The local efficacy of I-131 for F-18 FDG PET positive lesions in patients with recurrent or metastatic thyroid carcinomas

Purpose: The aim of this study was to evaluate the local efficacy of I-131 for F-18 fluorodeoxyglucose positron emission tomography (FDG PET)-positive lesions. Methods: Whole-body FDG PET/CT was performed on 37 patients (55 cases: 16 men, 21 women; age range: 24-82 years; mean age ± standard deviation: 60.5 ± 16.0 years) with differentiated thyroid cancer after total thyroidectomy. The metastatic or recurrent lesions were divided into 5 categories: primary tumor bed, lymph node, lung, bone, and other. The well-defined lesions were measured on CT, and the sizes were compared before and after radioactive iodine therapy. Results: The analysis was performed on 37 patients with 44 lesions (lymph node:24, lung:16, bone:4). Sixteen lesions (70%) were increased and 7 (30%) showed no change or reduction when there was positive accumulation on FDG PET/CT and negative accumulation on I-131 (F(+)I(-)) group. In the positive accumulation for both FDG PET/CT and I-131 (F(+)I(+)) group, 5 lesions (63%) were increased and 3 (37%) showed no change or reduction. There was no significant difference for the tendency to increase in size between the F(+)I(-) and the F(+)I(+) groups. Conclusions: Lesions which show positive accumulations on FDG PET/CT have a greater tendency to increase in size. FDG-avid lesions are resistant to radioactive iodine therapy with or without I-131 uptake.