Fine-needle aspiration in all thyroid incidentalomas at 18 F-FDG PET/CT: can EU-TIRADS revise the dogma?

OBJECTIVE: Focal thyroid incidentalomas (TIs) are observed in 2% of 18 F-FDG PET/CT representing malignancy in one-third of cases. Currently, due to the lack of evidence on their optimal management, guidelines suggest fine-needle aspiration cytology (FNAC). The study aim was to evaluate the role of ultrasound evaluation according to EU-TIRADS to assess the risk of TIs and inform FNAC prescriptions. DESIGN: We retrospectively reviewed 18 F-FDG PET/CT TIs recorded during the period 2014-2017. Enrolled were TIs with histological outcome and autonomous nodules. Cases with uncertain matching between 18 F-FDG PET/CT, ultrasound and histology were excluded. RESULTS: According to the selection criteria, 75 TIs, being 13 (17.3%) malignant and 62 (82.7%) benign, were included. Cancers had significantly higher SUVmax and SUVmax ratio (Mann-Whitney P < 0.01) than benign, and the most accurate cut-offs were >7.1 and >3.65, respectively. At ultrasound, the cancer rate was 0% in EU-TIRADS 2, 2.9% in EU-TIRADS 3, 4.2% in EU-TIRADS 4% and 78.6% in EU-TIRADS 5 (chi-squared P < 0.001). Sensitivity, specificity, PPV, NPV and accuracy for malignancy were 92%, 64%, 35%, 98% and 69% for SUVmax; 85%, 68%, 36%, 96% and 71% for SUVmax ratio; and 85%, 95%, 79%, 97% and 93% for EU-TIRADS, respectively. The absence of all these three features reached a specificity of 97.1%. CONCLUSIONS: EU-TIRADS, within a clinical careful approach, can discriminate with significant accuracy lesions at high risk of malignancy from those at low risk among TIs at 18 F-FDG PET/CT. Additionally, a centre-based threshold for SUV parameters should be useful for the initial assessment of these lesions during PET/CT reading and reporting.