Answer: Benign phyllodes tumor

Histology: This is a fibroepithelial neoplasm composed of a prominent stroma with incorporated benign epithelial ducts. The stroma is somewhat prominent in this case, as it pushed aside native ducts, and shows occasional mitotic activity along with increased cellularity. The stroma focally indents ducts to yield the intracanallicular pattern of growth. The borders of this tumor are ill-defined, as fibroblastic stromal tissue penetrates into the surrounding fat of the benign breast.

Discussion: This case falls upon the borderline of a fibroadenoma versus phyllodes tumor. In general, fibroadenomas are more circumscribed, and have uniform cellularity lacking stromal mitoses. They may grow in an intracanalicular pattern, though this pattern does not typically result in the leaf-like clefts which are characteristic of phyllodes tumor. Juvenile fibroadenomas can be cellular and mitotically active like phyllodes tumors. However, juvenile fibroadenomas typically occur in adolescent patients, and typically demonstrate a pericanalicular growth pattern with associated florid usual duct hyperplasia. It is the presence of increased stromal cellularity (particularly adjacent to ducts) and the elongated intracanallicular pattern (yielding leaf-like projections into cystic spaces) which distinguish a phyllodes tumor from a fibroadenoma. Most experts divide phyllodes tumors into those that are locally aggressive, non-metastasizing lesions, and those which are associated with a significant risk of metastasis. The former is sometimes separated into benign phyllodes tumor and borderline/low grade malignant phyllodes tumor. The latter, high grade malignant phyllodes tumors, are characterized by a high stromal mitotic rate (>5 per 10 high power field) and the presence of stromal overgrowth, recognized by the lack of duct epithelium within a low power (40X) microscopic field. High grade malignant phyllodes tumors typically also show prominent cytologic atypia. The current lesion lacks these aggressive features and hence falls within the spectrum of a benign phyllodes tumor. Phyllodes tumors should be completely excised to avoid the risk of local recurrence, which may relate to their infiltrative borders.