Complementary role of ¹⁸F-fluorodeoxyglucose positron emission tomography and ¹³¹I scan in the follow-up of post-therapy differentiated thyroid cancer

Background and Purpose: ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET) is a relatively new modality in the follow-up of patients with differentiated thyroid cancer (DTC) who have undergone total thyroidectomy and postoperative radioiodine therapy. The aim of this study was to assess the diagnostic value of FDG-PET, comparing it with ¹³¹I whole-body scan (WBS) and ²⁰¹T1 WBS. Methods: Fifteen selected patients with local invasive and/or aggressive DTC were included in this study. The follow-up period ranged from 1 to 12 years, with a mean ± standard error of 6 ± 3 years. FDG-PET was performed when patients were still receiving thyroxin therapy. Results: In the cervical region, residual cancer in two patients was demonstrated by FDG-PET, but could not be detected using ¹³¹I WBS or ²⁰¹T1 WBS. Pathology of the surgical specimen showed dedifferentiation of thyroid cancer in one of these patients. Metastatic cervical lymph nodes were detected using FDG-PET in three patients, but in only one patient using ¹³¹I WBS and in another one patient using ²⁰¹T1 WBS. Mediastinal metastases were detected using FDG-PET in three patients, ¹³¹I WBS in two patients, and ²⁰¹T1 WBS in one patient. Diffuse lung metastasis was detected only by ¹³¹I WBS in two patients. The use of FDG-PET in conjunction with computerized tomography provided useful diagnostic information about small nodular lesions of the lung which could not be localized by ¹³¹I WBS or ²⁰¹T1 WBS in three patients. In skeletal metastases, ¹³¹I WBS detected more metastatic lesions than FDG-PET or ²⁰¹T1 WBS, especially when the lesions were located in the pelvis. Conclusions: In the follow-up evaluation of patients with post-therapy DTC, FDG-PET was useful for detecting dedifferentiated lesions and was superior to ¹³¹I WBS in detecting residual cervical or mediastinal lesions and suspected small metastatic foci in the lung. FDG-PET was inferior to ¹³¹I WBS in detecting diffuse lung metastases and distant bone metastases.