Radiation Dosimetry of Theragnostic Pairs for Isotopes of Iodine in IAZA.

Theragnostic pairs of isotopes are used to infer radiation dosimetry for a therapeutic radiopharmaceutical from a diagnostic imaging study with the same tracer molecule labelled with an isotope better suited for the imaging task. We describe the transfer of radiation dosimetry from the diagnostic radioiodine isotope 123I, labelled for the hypoxia tracer molecule iodoazomycin arabinoside ([123I]IAZA), to isotopes 131I (therapeutic) and 124I (PET imaging). Uncertainties introduced by the dissimilar isotope half-lives are discussed in detail. Radioisotope dosimetries for [123I]IAZA were obtained previously. These data are used here to calculate residence times for 131I and 124I and their uncertainties. We distinguish two cases when extrapolating to infinity: purely physical decay (case A) and physical decay plus biological washout (case B). Organ doses were calculated using the MIRD schema with the OLIDNA/EXM code. Significant increases in some organ doses (in mSv per injected activity) were found for 131I and 124I. The most affected organs were the intestinal walls, thyroid, and urinary bladder wall. Uncertainty remained similar to 123I for case A but considerably greater for case B, especially for long biological half-lives (GI tract). Normal tissue dosimetries for IAZA must be considered carefully when substituting isotope species. A long biological half-life can significantly increase dosimetric uncertainties. These findings are relevant when considering PET imaging studies with [124I]IAZA or therapeutic administration of [131I]IAZA.

Rapid Standardized CT-Based Method to Determine Lean Body Mass SUV for PET-A Significant Improvement Over Prediction Equations.

In 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) studies, maximum standardized uptake value (SUVmax) is the parameter commonly used to provide a measurement of the metabolic activity of a tumor. SUV normalized by body mass is affected by the proportions of body fat and lean tissue, which present high variability in patients with cancer. SUV corrected by lean body mass (LBM), denoted as SUL, is recommended to provide more accurate, consistent, and reproducible SUV results; however, LBM is frequently estimated rather than measured. Given the increasing importance of a quantitative PET parameter, especially when comparing PET studies over time to evaluate disease response clinically, and its use in oncological clinical trials, we set out to evaluate the commonly used equations originally derived by James (1976) and Janmahasatian et al. (2005) against computerized tomography (CT)-derived measures of LBM. Methods: Whole-body 18F-FDG PET images of 195 adult patients with cancer were analyzed retrospectively. Representative liver SUVmean was normalized by total body mass. SUL was calculated using a quantitative determination of LBM based on the CT component of the PET/CT study (LBMCT) and compared against the equation-estimated SUL. Bland and Altman plots were generated for SUV-SUL differences. Results: This consecutive sample of patients undergoing usual care (men, n = 96; women, n = 99) varied in body mass (38-127 kg) and in Body Mass Index (BMI) (14.7-47.2 kg/m2). LBMCT weakly correlated with body mass (men, r2 = 0.32; women, r2 = 0.22), and thus SUV and SULCT were also weakly correlated (men, r2 = 0.24; women, r2 = 0.11). Equations proved inadequate for the assessment of LBM. LBM estimated by James' equation showed a mean bias (overestimation of LBM compared with LBMCT) in men (+6.13 kg; 95% CI 4.61-7.65) and in women (+6.32 kg; 95% CI 5.26-7.39). Janmahasatian's equation provided similarly poor performance. Conclusions: CT-based LBM determinations incorporate the patient's current body composition at the time of a PET/CT study, and the information garnered can provide care teams with information with which to more accurately determine FDG uptake values, allowing comparability over multiple scans and treatment courses and will provide a robust basis for the use of PET Response Criteria in Solid Tumors (PERCIST) in clinical trials.

Prolonged Response to Regorafenib in a Patient with Iodine Refractory Thyroid Cancer.

Thyroid cancer is the most common type of endocrine malignancy. Cornerstones of thyroid cancer treatment include surgery, radioactive iodine ablation, and thyroid stimulating hormone suppression. The National Comprehensive Cancer Network guidelines recommend two tyrosine kinase inhibitors for thyroid cancer patients who are non-responsive to iodine: sorafenib and lenvatinib. Another oral kinase inhibitor, regorafenib, is not considered standard of care treatment for differentiated thyroid cancer. The chemical structures of regorafenib and sorafenib differ by a single fluorine atom. Given the significant improvement in progression-free survival (PFS) of sorafenib compared to placebo demonstrated in the phase 3 DECISION trial, we report on a patient with iodine-refractory follicular thyroid cancer treated with regorafenib as part of a phase 1 clinical trial. A 75 year old woman was diagnosed with follicular thyroid carcinoma in 2006 and initiated on treatment with regorafenib in 2011. She has completed 76 cycles with stable disease and pulmonary metastases 34% smaller than baseline.

