Interobserver Reproducibility of the IAC Yokohama System for Breast FNAC: A Multirater Kappa Agreement Study

Abstract Background: The International Academy of Cytology Yokohama System for Reporting Breast Fine Needle Aspiration Cytopathology (IAC YSRB) aims to standardize breast fine needle aspiration cytology (FNAC) reporting by categorizing cytologic findings into five diagnostic groups. IAC YSRB is widely adopted; however, its reproducibility among cytopathologists with varying experience, especially in intermediate categories, remains a concern. Objective: To assess the interobserver reproducibility of the IAC YSRB among four cytopathologists using archived breast FNAC cases. Materials and Methods: This retrospective study included 239 breast FNAC cases reported in 2022 at a tertiary care center. Four cytopathologists with 20, 14, 5, and 3 years of experience independently reviewed smears and assigned each case to one of five IAC YSRB diagnostic categories. Observers were blinded to prior diagnoses and one another’s assessments. Interobserver agreement was analyzed using Fleiss’ kappa ( κ ) and pairwise κ values, with category-wise and overall concordance calculated. Results: Overall interobserver agreement was substantial ( κ = 0.69). The highest agreement was observed for malignant ( κ = 0.95) and benign ( κ = 0.72) categories. Atypical ( κ = 0.22) and suspicious ( κ = 0.26) categories showed fair agreement. Pairwise comparisons revealed higher concordance between more experienced cytopathologists ( κ = 0.80) than less experienced ones. Complete agreement among all four raters was achieved in 81% of cases. Disagreements were mainly concentrated in intermediate categories. Conclusion: IAC YSRB shows good interobserver reproducibility in breast FNAC reporting, particularly in benign and malignant categories. Interpretive variability in intermediate categories highlights the need for training, standardization, and ancillary tools to improve reliability.