Answer: Borderline/low grade malignant phyllodes tumor

Histology:
Histologic Description: This is a multinodular fibroepithelial neoplasm which has an infiltrative border as it protrudes into surrounding breast tissue. The stroma of the lesion is variably cellular, and the cellularity tends to be accentuated underneath the epithelium where mitoses are more evident. The lesion has a prominent intracanalicular pattern of growth, and protrusion of stroma projecting to cystic spaces gives it a leaf-like architecture. These are the typical features of a phyllodes tumor. The presence of highly cellular but non pleomorphic stroma and the absence of stromal overgrowth argue for a diagnosis of a borderline/low grade malignant phyllodes tumor.

Discussion: Fibroadenoma would lack the high cellularity and infiltrative border of the current lesion. The lesion has more cellularity and mitotic activity than should be seen in a benign phyllodes tumor, which is usually cytologically difficult to distinguish from a fibroadenoma. An overtly malignant phyllodes tumor is characterized by stromal atypia and stromal overgrowth (pure stroma in a 4X lower power field), which is not seen in the current case.

The current lesion has a significant change of local recurrence if not completely excised.