Efficacy of 177Lu Peptide Receptor Radionuclide Therapy for the Treatment of Neuroendocrine Tumors: A Meta-analysis.

OBJECTIVE: The purpose of this study was to assess the efficacy of Lu-labeled peptide receptor radionuclide therapy (PRRT) induction treatments for patients with unresectable metastatic neuroendocrine tumors. METHODS: MEDLINE, EMBASE, and Ovid were systematically searched with keywords "lutetium," "Lu-177," "PRRT," "neuroendocrine," and "prognosis." Studies evaluating treatment with Lu-labeled PRRT were assessed for disease response and/or disease control rate by Response Evaluation Criteria in Solid Tumors (RECIST) 1.0 or 1.1, modified RECIST, Southwest Oncology Group (SWOG), or modified SWOG criteria. Pooled proportions of disease response and control rates were calculated for both fixed- and random-effects models. RESULTS: Eighteen studies with 1920 patients were included (11 with 1268 patients using RECIST and 6 with 804 patients using SWOG). By RECIST criteria, the pooled disease response rate by random-effects model was 29.1% (95% confidence interval [CI], 20.2%-38.9%), and disease control rate was 74.1% (95% CI, 67.8%-80.0%). By SWOG criteria, the pooled disease response rate by random-effects model was 30.6% (95% CI, 20.7%-41.5%), and disease control rate was 81.1% (95% CI, 76.4%-85.4%). CONCLUSIONS: Induction therapy, typically 4 treatments, with Lu PRRT is an effective method of treating unresectable metastatic neuroendocrine tumors with significant disease response and control rates.

Pharmacokinetics and Scintigraphic Imaging of the Hypoxia-Imaging Agent [123I]IAZA in Healthy Adults Following Exercise-Based Cardiac Stress †.

The objective of this work is to evaluate the potential effect of cardiac stress exercise on the accumulation of [123I]IAZA, a radiopharmaceutical used to image focal tissue hypoxia, in otherwise normal myocardium in healthy volunteers, and to determine the impact of exercise on [123I]IAZA pharmacokinetics. The underlying goal is to establish a rational basis and a baseline for studies of focal myocardial hypoxia in cardiac patients using [123I]IAZA. Three healthy male volunteers ran the 'Bruce' treadmill protocol, a clinically-accepted protocol designed to expose myocardial ischemia in patients. The 'Bruce' criterion heart rate is 85% of [220-age]. Approximately one minute before reaching this level, [123I]IAZA (5.0 mCi/0.85 mg) was administered as a slow (1-3 min) single intravenous (i.v.) injection via an indwelling venous catheter. The volunteer continued running for an additional 1 min before being transferred to a gamma camera. Serum samples were collected from the arm contralateral to the administration site at pre-determined intervals from 1 min to 45 h post injection and were analyzed by radio HPLC. Pharmacokinetic (PK) parameters were derived for [123I]IAZA and total radioactivity (total[123I]) using compartmental and noncompartmental analyses. Whole-body planar scintigraphic images were acquired from 0.75 to 24 h after dosing. PK data and scintigraphic images were compared to previously published [123I]IAZA data from healthy volunteers rest. Following exercise stress, both [123I]IAZA and total[123I] exhibited bi-exponential decline profiles, with rapid distribution phases [half-lives (t1/2α) of 1.2 and 1.4 min, respectively], followed by slower elimination phases [t1/2ß of 195 and 290 min, respectively]. Total body clearance (CLTB) and the steady state volume of distribution (Vss) were 0.647 L/kg and 185 mL/min, respectively, for [123I]IAZA and 0.785 L/kg and 135 mL/min, respectively, for total[123I]. The t1/2ß, CLTB and Vss values were comparable to those reported previously for rested volunteers. The t1/2α was approximately 4-fold shorter for [123I]IAZA and approximately 3-fold shorter for total[123I] under exercise relative to rested subjects. The heart region was visualized in early whole body scintigraphic images, but later images showed no accumulated radioactivity in this region, and no differences from images reported for rested volunteers were apparent. Minimal uptake of radiotracer in myocardium and skeletal muscle was consistent with uptake in non-stressed myocardium. Whole-body scintigrams for [123I]IAZA in exercise-stressed healthy volunteers were indistinguishable from images of non-exercised volunteers. There was no evidence of hypoxia-dependent binding in exercised but otherwise healthy myocardium, supporting the conclusion that exercise stress at Bruce protocol intensity does not induce measurable myocardial hypoxia. Effects of exercise on PK parameters were minimal; specifically, the t1/2α was shortened, reflecting increased cardiac output associated with exercise. It is concluded that because [123I]IAZA was not metabolically bound in exercise-stressed myocardium, a stress test will not create elevated myocardial background that would mask regions of myocardial perfusion deficiency. [123I]IAZA would therefore be suitable for the detection of viable, hypoxic myocardium in patients undergoing stress-test-based diagnosis.

