Deciding on Fine Needle Aspiration Biopsy in Thyroid Incidentalomas in FDG-PET/CT: Should Ultrasonographic Evaluation or FDG Uptake Be in the Foreground?

Objectives: 18-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography (FDG-PET/CT) is a widespread imaging technique for whole-body scanning. Incidental lesions may be detected in thyroid gland and the importance and management of these lesions are still a matter of debate. The aims of this study were the evaluation of the diagnostic success of ultrasonography and FDG-PET/CT for predicting malignancy and contribution of these techniques for the decision of Fine Needle Aspiration Biopsy (FNAB) in incidental thyroid lesions detected in FDG-PET/CT. Methods: Patients who underwent FDG-PET/CT in Nuclear Medicine Unit in a single institution between January 2018 and December 2022 were screened for thyroid incidentaloma with increased focal FDG uptake. Imaging studies and pathology results of the patients with Focal Thyroid Incidentalomas (FTI) were reviewed retrospectively. Results: A total of 14.003 FDG-PET/CT reports of 8.259 patients were evaluated. In FDG-PET/CT imaging, 495 (6.0%) patients had increased uptake in thyroid gland, 383 (4.6%) patients had focal and 112 (1.4%) patients had diffuse FDG uptake. The rate of malignancy in FTIs was 19.2%. In the ROC curve analysis, regarding the prediction of malignancy in FTIs with FDG uptake, a SUVmax value of 5.5 and above predicts malignancy with a sensitivity of 71.4% and a specificity of 68.6% (AUC: 0.718, p=0.018, 95%CI: 0.564-0.872). The sensitivity of ACR-TIRADS-5 was 35.7% (95%CI: 14.6-61.7) and sensitivity of the combination of SUVmax>5.5 and ACR-TIRADS-5 was 30.0% (95%CI: 8.5-60.7). Conclusion: FDG-PET/CT findings can be used for determining malignancy risk and cutoff values such as 5.5 can be threshold for ordering FNAB. In patients with SUVmax less than 5.5, ultrasonographic risk classification criteria should be used for decision-making.