Clinical Utility of 18F-FDG PET/CT Concurrent with 131I Therapy in Intermediate–to–High-Risk Patients with Differentiated Thyroid Cancer: Dual-Center Experience with 286 Patients

Patients with differentiated thyroid carcinoma (DTC) are treated with 131 I therapy after total thyroidectomy or surgical resection of recurrent tumor. However, some recurrent DTC lesions are not iodineavid, which affects further treatment planning. The aim of this study was to evaluate the clinical benefit of 18F-FDG PET/CT performed concurrently with 131I therapy in DTC patients with intermediate to high risk. Methods: We retrospectively enrolled 286 DTC patients at 2 Korean medical centers who comprised 2 different patient groups: 28 patients who underwent adjuvant 131I treatment after curative surgical resection of recurrent tumor and 258 patients with intermediate to high risk who underwent 131I ablation after total thyroidectomy. 131I therapy and 18F-FDG PET/CT scanning were performed on the same day. Administration of L-thyroxine was withheld from all enrollees for 4 wk before 131I treatment. Results: In 39 patients (14%), 18F-FDG PET/CT detected additional recurrent or metastatic lesions that were not detected on the posttherapy 131I scan, and the treatment plan was changed for 30 patients (10%) based on such findings. Among the 28 patients receiving 131 I treatment after resection of recurrent tumor, PET/CT detected additional lesions in 46%, and treatment was changed in 43%. Assessing a subgroup of stage T3–T4N1 patients with tumor size . 2.0 cm, among 258 patients undergoing 131 I ablation after total thyroidectomy, we found that 25% had additional positive PET/CT results, and treatment changed for 17%. In contrast, 8% of stage T3–T4N1 patients with tumor size # 2.0 cm, 6% of stage T1–T2N1 patients, and 3% of stage T3–T4N0 patients had additional positive PET/CT findings. Conclusion: 18 F-FDG PET/CT performed concurrently with 131 I therapy detected additional lesions in 14% of DTC patients and was particularly helpful for detecting additional lesions in patients undergoing 131 I therapy after resection of recurrent tumor or in stage T3–T4N1 patients with tumor size . 2.0 cm.