Frequency of malignancy in thyroid incidentalomas detected by whole body ¹⁸F-FDG PET/CT

Background: Thyroid incidentalomas detected by ¹⁸F-Fludeoxyglucose (FDG) PET/CT are defined as an incidental and unexpected thyroid focus present in a patient studied for a non-thyroid disease. Aim: To assess the frequency of malignancy of nodular thyroid incidentalomas, and their association with ¹⁸F-FDG avidity (standard uptake value (SUV) max). Material and Methods: Whole body PET/CT performed from December 2008 to December 2017 were reviewed selecting those that showed nodular thyroid foci. Glands with diffuse increased uptake were excluded. Thyroid ultrasonography and fine-needle aspiration cytology (FNAC) performed after PET/CT were reviewed. Bethesda score and SUVmax were correlated. Results: Of 5,100 whole body ¹⁸F-FDG PET/CT, 119 showed a thyroid nodular uptake (2.3%). Forty eight percent of these patients were studied with FNAC or surgery, 50% of which (29/58) were confirmed as malignant. Benign nodules showed significantly lower ¹⁸F-FDG uptake (n = 20, SUVmax: 3.5 ± 1.7) than Bethesda V-VI (n=24, SUVmax: 8.2 ± 5.2) and thyroid metastases (n=5, SUVmax: 6.3 ± 2.1). The best cut-off value to distinguish between benign and malignant nodules was a SUVmax of 5.0, with a sensitivity of 76% (95% confidence intervals (CI) 56-90%), a specificity of 85% (95% CI 62 - 97%), and positive likelihood ratio of 5 (95% CI 1.8 - 14.6). The size of the thyroid nodule was not predictive of malignancy. Conclusions: Half of nodular thyroid incidentalomas detected by ¹⁸F-FDG PETC/CT are malignant. A nodule with a SUVmax ≥ 5,0 is highly suggestive of malignancy, regardless of its size