Answer: Phyllodes tumor, borderline

Histology: This is a fibroepithelial lesion characterized by a variably cellular stroma which in areas indents the epithelium to yield a leaf like or intracanalicular pattern. The extent of the stroma is variable, in that in some areas the epithelium is closely packed together while in some other areas it is widely separated by a stroma. The stroma demonstrates minimal cytologic atypia, but a significant mitotic rate is identified in areas. These are the typical features of a borderline phyllodes tumor.

Discussion: Fibroadenomas usually demonstrate evenly distributed stroma of uniform cellularity. The stroma may be cellular and demonstrate mitotic activity (as is typically seen in juvenile fibroadenomas), but variation in cellularity and extent of stromal proliferation are not typical of fibroadenoma, and favor phyllodes tumor. High grade malignant phyllodes tumors have areas of stromal overgrowth; that is, a 4x low power field contains only stroma which had outstripped the epithelium. These cases usually demonstrate advanced cytologic atypia in addition to mitotic activity, often with atypical forms. Neither the stromal overgrowth nor the marked cytologic atypia typically seen in a malignant phyllodes tumor was present in the current case. Metaplastic carcinoma may occasionally in encircle benign ducts, mimicking a fibroepithelial neoplasm. The absence in of immunoreactivity for cytokeratins, particularly high molecular weight cytokeratin, excludes that diagnosis.

Borderline phyllodes tumors have the potential for aggressive local recurrence if not completely excised. They have minimal if any potential for distant metastases, unless the tumor progresses to a high grade malignant form.