Thyroid Incidentalomas on (18)F-FDG PET/CT: Clinical Significance and Controversies

OBJECTIVE: The purpose of the current study is to examine the incidence and clinical significance of unexpected focal uptake of (18)F-fluorodeoxyglucose ((18)F-FDG) on positron emission tomography/computed tomography (PET/CT) in the thyroid gland of oncology patients, the maximum standardized uptake value (SUV(max)) of benign and malignant thyroid incidentalomas in these patients, and review the literature. METHODS: Seven thousand two hundred fifty-two (18)F-FDG PET/CT studies performed over four years, were retrospectively reviewed. Studies with incidental focal (18)F-FDG uptake in the thyroid gland were further analyzed. RESULTS: Incidental focal thyroid (18)F-FDG uptake was identified in 157 of 7252 patients (2.2%). Sufficient follow-up data (≥12 months) were available in 128 patients, of whom 57 (45%) had a biopsy performed and 71 had clinical follow-up. Malignancy was diagnosed in 14 of 128 patients (10.9%). There was a statistically significant difference between the median SUV(max) of benign thyroid incidentalomas (SUV(max) 4.8) vs malignant (SUV(max) 6.3), but the wide range of overlap between the two groups yielded no clinically useful SUV(max) threshold value to determine malignancy. CONCLUSION: (18)F-FDG positive focal thyroid incidentalomas occurred in 2.2% of oncologic PET/CT scans, and were malignant in 10.9% of 128 patients. This is the lowest reported malignancy rate in a North American study to date, and significantly lower than the average malignancy rate (35%) reported in the literature. Invasive biopsy of all (18)F-FDG positive thyroid incidentalomas, as recommended by some studies, is unwarranted and further research to determine optimal management is needed. There was no clinically useful SUV(max) cut-off value to determine malignancy and PET/CT may not be a useful imaging modality to follow these patients conservatively.