Phyllodes tumor (PT) is a rare fibroepithelial tumor of the breast, and these tumors are subdivided into benign, borderline, and malignant tumors. The criteria for their histologic diagnosis have been relatively well-described. However, the cytologic diagnosis presents more difficulties and only a few cytologic studies concerned with their subclassification have been published. The objective of the current study is to describe the cytologic features of benign, borderline and malignant PTs in an attempt to distinguish one from the others. Cytohistologic correlation for 11 benign, 3 borderline and 3 malignant PTs was performed. For all these cases, the preoperative fine needle aspiration (FNA) findings were available for review. The features we examined were a necrotic background, cellularity, stromal tissue fragments, stromal pleomorphism and atypism, dissociated stromal cells and mitosis. The overall diagnostic accuracy of FNA for the PT grading was 88.2% (15/17). Two benign PTs were cytologically misinterpreted as "atypical epithelial and stromal cells" and "highly suspicious for ductal carcinoma". Nevertheless, the cytologic diagnosis and the grading of PTs on FNA were relatively reliable. Semiquantitative analysis for the cellular stromal tissue fragments, stromal pleomorphism and atypism, dissociated stromal cells and mitosis might be helpful for subclassifing PTs on FNA. In the case of encountering a markedly necrotic background, special concern about degenerative change such as infarction is needed.
