Objective: Phyllodes tumor is a rare fibroepithelial neoplasia of the breast. Tumors are classified according to the histological criteria as benign, borderline, malignant groups. Criteria used to discriminate benign/malignant cases are stromal cytological atypia, stromal hypercellularity, mitotic rate, stromal overgrowth, and tumor margins. We designed this study retrospectively to see the validity of these criteria for our case group and investigate whether we could add any new criteria.

Material and Method: We included 22 cases diagnosed as phyllodes tumor from our archives. Pathology records were used for age, tumor site, macroscopic size. The Hematoxylin-Eosin stained slides were reviewed according to the defined criteria. Additionally leaf like growth pattern, stromal hyalinization, calcification, stromal giant cells, stromal neutrophil leukocyte, and mononuclear lymphocytic infiltrate were recorded qualitatively (present/absent) and numerically (mild, moderate, severe). We examined the differences between benign and malignant groups with the Mann-Whitney U and chi-square tests.

Results: The previously defined criteria were statistically significant but stromal giant cells, stromal hyalinization, calcification, stromal neutrophil and mononuclear lymphocytic infiltrate intensity were not statistically significant in determining the benign-borderlinemalignant cases.

Conclusion: Stromal hypercellularity, moderate/severe stromal cytological atypia, increased mitotic rate, stromal overgrowth and infiltrative tumor margins were observed in malignant phyllodes tumors. Malignant cases were also older and tumor size was bigger than benign ones.