Evaluation of Breast Lesions on Cytology Using International Academy of Cytology Yokohama Standardized Reporting System

Background Fine‐needle aspiration cytology (FNAC) remains widely used for the evaluation of palpable breast lesions, particularly in resource‐limited settings, though histopathology is the gold standard. The International Academy of Cytology (IAC) Yokohama System provides a standardized five‐tier reporting framework with defined risk of malignancy (ROM). This study aimed to evaluate its diagnostic performance, reproducibility, and applicability in a tertiary care setting. Materials and Methods A total of 548 breast FNACs from 533 patients were reclassified both prospectively and retrospectively according to the IAC Yokohama categories. Histological correlation was available in 228 cases. Interobserver agreement was assessed among two senior cytopathologists and one junior pathologist using unweighted and weighted kappa statistics. Diagnostic performance was calculated against histology using three definitions of test positivity (Cat‐5 only; Cat‐4 & 5; Cat‐3–5). Results A total of 548 FNACs from 533 patients were reclassified using the IAC Yokohama system: Cat‐1 (36, 6.6%), Cat‐2 (318, 58.0%), Cat‐3 (14, 2.6%), Cat‐4 (25, 4.6%), and Cat‐5 (155, 28.3%). Histological correlation was available in 228 cases. The observed ROM was 50% for Cat‐1, 7.9% for Cat‐2, 45.5% for Cat‐3, 93.8% for Cat‐4, and 100% for Cat‐5. Diagnostic accuracy improved with reclassification: in Group A (Cat‐5 positive), sensitivity increased to 78.4% and specificity was 100%; in Group B (Cat‐4 & 5 positive), sensitivity was 90.9% and specificity was 97.9%; and in Group C (Cat‐3–5 positive), sensitivity reached 94.7% with a specificity of 89.6%. ROC analysis confirmed superior discrimination for the Yokohama system (AUC 0.94 vs. 0.88 for the original). Interobserver reproducibility was excellent, with weighted kappa values of 0.954 (P1 vs. P2), 0.942 (consensus vs. P3), and 0.939 (P2 vs. P3). Agreement was highest in benign and malignant categories and lowest in atypical and suspicious groups. Conclusion The IAC Yokohama System showed high diagnostic accuracy, excellent reproducibility, and reliable risk stratification. By reducing false negatives and minimizing equivocal reporting, it improved alignment with histology compared with the conventional descriptive system, supporting its routine use in breast cytology practice.