Atypical epithelioid cell myofibroblastoma of the breast with multinodular growth pattern: a potential pitfall of malignancy.

Myofibroblastoma (MFB) of the breast is the prototypical benign spindle cell tumor arising from the mammary stroma. Over the last two decades, several morphological variants of this tumor have been recognized. Epithelioid cell MFB is composed predominantly of neoplastic elements with epithelioid morphology. It represents a potential diagnostic pitfall of malignancy, especially when evaluating small biopsies. We report a unique case of a mammary epithelioid cell MFB composed of large mono- to multi-nucleated cells showing mild to moderate nuclear pleomorphism, predominantly arranged in a multinodular growth pattern. This tumor needs to be distinguished from invasive apocrine, oncocytic, pleomorphic lobular carcinoma, as well as metastases. Immunohistochemistry revealed the fibroblastic/myofibroblatic (positivity for vimentin, desmin, CD34 and focally for α-smooth muscle actin) nature of proliferating cells, and therefore was crucial for a correct diagnosis.