Background: Atypical breast cytology is a poorly understood heterogeneous category with limited clinical utility but significant implications for patient management. Objective: To provide an insight into the true nature of atypical breast cytology in screening-detected (asymptomatic) and symptomatic settings, and find strategies for reducing the use of this diagnostic category. Materials and Methods: A total of 6,415 breast cytology samples were processed between January 2004 and December 2008. An atypical cytological diagnosis was rendered in 256 (4%) of the cases. A blind microscopic review of the atypical cases was conducted and results were correlated with subsequent histological and/or clinical outcomes. Results: Follow-up information by histology was available in 85.5%, by repeat fine-needle aspiration (FNA) in 3.5% and by imaging or clinical follow-up in 10.2% of the cases. Two patients (0.8%) were lost to follow-up. Of the 254 cases with follow-up, 62.6% were benign and 37.4% were malignant. The benign to malignant ratios were 1:1 and 2:1 in the screening and symptomatic groups, respectively. The atypical category in the screening population mostly yielded fat necrosis, complex sclerosing lesions and low- to intermediate-grade carcinoma on follow-up. The main outcomes in the symptomatic group were papilloma, fibroadenoma, ductal carcinoma in situ and lobular carcinoma. Preanalytical (suboptimal samples) factors were encountered in 34.8% and interpretative factors in 65.2% of the cases. Uncertainty about cellular morphology was attributed to such a diagnosis in 38 (14.8%) of the cases, architectural complexity in 137 (53.5%) and morphology and architecture in 70 (27.3%); 4.3% of cases were considered nondiagnostic. Conclusion: The atypical category is a necessary diagnosis but of limited use from a patient management perspective. Some preanalytical factors such as poor sample quality can be minimized by the involvement of cytopathologists in the FNA procedure. The use of the atypical category is partly dependent on the experience and confidence of the reporting pathologist. Assigning a case to this category is also likely to be unduly influenced by clinical or radiological findings. Our study indicates that the use of the atypical category can be reduced by up to 40% by appreciating these contributing factors. The practical utilization of the atypical category in breast cytology remains subjective and further study is required to identify useful objective criteria.

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