Clinical significance of standardized uptake values in thyroid incidentaloma discovered by F-18 fluorodeoxyglucose positron emission tomography/computed tomography

Aim of Study: While using F-18 fluorodeoxyglucose (FDG) positron emission tomography computed tomography (PET/CT) for other than thyroid disease, an increased frequency of incidentally discovered areas of focally or diffuse increased uptake within the thyroid gland can be seen. We aimed to find the focal thyroid FDG uptake and compare the maximum standardized uptake value (SUVmax) results with cytology and histology results. Materials and Methods: We examined PET scan reports for all patients undergoing FDG PET/CT investigation over a 10-year period in a single center. Twelve thousand seven hundred and ninety-six patients underwent FDG PET/CT scanning in one PET/CT unit. Within this group, 526 patients had diffuse, focal, or multifocal FDG uptake. About 305 of 526 patients (57.9%) showed diffuse FDG uptake and 221 (42%) showed focal uptake on thyroid gland. Results: The malignant group thyroid nodule sizes were between 8 and 39 mm (21.1 mm average, standard deviation [SD] +/- 7.3) on ultrasonography (USG) examination. These nodules have SUVmax values between 2.3 and 31.2 (average 8.8 SD +/- 5.7). Benign group thyroid nodule sizes were between 5 and 46 mm (average 18.3 mm, SD +/- 5.8) on USG examination. There were no significant correlations between SUVmax of the incidental focal thyroid lesions seen on FDG PET/CT and fine needle aspiration biopsy results. Conclusion: There is a relatively high possibility of a malignant lesion in thyroid incidentaloma. FDG uptake of these lesions is not a useful tool in absolute discrimination between malignancy and benign lesion. The presence of primary or secondary malign lesion is diagnosed in 34.1% of the patients who are found to have incidental focal FDG uptake within thyroid gland in PET/CT scans, but we suggest that the thyroid incidentalomas detected on FDG PET/CT should be further examined with USG and scintigraphy.