The prevalance of malignancy and other pathologies in incidental thyroid lesions detected by F-18 FDG PET/BT; a cross-sectional study

Aim: To determine the prevalence and the causative pathologic conditions of F-18 FDG PET thyroid incidentalomas. Material and Methods: The images of the patients referred with a known oncologic disease were evaluated prospectively for the presence of any thyroid uptake. A diagnostic algorithm was performed by an endocrinologist following a through physical examination in patients with incidentalomas. The final diagnosis was made by USG and biochemical tests or FNAB and/or a total thyroidectomy in patients with nodules. A clinical diagnosis was made in patients without any thyroidal nodule or in patients with a thyroidal nodule that is smaller than one centimeter and in whom a FNAB can?t be performed.Results: A thyroid incidentaloma was detected in 178 of the 4204 patients (4.2%). One hundred and sixteen of them were included in the study. Three different uptake patterns were observed; focal uptake in 68 (59%), diffuse uptake in 35 (30%) and diffuse-focal uptake in 13 (11%). We did not diagnose any malignancy in the diffuse uptake group. A malignant thyroid nodule was diagnosed in 29% and 33% of the focal and diffuse-focal uptake groups respectively. The SUVmax values of the nodules were significantly higher in the patients with thyroid carcinoma than the patients with benign nodules (p=0.006). The calculated cut-off value of SUVmax for malignancy was 3.5. The sensitivity and the specificity values for this cut-off value were 73% and 66%, respectively. A malignant nodule was diagnosed with total thyroidectomy in two patients in whom the FNAB revealed a benign disease. Conclusion: A F-18 FDG PET incidentaloma is not a rare finding in oncology patients. A malignant nodule was present in one out of three in patients with focal or diffuse-focal uptake patterns. A SUVmax value of 3.5 can be applied as a cut-off value for the differentiation of benign versus malignant lesion. A benign cytology on FNAB in patients with focal or diffusefocal thyroid incidentalomas may underestimate a thyroid malignancy.Keywords: F-18 FDG PET, thyroid, incidentaloma, malignancy