18F-Fluorodeoxyglucose uptake in thyroid from positron emission tomogram (PET) for evaluation in cancer patients:: High prevalence of malignancy

Objectives. To investigate the prevalence of incidental thyroid F-18-fluorodeoxyglucose (FDG) uptake in positron emission tomogram (PET) scan for evaluation in cancer patients and the role of standard uptake value (SUV) measurement in differentiation of thyroid malignancy from benign disease. Study Design. Retrospective single-center study. Methods. Four thousand one hundred thirty-six subjects had been given FDG-PET scan for evaluation of known malignancies not associated with thyroid. The maximum SUVs of the thyroid lesions were recorded and reviewed. Fine needle aspiration was performed in patients with definite nodule by palpation or ultrasonography. Results: Ninety-four (2.2%) were identified to have focal (n = 45, 1.1%) or diffuse (n = 45, 1.1%) thyroid FDG uptake. The incidence of focal or diffuse thyroid FDG uptake was higher in women (2.7% or 1.9%) than in men (0.4% or 0.7%). Cytologic diagnosis was available in 32 of 45 focal thyroid FDG uptakes. In 16 (50%) patients, the tumor was found to be malignant; 14 were papillary thyroid carcinoma (surgically confirmed in 7 cases), 2 were metastatic tumor from breast and esophagus. Sixteen were cytologically diagnosed as follicular cell lesions: follicular neoplasm (n = 2), nodular hyperplasia (n = 7), indeterminate follicular lesion (n = 7). There was no significant difference in maximum SUV between benign and malignant nodules. From 45 patients with diffuse thyroid FDG uptake, presumptive diagnosis of chronic thyroiditis was possible in 34 patients by clinical and laboratory findings. Conclusion: Our data suggest that a cytologic diagnosis of focal thyroid FDG-PET incidentaloma regardless of SUV is mandatory considering the very high prevalence of thyroid malignancy.