Postoperative fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography: an important imaging modality in patients with aggressive histology of differentiated thyroid cancer.

Background: Aggressive histopathologic subtypes of differentiated thyroid cancer (DTC) are fluorodeoxyglucose (FDG)-avid tumors and are at high risk for persistent/recurrent disease. In these patients, fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) is performed in cases of suspicion of recurrence based on thyroglobulin (Tg) levels or thyroglobulin antibodies (TgAb). The goals of this study were to evaluate the sensitivity of systematic postoperative FDG-PET/CT and to identify risk factors for abnormal FDG-PET/CT.

Methods: Single-center retrospective study of 38 consecutive patients (16 males, 22 females; mean age, 57 years) with aggressive histology DTC, without known persistent disease at the time of postoperative radioactive iodine (RAI) ablation. The most frequent aggressive histologic subtypes were tall cell papillary carcinoma (45%) and poorly differentiated carcinoma (42%).

Results: A total of 86 lesions were found in 20 (53%) patients, distributed in 33 organs. FDG-PET/CT and the postablation whole-body scan (RAI WBS) showed persistent disease in 15 and 12 patients, respectively. FDG-PET/CT was more sensitive than WBS for the detection of individual lesions (69% vs. 59%). Both imaging techniques were complementary with 41% of the lesions detected only by FDG-PET/CT and 31% only by RAI WBS. The only risk factor of abnormal FDG-PET/CT was a stimulated Tg level (Tg/TSH) measured at ablation >10 ng/mL with persistent disease showing FDG uptake in 72% of the patients with a Tg/TSH >10 ng/mL and in 10% of the patients with Tg/TSH ≤10 ng/mL.

Conclusion: Postoperative FDG-PET/CT should be performed routinely in patients with aggressive histology DTC.