Answer: Phyllodes tumor

Histology: The lesion has a prominent leafy architecture, and cellularity which is accentuated underneath the epithelium, support the diagnosis of phyllodes tumor. In the stroma of the phyllodes tumor are clusters of epitheloid cells which simulate the morphology of an epithelial neoplasm such as invasive lobular carcinoma. These cells are, however, non-immunoreactive for cytokeratin but do label for CD34. Hence, these cells are best characterized as epithelioid myofibroblasts, which may be seen in a variety of mammary lesions such as epithelioid myofibroblastoma or fibroepithelial lesions such as this. This lesion was incompletely excised, and hence re-excision was performed a few months later. The excision demonstrated stromal overgrowth by sheets of malignant stromal cells, support the diagnosis of a malignant phyllodes tumor.

Discussion: Invasive lobular carcinoma involving a phyllodes tumor should label for cytokeratins and not for CD34. Stromal giant cells may be seen in fibroadenoma; however, these cells are usually not numerous, and a fibroadenoma lacks the architectural features of the current case (specifically the prominent intracanalicular patterns and subepithelial stromal cellularity).