Adjunctive role of focal FDG Uptake on PET/CT in assessing thyroid malignancy beyond sonography

Introduction Focal fluorodeoxyglucose (FDG) uptake on positron emission tomography/computed tomography (PET/CT) has been associated with thyroid malignancy. However, its incremental value beyond ultrasonographic risk stratification remains unclear. We evaluated whether focal FDG uptake and the maximum standardized uptake value (SUVmax) improve the prediction of malignancy when incorporated into ultrasonographic risk stratification systems. Methods This retrospective cohort study included 464 thyroid nodules from 454 individuals who underwent ultrasonography, biopsy, and PET/CT between 2017 and 2022 at our tertiary referral hospital. PET/CT had been performed for cancer work-up in 96.6% of nodules. Nodules were classified according to the American College of Radiology Thyroid Imaging Reporting and Data System (ACR-TIRADS) and the Korean Thyroid Imaging Reporting and Data System (K-TIRADS). Diagnostic performance was assessed using receiver operating characteristic analysis and the area under the curve (AUC). Incremental value was evaluated using the net reclassification improvement (NRI) and decision curve analysis. Results Among 464 nodules, 221 (47.6%) were malignant. The malignancy rate was higher in nodules with focal FDG uptake than in those without (52.3% vs. 24.4%). Adding focal FDG uptake increased the AUC for both ACR-TIRADS and K-TIRADS; however, the overall NRI was not improved. The absence of focal FDG uptake modestly improved the reclassification of benign nodules (NRI_nonevent = 0.070 for ACR-TIRADS and 0.045 for K-TIRADS). SUVmax was positively associated with malignancy risk without a clear diagnostic cutoff. Incorporating SUVmax improved the AUC and the overall NRI (0.093 for ACR-TIRADS and 0.077 for K-TIRADS), mainly through reclassification of benign nodules (NRI_nonevent = 0.271 and 0.129), whereas reclassification of malignant nodules worsened (NRI_event = −0.178 and −0.052). Conclusions Focal FDG uptake and SUVmax on PET/CT provided only modest incremental value beyond ultrasonographic risk stratification, largely confined to the reclassification of benign nodules. Nevertheless, the absence of focal FDG uptake did not reliably exclude malignancy in nodules with intermediate- or high-risk sonographic features, given the high malignancy rate observed in these FDG-nonavid nodules (41.4%–77.8%).