Background: The new Bethesda system for reporting thyroid cytopathology (TBSRTC) was a major step towards standardization, reproducibility and clarity of communication in thyroid fine needle aspiration cytology (FNAC). This system is used worldwide, including in Thailand, and the Department of Pathology at Rajavithi Hospital has been using this system for three years.
Objective: To evaluate the Bethesda system for reporting thyroid cytopathology at Rajavithi Hospital and assess the malignancy
risk and diagnostic accuracy of thyroid FNAC.
Material and Method: The author retrospectively reanalyzed the thyroid FNAC reports for three years (January 2013 to December 2015). The reports were reclassified into each Bethesda system classes, and the malignancy risks for each category were calculated and compared with the follow-up histopathology reports. The statistical parameters of diagnostic accuracy for FNAC were also analyzed.
Results: A final total of 13,371 reports were included as follows: non-diagnostic/unsatisfactory (ND/UNS) 32.2%; benign 46.9%; atypical follicular lesion of undetermined significance (AFLUS) 4.8%; follicular neoplasm or suspicious for follicular neoplasm (FN/SFN) 7.8%; suspicious for malignancy (SM) 4.4%; and malignant 3.9%. The rates of malignancy reported on 358 cases with follow-up histopathology were ND/UNS, 22.1%; benign, 2.8%; AFLUS, 46.2%; FN/SFN, 37.3%; SM 74.3%; and malignant 90.0%. The sensitivity, specificity, positive predictive value, negative predictive value and accuracy of FNAC were 74.7%, 90.3%, 81.5%, 86.2% and 84.6%, respectively.
Conclusion: The thyroid FNAC interpretation, using the Bethesda system in routine service allowed a more specific cytological diagnosis. The distribution of cases using the Bethesda categories differed from previous studies, but the malignancy rate of ND/UNS and AFLUS case were higher, the overall accuracy rate was similar with high yield. The universal Bethesda classification is a useful application for general practice in institutes without specialist pathologists and may decrease interlaboratory disagreements and variability.

Keywords: Thyroid FNAC, Bethesda system, Cytology, Fine-needle aspiration