Comparison of 30 mCi and 50 mCi I-131 doses for ablation of thyroid remnant in papillary thyroid cancer patients.

PURPOSE OF THE STUDY: To compare efficacy of thyroid remnant ablation using 30 mCi or 50 mCi 131-I in papillary thyroid cancer patients. MATERIALS AND METHODS: Five hundred and fifteen consecutive patients with Tumor-Node-Metastasis (TNM) stages T1-T3 N1/N0/NX receiving either 30 mCi or 50 mCi I-131 were analyzed for the effectiveness of remnant ablation using rhTSH-stimulated serum thyroglobulin. One hundred and five consecutive patients receiving 100 mCi I-131 were analyzed for the incidence of radiation thyroiditis and sialadenitis. RESULTS AND CONCLUSIONS: Doses of 30 mCi and 50 mCi were equally effective for low- and moderate-risk disease but 30 mCi was less effective for T1T2NX disease, and 50 mCi was less effective for T3 compared to T1T2 disease. Low dose radiation hypersensitivity or unknown more extensive disease may have accounted for observed differences. Radiation thyroiditis and sialadenitis were more common in a comparison series of 100 mCi dose compared to 30 mCi, but not more common than in 50 mCi doses.

131I-MIBG Therapy in a Metastatic Small Bowel Neuroendocrine Tumor Patient Undergoing Hemodialysis.

Systemic radioisotope therapy with I-metaiodobenzylguanidine (I-MIBG) is an effective form of targeted therapy for neuroendocrine tumors. One of the absolute contraindications to administering I-MIBG therapy listed in the 2008 European Association of Nuclear Medicine guidelines is renal insufficiency requiring dialysis, although this contraindication is not evidence based. We describe a 68-year-old woman with a metastatic small bowel neuroendocrine tumor who developed renal insufficiency requiring hemodialysis. Imaging and dosimetry with I-MIBG were performed and showed that the radiation doses to the whole body and lungs were within safe limits. She was treated with 1820 MBq of I-MIBG with no short-term adverse reactions.

Clinical Trial with Sodium 99mTc-Pertechnetate Produced by a Medium-Energy Cyclotron: Biodistribution and Safety Assessment in Patients with Abnormal Thyroid Function.

A single-site prospective open-label clinical study with cyclotron-produced sodium 99mTc-pertechnetate (99mTc-NaTcO4) was performed in patients with indications for a thyroid scan to demonstrate the clinical safety and diagnostic efficacy of the drug and to confirm its equivalence with conventional 99mTc-NaTcO4 eluted from a generator. Methods:99mTc-NaTcO4 was produced from enriched 100Mo (99.815%) with a cyclotron (24 MeV; 2 h of irradiation) or supplied by a commercial manufacturer (bulk vial eluted from a generator). Eleven patients received 325 ± 29 (mean ± SD) MBq of the cyclotron-produced 99mTc-NaTcO4, whereas the age- and sex-matched controls received a comparable amount of the generator-derived tracer. Whole-body and thyroid planar images were obtained for each participant. In addition to the standard-energy window (140.5 keV ± 7.5%), data were acquired in lower-energy (117 keV ± 10%) and higher-energy (170 keV ± 10%) windows. Vital signs and hematologic and biochemical parameters were monitored before and after tracer administration. Results: Cyclotron-produced 99mTc-NaTcO4 showed organ and whole-body distributions identical to those of conventional 99mTc-NaTcO4 and was well tolerated. All images led to a clear final diagnosis. The fact that the number of counts in the higher-energy window was significantly higher for cyclotron-produced 99mTc-NaTcO4 did not influence image quality in the standard-energy window. Image definition in the standard-energy window with cyclotron-produced 99mTc was equivalent to that with generator-eluted 99mTc and had no particular features allowing discrimination between the 99mTc production methods. Conclusion: The systemic distribution, clinical safety, and imaging efficacy of cyclotron-produced 99mTc-NaTcO4 in humans provide supporting evidence for the use of this tracer as an equivalent for generator-eluted 99mTc-NaTcO4 in routine clinical practice.

