Fibroadenoma mimicking papillary carcinoma on ThinPrep of fine-needle aspiration of the breast

OBJECTIVE: To compare and contrast benign and malignant lesions of the breast that have similar appearances on fine-needle aspiration cytology and that constitute diagnostic pitfalls. DESIGN: The cytology files (dated November 1995 through May 1998) of the Beth Israel Deaconess Medical Center were searched to identify cases of breast fine-needle aspiration biopsies that were highly cellular and composed of bland-appearing spindle/columnar cells and that could represent either epithelial or stromal cells; these cases were reported as indeterminate (atypical/suspicious) and had subsequent excisional biopsies taken. RESULTS: Four such cases were found. Two were fibroadenomas and 2 were papillary carcinomas. Their appearances were strikingly similar on aspiration cytology. All cases were prepared with the ThinPrep method. On microscopic examination, all 4 cases were hypercellular and had many single cells and clusters of columnar/elongate cells. Immunocytochemistry proved these cells to be of epithelial origin. At least occasional bipolar stromal cells were seen in the background. The only appreciable difference between the benign and malignant cases was more significant nuclear atypia, which was barely discernible, in the malignant cases. Immunocytochemistry for smooth muscle actin was helpful in 2 cases that had sufficient material. CONCLUSIONS: Some cases of fibroadenomas and papillary carcinomas can be very difficult, if not impossible, to distinguish on fine-needle aspiration cytology. Immunocytochemistry may be helpful if sufficient material is available. To avoid false-negative or false-positive diagnosis on cytology, it is best to report such cases as atypical or suspicious with final diagnosis pending excisional biopsy.