Pitfall of Cyst Fluid Only

The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) was developed to address terminology and other issues related to thyroid fine-needle aspiration cytology (FNAC) [1, 2]. According to the system, recommended diagnostic categories are classified into six groups: “nondiagnostic or unsatisfactory (ND/UNS),” “benign,” “atypia of undetermined significance or follicular lesion of undetermined significance,” “follicular neoplasm or suspicious for a follicular neoplasm,” “suspicious for malignancy,” and “malignant.” ND/UNS applies to specimens that are unsatisfactory owing to the following: (1) fewer than six groups of well-preserved, well-stained follicular cell groups with ten cells each; (2) poorly prepared, poorly stained, or obscured follicular cells; or (3) cyst fluid, with or without histiocytes, and fewer than six groups of ten benign follicular cells. Cystic papillary thyroid carcinoma cannot be excluded in the last “cyst fluid only (CFO)” scenario, resulting in these cases being classified as ND/UNS [1, 2]. In reporting system of Japanese Society of Thyroid Surgery in 2015, however, CFO is handled differently from TBSRTC.