Metastatic Insulinoma Pancreatic Neuroendocrine Tumor Treated With 177Lu-DOTATATE Induction and Maintenance Peptide Receptor Radionuclide Therapy: A Suggested Protocol.

A 70-year-old woman presented with frequent episodes of hypoglycemia. Imaging revealed a 6-cm pancreatic mass with several liver lesions. The pancreatic mass was resected and confirmed to be a well-differentiated insulinoma. Surgery improved but did not resolve her hypoglycemic episodes, and she was referred for peptide receptor radionuclide therapy with 177Lu-DOTATATE to treat her residual disease. A modified protocol with a continuous IV dextrose infusion was used, and the treatments were well tolerated. After 4 induction and 2 maintenance treatments, her hypoglycemic symptoms resolved completely and her disease stabilized. She has been progression free for 24 months.

Chest X-ray Artifact Caused by Bilateral 99mTc-Antimony Trisulfite Injection for Sentinel Node Imaging in a Patient With Breast Cancer.

A 52-year-old woman diagnosed with invasive ductal carcinoma of both breasts had a chest x-ray for preoperative assessment. A striking artifact was noted by the x-ray technologist, who, as a result, became very concerned about radiation exposure from the patient. The patient had undergone bilateral sentinel lymph node injections in the nuclear medicine department with Tc-antimony trisulfite colloid just 2 hours before the chest x-ray. Radiation exposure to the x-ray technologist was determined to be similar to 8 hours of naturally occurring background radiation (∼2.96 µSv).

Palliation of Extensive Metastatic Bone Disease With 223Ra-Dichloride α-Particle Therapy in a Patient With Malignant Hereditary Paraganglioma-Pheochromocytoma Syndrome With SDHB Mutation.

A 26-year-old woman with a 5-year history of metastatic paraganglioma due to hereditary paraganglioma-pheochromocytoma syndrome with SDHB mutation, who had failed multiple treatment regimens and had transfusion dependent pancytopenia, presented with progressive liver and bone metastases. She was unable to sleep due to painful skull metastases and had severe weakness in her extremities that limited her mobility and daily activities. She was treated with 2 doses of Ra-dichloride (Xofigo, Ra) and had a dramatic improvement in pain control, mobility, and overall quality of life for 8 weeks, before passing away from pulmonary hemorrhage.

Combined Treatment With 131I-MIBG and Sunitinib Induces Remission in a Patient With Metastatic Paraganglioma Due to Hereditary Paraganglioma-Pheochromocytoma Syndrome From an SDHB Mutation.

A 22-year-old woman with rapidly progressing metastatic paraganglioma due to hereditary paraganglioma-pheochromocytoma syndrome from an SDHB mutation, who recurred after neoadjuvant chemotherapy, was found to be MIBG avid. She was treated with 2 I-MIBG treatments and concurrent sunitinib, achieving a complete response. She was in full remission for 9 months before developing bone metastases.

Ectopic Corticotropin-Producing Neuroendocrine Tumor of the Pancreas Treated With 177Lu DOTATATE Induction and Maintenance Peptide Receptor Radionuclide Therapy.

A 57-year-old woman diagnosed with ectopic Cushing syndrome was found to have a 111In-octreotide-avid corticotropin-producing pancreatic neuroendocrine tumor with liver metastases. She was treated with 4 induction and 4 maintenance cycles of 177Lu-DOTATATE, which normalized her serum corticotropin levels and dramatically reduced the size of the pancreatic primary and liver metastases.

Orbital Metastases of Neuroendocrine Tumors Treated With 177Lu-DOTATATE PRRT or 131I-MIBG Therapies.

