Objective: To study the cytologic features of phyllodes tumor (PT) of the breast and determine the accuracy of their subclassification in fine needle aspirates.
Study design: Eighty cases of histologically diagnosed PT between 1982 and 1997 with a previous fine needle aspiration (FNA) were evaluated. The FNA smears of each case were reviewed without knowledge of the initial cytologic diagnosis and subclassified into benign, borderline or malignant PT.
Results: Benign PTs were characterized by a dimorphic mixture of stromal and epithelial cells. The stromal fragments showed mild to moderate cellularity with absent to minimal pleomorphism and no mitosis. There were occasional, if any, single stromal cells. Borderline PTs had stromal fragments with moderately cellular stroma exhibiting moderate pleomorphism. Two additional features were the presence of single stromal cells and an occasional mitosis in the stromal fragments/single cells. Aspirates from malignant PT were very cellular, with a high stromal/epithelial ratio and marked stromal cellularity. The stromal cells were highly pleomorphic, with frequent mitosis and atypical single stromal cells in the background. Fifty-seven of the 80 histologically documented cases (71.3%) were diagnosed as PT on FNA (40 benign, 10 borderline and 7 malignant). In 81% (46 of 57 PTs), good cytohistologic correlation (32 benign, 8 borderline and 6 malignant) was observed. In another eight cases, one grade differentiation between cytologic and histologic grade was observed. Six of the nine malignant PTs on histology were correctly subclassified on cytology. There were one false positive and two false negative cases.
Conclusion: Cytologic diagnosis and grading of PT on FNA is possible. Special care should be undertaken in interpreting phyllodes fragments, cellularity of stroma, pleomorphism and mitosis. Single stromal cells are also important morphologic criteria for subclassification. Multiple-site aspiration is advisable to avoid diagnostic errors.