Thyroid incidentalomas detected on ¹⁸F-fluorodeoxyglucose positron emission tomography with computed tomography in cancer patients: imaging clues for further evaluation

Objectives: To evaluate the malignancy risk of thyroid incidentalomas (TI) detected on ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) in patients with cancer and identify factors guiding further evaluation. Materials and methods: Among 71,878 patients who underwent ¹⁸F-FDG PET/CT between Jan 2019 and Dec 2022, 2185 patients (2202 nodules) received ultrasound for focal thyroid uptake. Final diagnoses were made for 233 patients (234 nodules). The maximum standardized uptake value (SUVmax) on PET, CT features, and cytologic or histopathological results were reviewed. Receiver operating characteristic curve analysis determined optimal thresholds for age, nodule size, and SUVmax. Logistic regression identified independent predictors of malignancy. Results: The malignancy rate of PET/CT-detected TI was 50.4% (118/234). Malignant nodules were associated with younger age (p = 0.03), smaller size (p < 0.001), higher SUVmax (p < 0.001), homogeneous density (p = 0.03), and irregular CT margins (p < 0.001). In multivariable analysis, smaller size (< 1.5 cm; odds ratio [OR], 2.65; 95% CI: 1.45–4.85; p = 0.001), irregular CT margin (OR, 3.04; 95% CI: 1.62–5.70; p = 0.001), and higher SUVmax (≥ 6.25; OR, 2.55; 95% CI: 1.44–4.52; p = 0.001) remained independent predictors of malignancy. Conclusion: In our retrospective cohort, the risk of malignancy in TI detected on ¹⁸F-FDG PET/CT increases with smaller nodule size, higher SUVmax, irregular CT margins, and younger patient age. Combining these imaging and clinical parameters improves malignancy prediction. In oncologic patients, selective ultrasound evaluation of such nodules may facilitate diagnosis while avoiding unnecessary procedures. Key Points: Question Which clinical and imaging features predict malignancy in TI detected by 18F-FDG PET/CT in cancer patients? Findings Malignancy was associated with smaller nodule size (< 1.5 cm), higher SUVmax (≥ 6.25), and irregular margins on CT among 234 cases. Clinical relevance Cancer patients with metabolically active thyroid nodules showing irregular CT margins may benefit from selective ultrasound to enable early diagnosis and appropriate management.