Objective: The Bethesda System for Reporting Thyroid Cytopathology is now widely used as the standard reporting system for fine-needle aspiration cytology (FNAC). Recently, several studies have attempted to subcategorize the atypia of undetermined significance (AUS)/follicular lesion of undetermined significance (FLUS) category. We aimed to analyze the significance of a subcategory of AUS/FLUS showing both cytologic and architectural atypia (AUS/FLUS-C&A). Study Design: From April 2011 to May 2014, 18,091 patients underwent FNAC at Samsung Medical Center. For those patients we analyzed the clinical significance of the subcategory AUS/FLUS-C&A. Results: One hundred and sixty-three patients were diagnosed as AUS/FLUS-C&A. Of 71 cases with subsequent histologic confirmation, 47 (66.2%) were diagnosed with papillary thyroid carcinoma (PTC). Of the 47 PTC cases, 32 (68.1%) were follicular variant-PTC. A significant difference in the PTC rate (58.3 vs. 82.6%) and PTC size (average: 1.8 and 0.9 cm) was noted between circumscribed lesions and infiltrative lesions on ultrasonography. Conclusion: We demonstrated that the subcategory of AUS/FLUS-C&A has considerable clinical implications and one should be aware of the cytological and ultrasonographic features.

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