Thyroid fine-needle aspiration (FNA) is the gold standard method for the evaluation of thyroid nodules following an initial risk assessment by ultrasound (US).1 The Bethesda System for Reporting Thyroid Cytopathology (BSRTC) was established with the aim to facilitate uniform communication among cytopathologists by reporting 6 standardized diagnostic categories: I— nondiagnostic; II—benign; III—atypia of undetermined significance (AUS); IV—follicular neoplasm (FN); V—suspicious for malignancy (SFM); and VI—malignant.2 Since published, BSRTC has been recognized by the American Thyroid Association (ATA)3 and has been accepted as a consensus paper worldwide,1,2,4,5 by offering a simple and practical teer system. Nevertheless, despite its simplicity and clarity, cytology analysis remains observer-dependent, especially among the indeterminate categories.6

Following its publication, ample studies from different centers worldwide have reported significant variability both in distribution and malignancy rates of the BSRTC, reporting mild-moderate agreement which was shown even among experts.7,8 This was also acknowledged in the 2 BSRTC revisions in 20174 and the recently published 2023 BSRTC.2 Yet, whether the reported BSRTC categories and their malignancy rate may change over time since introduced in a given dedicated workgroup, including US experts and dedicated pathologists has hardly been investigated up to date.9 In former retrospective studies reporting changes in the distribution of BSRTC categories over several years, results were limited to indeterminate cytology only9 or to a comparison of distribution made between the time prior to the BSRTC implantation in 2009 and the time following.10, 11, 12, 13 Proving the consistency of BSRTC through time would further support the reliability of the BSRTC as the gold standard for thyroid cytopathology.

The aim of this study was to examine the consistency of benign, AUS, FN, SFM, and malignant category (BSRTC II-VI) distributions between the years 2010 and 2018 in a high-volume thyroid center in a single institution. A secondary goal was to examine the calculated rate of malignancy (ROM) for each BSRTC category following surgery. To our knowledge, this is the first study to examine the reproducibility of BSRTC distribution over such an extended time period, observing changes prior to and following the revised 2015 ATA guidelines and 2017 BSRTC implantation.