Follow-Up Strategies for Thyroid Nodules with Benign Cytology on Ultrasound-Guided Fine Needle Aspiration: Malignancy Rates of Management Guidelines Using Ultrasound Before and After the Era of the Bethesda System.

To evaluate differences in malignancy rates and consequent follow-up strategies for cytologically benign thyroid nodules before and after the introduction of the Bethesda system according to the risk stratification categories of four thyroid management guidelines. This retrospective study was approved by our institutional review board. In this study, 1716 thyroid nodules with initially benign cytologic diagnosis at ultrasound-guided fine needle aspiration (US-FNA) in 1695 patients were included: 1187 nodules from the pre-Bethesda period and 529 nodules from the post-Bethesda period. Based on US features, the thyroid nodules were categorized into the final assessment categories of the 2015 American Thyroid Association (ATA), the 2016 American Association of Clinical Endocrinologists, American College of Endocrinology, and Associazione Medici Endocrinologi, the American College of Radiology Thyroid Imaging Reporting and Data System (ACR-TIRADS), and the European Thyroid Association guidelines for Ultrasound Malignancy Risk Stratification systems. Estimated malignancy rates before and after propensity score matching according to follow-up intervals were obtained. Of the 1716 thyroid nodules benign on initial US-FNA, the malignancy rate was 3.2% (38 of 1187) in the pre-Bethesda period and 2.6% (14 of 529) in the post-Bethesda period ( = 0.641). The 2015 ATA high suspicion pattern and the ACR-TIRADS category 5 had high estimated malignancy rates of >5% in the post-Bethesda period (6.52 and 8.57, respectively). Positive findings that indicated US-FNA in the ACR-TIRADS had estimated malignancy rates of 5.26 and 5.67, respectively, while the remaining guidelines had estimated malignancy rates of <5% in both periods. Immediate diagnostic intervention after benign cytologic diagnosis may not be necessary regardless of the cytologic criteria applied, but it can be considered for the highly suspicious categories in the 2015 ATA or the ACR-TIRADS for benign cytologic diagnosis of the Bethesda system.