124I PET/CT in Patients with Differentiated Thyroid Cancer: Clinical and Quantitative Image Analysis

Background: Although radioactive iodine (RAI) imaging/therapy is one of the earliest applications of theranostics, there remain a number of unresolved clinical questions as to the optimization of diagnostic techniques/protocols and improvements in patient-specific treatment planning strategies. The objectives of this study were to determine the imaging characteristics and clinical feasibility of I-124 positron emission tomography/computed tomography (PET/CT) for the determination of extent of disease and evaluation of RAI kinetics in its physiologic and neoplastic distribution in patients with differentiated thyroid cancer (DTC). Methods: The study was designed as a prospective phase II diagnostic trial of patients with confirmed DTC. Following adequate preparation, patients received 2mCi I-124 in liquid form and sequential whole-body PET/CT imaging was performed at five time points (2-4h, 246h, 486h, 72 +/- 6h, and 96 +/- 6h post-administration). All patients who had I-124 imaging subsequently underwent RAI treatment with I-131, with administered activities ranging from 100 to 300mCi. Post-treatment scans were obtained 5-7 days after RAI treatment. A by-patient and by-lesion analysis of the I-124 images was performed and compared with the post-treatment I-131 scans as well as F-18 FDG PET/CT images. Quantitative image analysis was also performed to determine the total functional volume (mL), activity per functional volume (Ci/mL), and cumulated activity (Ci/h) for remnants, salivary glands, and nodal metastases. Results: Fifteen patients (6 women; M-age=57 years; range 29-91 years) were enrolled into the study. Forty-six distinct lesions were identified in these 15 patients on I-124 PET/CT images, with a sensitivity of 92.5%. In addition, I-124 identified 22.5% more foci of RAI-avid lesions compared with the planar I-131 post-treatment scans. This study demonstrates different kinetic profiles for normal thyroid remnants (peaked at 24h with mono-exponential clearance), salivary glands (peaked at 4h with bi-exponential clearance), and metastatic lesions (protracted retention), as well as individual variations in functional volumes and thus cumulated activities. Conclusions: I-124 PET/CT is a valuable clinical imaging tool/agent, both in determining the extent of disease in the setting of metastatic DTC and in the functional volumetric and kinetic evaluation of target lesions.