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Objective:
The increasing usage of PET/CT for detection and monitoring of malignancy has led to an increase in incidental detection of thyroid nodules. Nodules which demonstrate increased avidity for 2-[18F]-fluoro-2-deoxy-D-glucose (FDG) have been shown to carry a high incidence of malignancy and warrant further investigation. Thyroid ultrasound is the preferred modality for the assessment of thyroid nodules, and the American College of Radiology Thyroid Imaging Reporting and Data System (ACR TIRADS) is a classification system which risk stratifies thyroid nodules based on their sonographic appearance. However, at present there has been limited research in the risk stratification of nodules which are FDG avid, thus this study aims to evaluate the efficacy of the ACR TIRADS classification in the risk stratification of such nodules.

Materials and Methods:
Data was collected retrospectively for FDG-avid thyroid nodules which were identified on PET/CT between January 2012 and July 2023. All nodules underwent sonographic assessment and were characterized using the TIRADS classification, and subsequently underwent ultrasound guided fine needle aspirate (FNA) biopsy. Nodules were divided into low risk, which are nodules that do not meet the TIRADS criteria for FNA, and high risk, for which FNA is recommended. Ultrasound features that comprise the TIRADS criteria were also individually assessed. Cytological findings were classified using the Bethesda reporting system. Nondiagnostic samples (Bethesda class I) were excluded. Remaining samples were divided into two groups; benign (Bethesda class II) or suspicious for malignancy/malignant (Bethesda class III or above).

Results:
There were 44 nodules in the low- and high risk-groups, respectively. Thirty-six percent of low-risk nodules and 45% of high-risk nodules were malignant, respectively (<em>p</em> = 0.516). The sensitivity and specificity of TIRADS for detection of malignant nodules was 56% and 54%, respectively. There were no malignant TIRADS 1 or 2 nodules. The presence of any suspicious sonographic feature was 100% sensitive with a 1.0 NPV. The sonographic feature of solid composition was the most sensitive predictor (86%) of malignancy, whereas nodules which were taller than wide, had low echogenicity or irregular margins all demonstrated high specificity of 92%, 90%, and 96%, respectively.

Conclusion:
FDG-avid nodules which are classified as TIRADS 1 or 2 or have no suspicious ultrasound features have a 0% incidence of malignancy, and thus may not require further assessment with FNA when detected incidentally. FDG-avid nodules which are TIRADS 3 or above should undergo FNA regardless of size due to the high risk of malignancy and poor sensitivity of the TIRADS classification system.