Neuroendocrine tumors have a propensity to metastasize to the orbit, although the reason is unknown. A review of 251 neuroendocrine tumors treated with Lu-DOTATATE or I-MIBG at our institution since 2003 revealed 4 patients with orbital metastases (1.6%), 2 treated with Lu-DOTATATE and 2 with I-MIBG. Of these 4 patients, 1 patient was symptomatic with diplopia and eye pain, and 2 patients had physical signs of orbital involvement. The symptomatic orbital metastasis improved with Lu-DOTATATE therapy, and proptosis improved with I-MIBG therapy in 1 of 2 patients. We present the imaging findings of these 4 patients, as well as their management.

Cardiac metastases of neuroendocrine tumors treated with 177Lu DOTATATE peptide receptor radionuclide therapy or 131I-MIBG therapy.

Neuroendocrine tumors have a propensity to metastasize to the heart, although the reason for this remains unknown. A review of 251 neuroendocrine tumor patients treated with Lu DOTATATE peptide receptor radionuclide therapy or I-MIBG therapy at our institution since 2003 revealed 2 patients with cardiac metastases (incidence, 0.8%), one treated with Lu DOTATATE and one with I-MIBG. We present the imaging findings of these 2 patients, as well as their management and responses to therapy.

Resolution of Hyperreninemia, Secondary Hyperaldosteronism, and Hypokalemia With 177Lu-DOTATATE Induction and Maintenance Peptide Receptor Radionuclide Therapy in a Patient With Pancreatic Neuroendocrine Tumor.

A 54-year-old woman presented with a history of nausea, vomiting, diarrhea, and recurrent episodes of severe hypokalemia requiring hospitalization. Imaging revealed a pancreatic mass with liver metastases, histologically confirmed to be a neuroendocrine tumor. Elevated active renin and aldosterone levels were identified, and the patient was treated with 4 induction cycles of Lu-DOTATATE, which resolved the diarrhea, nausea, and hypokalemia, and normalized the renin and aldosterone levels. After 3 additional maintenance Lu-DOTATATE treatments, the pancreatic tumor had decreased in size, was deemed operable, and was resected. She remains on maintenance Lu-DOTATATE therapy with progression-free survival of 45 months thus far.

The Challenges of Treating Paraganglioma Patients with (177)Lu-DOTATATE PRRT: Catecholamine Crises, Tumor Lysis Syndrome and the Need for Modification of Treatment Protocols.

PURPOSE: A high percentage of paragangliomas express somatostatin receptors that can be utilized for targeted radioisotope therapy. The aim of this study was to describe and discuss the challenges of treating these tumors with (177)Lu-[DOTA(0),Tyr(3)]octreotate (DOTATATE) radioisotope therapy using established protocols. METHODS AND RESULTS: Three paraganglioma patients were treated with 4-5 cycles of (177)Lu-DOTATATE and were evaluated for side effects and response to therapy. Two of the three patients developed severe adverse reactions following their first (177)Lu-DOTATATE treatment. One patient developed a catecholamine crisis and tumor lysis syndrome within hours of treatment, requiring intensive care unit (ICU) support, and another developed a catecholamine crisis 3 days after treatment, requiring hospitalization. The treatment protocols at our institution were subsequently modified by increasing the radioisotope infusion time from 15 to 30 min, as recommended in the literature, to 2-4 h and by reducing the administered dose of (177)Lu-DOTATATE. Subsequent (177)Lu-DOTATATE treatments utilizing the modified protocols were well tolerated, and response to therapy was achieved in all three patients, resulting in significantly improved quality of life. CONCLUSION: (177)Lu-DOTATATE is an exciting new therapeutic option in the management of paragangliomas; however, current treatment protocols described in the literature may need to be modified by lengthening the infusion time and/or lowering the initial treatment dose to prevent or reduce the severity of adverse reactions.

Glucagonoma Pancreatic Neuroendocrine Tumor Treated With 177Lu DOTATATE Induction and Maintenance Peptide Receptor Radionuclide Therapy.

A 56-year-old man presented with a history of 2 prior resections of a recurrent pancreatic glucagonoma in the past 4 years. Workup revealed new liver and abdominal nodal metastases with a rising serum glucagon level. He was started on peptide receptor radionuclide therapy with Lu DOTATATE, and his disease stabilized, while his glucagon levels decreased and also stabilized. After 4 induction and 2 maintenance cycles, he remains progression free for 23 months.

In Vivo Evaluation of ¹8F-SiFAlin-Modified TATE: A Potential Challenge for 68Ga-DOTATATE, the Clinical Gold Standard for Somatostatin Receptor Imaging with PET.

