Changes in thyroid nodule cytology rates after institutional implementation of the Thyroid Imaging Reporting and Data System

Background: The American College of Radiology Thyroid Imaging Reporting and Data System for ultrasound classification of malignancy risk was developed to better triage thyroid nodules for fine-needle aspiration biopsy. To examine further, we compared thyroid cytologic classification rates in nodules before and after institutional Thyroid Imaging Reporting and Data System implementation. Methods: Cytology diagnoses by Bethesda criteria (categories I–VI) from January 2014 to October 2021 were retrieved; observed changes in yearly category frequency were analyzed by linear regression; and pooled cohorts of pre– (2014–2018) and post–Thyroid Imaging Reporting and Data System (2019–2021) cytology call rates were compared. Results: Overall, 7,413 cytologic specimens were included (range/year 715–1,444). From 2014 to 2021, the proportion of benign (Bethesda category II) diagnosis per year declined stepwise from 49.7% to 19.4%, and atypia of undetermined significance/follicular lesion of undetermined significance (Bethesda category III) increased sequentially from 21.3% to 51.5%. Between 2014 and 2021, Bethesda category III diagnosis increased on average by 4.8% per year (95% confidence internal, 3.29–5.54; P <.001) and Bethesda category II results decreased on average by 4.4% per year (95% confidence interval, 6.29–3.42; P <.001). When comparing pre– and post–Thyroid Imaging Reporting and Data System, the proportion of Bethesda category II cytology results decreased (43.1% vs 21%; P =.001) while Bethesda category III (28.3% vs 47.7%; P =.002) and Bethesda category V (1.1% vs 1.7%; P =.015) results increased. Conclusion: After implementation of American College of Radiology Thyroid Imaging Reporting and Data System ultrasound criteria, we observed a 2.5-fold decline in the rate of benign cytology and an increase in the proportion of atypia of undetermined significance/follicular lesion of undetermined significance results.