FNA indication according to ACR-TIRADS, EU-TIRADS and K-TIRADS in thyroid incidentalomas at ¹⁸F-FDG PET/CT

Focal thyroid incidentaloma (TI) occurs in a 2% of ¹⁸F-FDG PET/CT and about one-third of TIs is cancer. Due to the lack of evidence on the optimal management of TI, current guidelines suggest performing fine-needle aspiration cytology (FNA). The study aim was to evaluate the reliability of ACR-TIRADS, EU-TIRADS, and K-TIRADS in indicating FNA in TIs.

We retrospectively reviewed ¹⁸F-FDG PET/CT TIs recorded during the period 2016-2019. Enrolled were TIs with histologic outcome and autonomous nodules. Cases with uncertain matching between ¹⁸F-FDG PET/CT, US/scintiscan and histology were excluded.

Eighty TIs at ¹⁸F-FDG PET/CT (median size 17 mm, median SUVmax 7.85) were included; a 26.2% was cancer. The percentage of nodules classified as high risk according to ACR-TIRADS, EU-TIRADS, and K-TIRADS was 20%, 30%, and 29.8%, respectively. The cancer prevalence in high-risk class was 56.2%, 66.7%, and 65.2% in ACR-TIRADS, EU-TIRADS, and K-TIRADS, respectively. ACR-TIRADS had the lowest number of cases with FNA indication (48%) and the K-TIRADS, the highest one (75%). Evaluating the reliability of the three systems in indicating FNA, we found a 100% sensitivity and NPV for EU-TIRADS and K-TIRADS; while all the three systems showed poor specificity and PPV.

All TIRADSs were reliable to stratify the risk of cancer in focal TI. Comparing their reliability in indicating FNA, we found a good performance of EU-TIRADS and K-TIRADS. Considering the high cancer percentage expected in this setting of patients, those TIRADS with higher propensity to indicate FNA should be preferred.