–1774.
85. Zeisel SH. Dietary choline: Biochemistry, physiology and pharmacology. Annu Rev Nutr. 1981;1:95–121.
86. Ackerstaff E, Pflug BR, Nelson JB, et al. Detection of increased choline compounds with proton nuclear magnetic resonance spectroscopy subsequent to malignant transformation of human prostatic epithelial cells. Cancer Res. 2001;61:3599–3603.
87. Müller SA, Holzapfel K, Seidl C, et al. Characterization of choline uptake in prostate cancer cells following bicatulamide and docetaxel treatment. Eur J Nucl Med Mol Imaging. 2009;36(9):1434–1442.
88. Husarik DB, Miralbell R, Dubs M, et al. Evaluation of [(18)F]-choline PET/CT for staging and restaging of prostate cancer. Eur J Nucl Med Mol Imaging. 2008;35(2):253–263.
89. Soyka JD, Muster MA, Schmid DT, et al. Clinical impact of 18F-choline PET/CT in patients with recurrent prostate cancer. Eur J Nucl Med Mol Imaging. 2012;39(6):936–943.
90. Zalesky M, Urban M, Smerhovský Z, et al. Value of power doppler sonography with 3D reconstruction in preoperative diagnostics of extraprostatic tumor extension in clinically localized prostate cancer. Int J Urol. 2008;15(1):68–75.
91. Briganti A, Blute ML, Eastham JH, et al. Pelvic lymph node dissection in prostate cancer. Eur Urol. 2009;55(6):1251–1265.
92. Heidenreich A, Bastian PJ, Bellmunt J, et al. EAU Guidelines on Prostate Cancer. Part 1: Screening, Diagnosis, and Local Treatment with Curative Intent-Update 2013. Eur Urol. 2014;65(1):124–137.
93. Harisinghani MG, Barentsz J, Hahn PF, et al. Noninvasive detection of clinically occult lymph-node metastases in prostate cancer. N Engl J Med. 2003;348(25):2491–2499.
94. Schiavina R, Scattoni V, Castellucci P, et al. (11)C-choline positron emission tomography/computerized tomography for preoperative lymph-node staging in intermediate-risk and high-risk prostate cancer: Comparison with clinical staging nomograms. Eur Urol. 2008;54:392–401.
95. Beheshti M, Imamovic L, Broinger G, et al. 18F choline PET/CT in the preoperative staging of prostate cancer in patients with intermediate or high risk of extracapsular disease: A prospective study of 130 patients. Radiology. 2010;254(3):925–933.
96. Poulsen MH, Bouchelouche K, Høilund-Carlsen PF, et al. [(18) F]fluoromethylcholine (FCH) positron emission tomography/computed tomography (PET/CT) for lymph node staging of prostate cancer: A prospective study of 210 patients. BJU Int. 2012;110:1666–1671. doi:10.1111/j.1464-410X.2012.11150.x
97. ESMO Guidelines Task Force. ESMO minimum clinical recommendations for diagnosis, treatment and follow-up of prostate cancer. Ann Oncol. 2005;16(suppl 1):i34–i36.
98. Choueiri TK, Dreicer R, Paciorek A, et al. A model that predicts the probability of positive imaging in prostate cancer cases with biochemical failure after initial definitive local therapy. J Urol. 2008;179(3):906–910.
99. Picchio M, Messa C, Landoni C, et al. Value of [11C]choline-positron emission tomography for re-staging prostate cancer: A comparison with [18F]fluorodeoxyglucose-positron emission tomography. J Urol. 2003;169:1337–1340.
100. Krause BJ, Souvatzoglou M, Tuncel M, et al. The detection rate of [(11)C]Choline-PET/CT depends on the serum PSA-value in patients with biochemical recurrence of prostate cancer. Eur J Nucl Med Mol Imaging. 2008;35:18–23.
101. Giovacchini G, Picchio M, Coradeschi E, et al. Predictive factors of [11C]choline PET/CT in patients with biochemical failure after radical prostatectomy. Eur J Nucl Med Mol Imaging. 2010;37(2):301–309.
102. Castellucci P, Fuccio C, Nanni C, et al. Influence of trigger PSA and PSA kinetics on 11C-choline PET/CT detection rate in patients with biochemical relapse after radical prostatectomy. J Nucl Med. 2009;50(9):1394–1400.
103. Castellucci P, Fuccio C, Rubello D, et al. Is there a role for (11)C-Choline PET/CT in the early detection of metastatic disease in surgically treated prostate cancer patients with a mild PSA increase <1.5ng/ml? Eur J Nucl Med Mol Imaging. 2011;38(1):55–63.
104. Sossi V, Ruth TJ. Micropet imaging: In vivo biochemistry in small animals. J Neural Transm. 2005;112(3):319–330.
105. Aide N, Labiche A, Herlin P, et al. Usefulness of automatic quantification of immunochemical staining on whole tumor sections for correlation with oncological small animal PET studies: An example with cell proliferation, glucose transporter 1 and FDG. Mol Imaging Biol.2008;10(5):237–244.
106. Su H, Bodenstein C, Dumont RA, et al. Monitoring tumor glucose utilization by positron emission tomography for the prediction of treatment response to epidermal growth factor receptor kinase inhibitors. Clin Cancer Res. 2006;12(19):5659–5667.
107. Nanni C, Di Leo K, Tonelli R, et al. FDG small animal PET permits early detection of malignant cells in a xenograft murine model. Eur J Nucl Med Mol Imaging. 2007;34(5):755–762.
108. Apisarnthanarax S, Alauddin MM, Mourtada F, et al. Early detection of chemoradioresponse in esophageal carcinoma by 3′-deoxy-3′-3H-fluorothymidine using preclinical tumor models. Clin Cancer Res. 2006;12(15):4590–4597.
109. Ambrosini V, Nanni C, Pettinato C, et al. Assessment of a chemically induced model of lung squamous cell carcinoma in mice by 18F-FDG small-animal PET. Nucl Med Commun. 2007;28(8):647–652.
110. Putzer D, Gabriel M, Henninger B, et al. Bone metastases in patients with neuroendocrine tumor: 68Ga-DOTA-Tyr3-octreotide PET in comparison to CT and bone scintigraphy. J Nucl Med. 2009;50(8):1214–1221.
111. Gary JR Cook C P, Chua S, et al. 18F-fluoride PET: Changes in uptake as a method to assess response in bone metastases from castrate-resistant prostate cancer patients treated with 223Ra-chloride (Alpharadin). EJNMMI Res. 2011;1:4.