Objectives
To review the performance and utility of the International Academy of Cytology (IAC) Yokohama System for Reporting Breast Fine Needle Aspiration Biopsy (FNAB) Cytology five category stratification and evaluate the impact of rapid onsite evaluation (ROSE).

Method
A retrospective analysis of breast FNAB cytology cases with matched histopathological results at a single institution over a 32 months period using a structured reporting system with 5 diagnostic categories (“inadequate/insufficient,” “benign,” “atypical,” “suspicious of malignancy” and “malignant”) closely paralleling the proposed IAC System.

Results
Of 2,696 breast FNAB cases, there were 579 with matched histopathology and 456 of these had ROSE. ROSE decreased the number in the “insufficient” category (17.1% without ROSE to 4.0% with ROSE) and increased the number in the “malignant” (17.9 to 39.0%) with a lesser impact on the “atypical,” “benign” and “suspicious of malignancy” categories. The performance data showed a positive predictive value of 96.4%, negative predictive value of 97.6%, and a risk of malignancy of a FNAB categorized as “insufficient” to be 2.6%, “benign” 1.7%, “atypical” 15.7%, “suspicious of malignancy” 84.6%, and “malignant” 99.5%.

Conclusion
Breast FNAB is an accurate test enabling effective diagnosis of breast lesions. ROSE improved the performance by decreasing the proportion of “insufficient” and “atypical” and increasing the “suspicious of malignancy” and “malignant” diagnoses and enabling immediate triage for further biopsy where necessary.