UNLABELLED: Radiolabeled peptides for tumor imaging with PET that can be produced with kits are currently in the spotlight of radiopharmacy and nuclear medicine. The diagnosis of neuroendocrine tumors in particular has been a prime example for the usefulness of peptides labeled with a variety of different radionuclides. Among those, (68)Ga and (18)F stand out because of the ease of radionuclide introduction (e.g., (68)Ga isotope) or optimal nuclide properties for PET imaging (slightly favoring the (18)F isotope). The in vivo properties of good manufacturing practice-compliant, newly developed kitlike-producible (18)F-SiFA- and (18)F-SiFAlin- (SiFA = silicon-fluoride acceptor) modified TATE derivatives were compared with the current clinical gold standard (68)Ga-DOTATATE for high-quality imaging of somatostatin receptor-bearing tumors. METHODS: SiFA- and SiFAlin-derivatized somatostatin analogs were synthesized and radiolabeled using cartridge-based dried (18)F and purified via a C18 cartridge (radiochemical yield 49.8% ± 5.9% within 20-25 min) without high-performance liquid chromatography purification. Tracer lipophilicity and stability in human serum were tested in vitro. Competitive receptor binding affinity studies were performed using AR42J cells. The most promising tracers were evaluated in vivo in an AR42J xenograft mouse model by ex vivo biodistribution and in vivo PET/CT imaging studies for evaluation of their pharmacokinetic profiles, and the results were compared with those of the current clinical gold standard (68)Ga-DOTATATE. RESULTS: Synthetically easily accessible (18)F-labeled silicon-fluoride acceptor-modified somatostatin analogs were developed. They exhibited high binding affinities to somatostatin receptor-positive tumor cells (1.88-14.82 nM). The most potent compound demonstrated comparable pharmacokinetics and an even slightly higher absolute tumor accumulation level in ex vivo biodistribution studies as well as higher tumor standardized uptake values in PET/CT imaging than (68)Ga-DOTATATE in vivo. The radioactivity uptake in nontumor tissue was higher than for (68)Ga-DOTATATE. CONCLUSION: The introduction of the novel SiFA building block SiFAlin and of hydrophilic auxiliaries enables a favorable in vivo biodistribution profile of the modified TATE peptides, resulting in high tumor-to-background ratios although lower than those observed with (68)Ga-DOTATATE. As further advantage, the SiFA methodology enables a kitlike labeling procedure for (18)F-labeled peptides advantageous for routine clinical application.

Role for (11)C-choline PET in active surveillance of prostate cancer.

INTRODUCTION: Active surveillance (AS) is an increasingly popular management strategy for men diagnosed with low-risk indolent prostate cancer. Current tests (prostate-specific antigen [PSA], clinical staging, and prostate biopsies) to monitor indolent disease lack accuracy. (11)C-choline positron emission tomography (PET) has excellent detection rates in local and distant recurrence of prostate cancer. We examine (11)C-choline PET for identifying aggressive prostate cancer warranting treatment in the AS setting. METHODS: In total, 24 patients on AS had clinical assessment and PSA testing every 6 months and (11)C-choline PET and prostate biopsies annually. The sensitivity and specificity to identify prostate cancer and progressive disease (PD) were calculated for each (11)C-choline PET scan. RESULTS: In total, 62 biopsy-paired, serial (11)C-choline PET scans were analyzed using a series of standard uptake value-maximum (SUVmax) cut-off thresholds. During follow-up (mean 25.3 months), 11 of the 24 low-risk prostate cancer patients developed PD and received definitive treatment. The prostate cancer detection rate with (11)C-choline PET had moderate sensitivity (72.1%), but low specificity (45.0%). PD prediction from baseline (11)C-choline PET had satisfactory sensitivity (81.8%), but low specificity (38.5%). The addition of clinical parameters to the baseline (11)C-choline PET improved specificity (69.2%), with a slight reduction in sensitivity (72.7%) for PD prediction. CONCLUSIONS: Addition of (11)C-choline PET imaging during AS may help to identify aggressive disease earlier than traditional methods. However, (11)C-choline PET alone has low specificity due to overlap of SUV values with benign pathologies. Triaging low-risk prostate cancer patients into AS versus therapy will require further optimization of PET protocols or consideration of alternative strategies (i.e., magnetic resonance imaging, biomarkers).