Revised Bethesda System for Reporting Thyroid Cytology: Lessons learned from an appraisal of 5 years of experience in a central hospital

OBJECTIVE: The Bethesda System for Reporting Thyroid Cytology (BSFRTC) is widely adopted in the management of thyroid nodules. The system was updated in 2017, and its impact is the subject of this paper. METHODS: All thyroid fine needle aspirations from 2016-2020 using the BSFRTC, with follow-up surgical pathology, were reviewed. The risk of neoplasia (RON), risk of malignancy (ROM), RON/ROM ratio, and surgical follow-up rate were determined for each diagnostic category with cytohistological correlation. ROM was calculated in two separate manners, with non-invasive follicular tumours with papillary-like nuclear features (NIFTP) counted as malignant or non-malignant. Sensitivity, specificity, negative and positive predictive values were determined for indeterminate categories: atypia of undetermined significance (AUS), suspicious for follicular neoplasm (SFN), and suspicious for malignancy (SFM). RESULTS: RON, ROM, and the surgical follow-up rate increased steadily from the benign through intermediate to malignant categories. The omission of NIFTP from malignant lesions reduced the calculated ROM in indeterminate categories and improved the stratification between AUS and SFN. ROM in AUS was distinct from SFN. AUS has a well-balanced sensitivity and specificity favouring a screening rather than a diagnostic category. The calculated RON/ROM was significantly higher in AUS (1.56), compared to SFN (1.03) and SM (1.05), in agreement with current BSRTC management recommendations. CONCLUSIONS: AUS is an important screening category and should remain with the addition of subcategorisation. RON and surgical follow-up rates are essential quality indicators. The RON/ROM ratio could be utilised to determine appropriate management for each diagnostic category on an institutional basis.