The Yokohama System for Breast Cytology in Japan: A Multicenter Comparison of the International Academy of Cytology Yokohama System and the Conventional Japanese System for Reporting Breast Fine‐Needle Aspiration Biopsy

Background The International Academy of Cytology (IAC) recently introduced the Yokohama System (IAC‐YS) for reporting breast fine‐needle aspiration biopsy (FNAB) cytology. The Working Group on the Accuracy of Breast Fine‐Needle Aspiration Cytology of the Japanese Society of Clinical Cytology aimed to categorize breast FNAB specimens according to the IAC‐YS, assess the risk of malignancy (ROM) for each category, and evaluate the diagnostic yield of breast FNAB in Japan. Methods Breast FNAB specimens obtained between January 2010 and December 2019 from nine cooperating facilities were retrospectively reclassified following the IAC‐YS. The ROM for each category was calculated and compared with that using the conventional Japanese diagnostic system (CJS). Diagnostic yield was evaluated using a five‐category approach to distinguish benign from malignant samples. Results In total, 1727 samples from 1722 patients were distributed as follows: Category 1 (insufficient material), 21.3%; Category 2 (benign), 38.9%; Category 3 (atypical), 6.9%; Category 4 (suspicious for malignancy), 10.0%; and Category 5 (malignant), 23.0%. Histological follow‐up was available for 856 cases (49.5%). The ROM was 13.1%, 4.8%, 26.9%, 84.3%, and 94.5% for Categories 1–5, respectively. Conclusions IAC‐YS showed a higher proportion of cases of Category 1 and a lower proportion of cases of Category 3 than the CJS. Despite these differences, both systems were equivalent in their impact on daily medical care, although ROM differed for the content of Categories 1 and 3. Quality assurance parameters for our data based on IAC‐YS are comparable to those of CJS.