The B ethesda system for reporting thyroid cytopathology: an institutional experience of the outcome of indeterminate categories

OBJECTIVE: In this study, we investigated the efficacy of the Bethesda system (TBS) for reporting thyroid cytopathology in determining the risk of malignancy for indeterminate cases. These cases comprised atypia or follicular lesion of undetermined significance (AUS/FLUS), follicular neoplasia or suspicious for follicular neoplasia (FN/SFN) and suspicious for malignancy (SM) categories. AUS/FLUS cases were further subcategorized according to their patterns, and the malignancy rate for each subcategory was calculated. METHODS: A total of 6310 thyroid fine needle aspiration (FNA) cases were analysed and 655 indeterminate cases were enrolled in the study. AUS/FLUS cases were further subcategorized into four patterns as follows: papillary thyroid carcinoma (PTC) pattern, microfollicle pattern, atypical cell pattern and Hürthle cell pattern. RESULTS: Malignancy rates for AUS/FLUS, FN/SFN and SM were 18.9%, 45.7% and 71%, respectively. Malignancy rates for AUS/FLUS subcategories were as follows: PTC pattern, 28%; microfollicle pattern, 6.9%; atypical cell pattern, 22.2%; and Hürthle cell pattern, 0%. CONCLUSION: The Bethesda system categorizes thyroid FNAs with increasing risk of malignancy as advocated. However, our findings suggest that the risk of malignancy for AUS/FLUS might be higher than estimated. Particularly, AUS/FLUS cases with focal features suggestive of PTC seem to be associated with a much higher risk of malignancy than other patterns.