Differences in Clinical Features Between Subcategories of "Atypia/Follicular Lesion of Undetermined Significance".

Within the Bethesda System for Reporting Thyroid Cytopathology (TBSRTC), category III (atypia or follicular lesion of undetermined significance (AUS/FLUS)) comprises specimens with heterogeneous features and the need for subcategorization has been reported. We compared the clinical features of two subgroups from within the category of AUS/FLUS to determine precise guidelines for clinicians who explain the results of fine-needle aspiration (FNA) to patients. Retrospective review was performed including data from all patients who underwent FNA with results reported as AUS/FLUS at a single tertiary hospital from January 2010 to August 2014. The results of repeated FNA, core needle biopsy, and diagnostic surgery were analyzed. Of the 903 patients with results categorized as AUS/FLUS, 500 were subcategorized as cellular atypia and 160 as architectural atypia. On repeated biopsy, diagnostic results were obtained for 78.9% patients of the cellular atypia group, compared to only 54.3% of the architectural atypia group (p < 0.0001). The rate of neoplasm or malignancy was also significantly higher in the cellular atypia group compared to the architectural atypia group (51.8 vs. 25.2%, p < 0.0001, 48.2 vs. 14.2%, p < 0.0001). Therefore, clinicians can inform patients with architectural atypia that the risk of malignancy is low. However, the high rate of non-diagnostic results on repeated biopsy makes clinical decisions difficult. The data of the present study revealed the necessity of subcategorization of category III of TBSRTC in the future.