Answer: Low grade malignant phyllodes tumor

Histology: This lesion is a fibroepithelial lesion with a benign epithelial component indented by proliferating stroma. The stroma has mitotic activity and is cellular, and resembles a low grade fibroblastic sarcoma. The lesion has an infiltrative border with secondary nodules at its periphery. Stromal overgrowth and significant cytologic atypia are lacking. Hence, this lesion falls into the low grade malignant/borderline category of phyllodes tumors.

Discussion: Fibroadenoma may have a prominent intracanicular pattern but would lack the high cellularity of a phyllodes tumor. The distinction between a fibroadenoma, cellular type and a benign phyllodes tumor is often difficult; many recommend close follow-up for patients with borderline lesions. High grade malignant phyllodes tumors demonstrate stromal overgrowth (defined by absence of epithelium within a 4X objective low power field), significant cytologic atypia and mitotic activity. These lesions have a significant (approximately 25%) chance of metastasis. Low grade malignant phyllodes tumors are typically locally aggressive neoplasms that require wide excision. Hematogenous metastasis is extremely rare in well-sampled and characterized low grade malignant lesions.