Objective: In nearly 20%-30% of patients with differentiated thyroid carcinoma (DTC) relapse and 7% of them die during the next 10 years after initial diagnosis. In 10%-30% of patients with DTC after ablation therapy during the follow-up show a negative iodine-131 (131I) whole-body screening test (131I WBS) and increased serum thyroglobulin (Tg) level. Loss of ability of DTC metastatic lesions to trap 131I is associated with pure survival and often aggressive disease. Several studies have shown that in DTC cases non trapping 131I, fluorine-18-fluorodeoxyglucose positron emission tomography (18F-FDG PET) can detect recurrence or metastases with high sensitivity (80%-90%). The purpose of this study was to investigate the clinicopathologic features and other related risk factors of patients with DTC having elevated Tg levels and negative 131I WBS in which recurrence was detected by 18F-FDG PET/CT. We tried to study and stratify patients in this grey zone who could benefit from 18F-FDG PET/CT for the detection of metastases/recurrence according to predefined risk factors not investigated by other researchers.
Subjects and methods: We studied retrospectively 165 DTC patients with elevated Tg levels and a negative 131I WBS during their follow-up between 2004-2015. Metastases/recurrence was found in 49% of the patients on restaging with 18F-FDG PET/CT and were compared with nonmetastatic group according to predefined risk factors. These factors were also evaluated in true positive and false negative cases.
Results: The sensitivity and specificity of 18F-FDG PET/CT for detecting recurrent/metastatic disease were 90% and 98.5%, respectively. No apparent predefined risk factor impacting a false negative 18F-FDG PET/CT was found. Findings in follicular carcinoma, Hürtle cell carcinoma and papillary carcinoma were not different from positive PET findings. The variants of papillary carcinoma also had no statistically difference with regard to 18F-FDG results.
Conclusion: The most important factors affecting a true positive 18F-FDG PET/CT study were: ETE, high total 131I dose and the SUVmax values over 4.5.