Mammary Myofibroblastoma with Unusual Morphological and Immunohistochemical Features

Mammary myofibroblastoma (MFB) is a rare mesenchymal tumor, derived from mammary stromal fibro/myofibroblasts, which has various morphological features and characteristic immunohistochemical staining. The epithelioid morphologic variant is defined, accordingly, as a proliferation of exclusively or predominantly (>50%) epithelioid cells, variably embedded in a myxoid to fibrous stroma. These histological and cytological features may pose a diagnostic challenge mainly with metaplastic carcinoma and invasive lobular carcinoma of the breast. Thus, immunohistochemical staining by myofibroblastic markers is helpful for confirming diagnosis. Herein, we present a case of MFB in a 43-year-old female. This case report emphasizes the role of immunohistochemistry as gold standard in the diagnosis of MFB. This case is also being presented because of its unusual radiologic findings, its epithelioid histologic variant mimicking malignancy, and its uncommon immunohistochemical phenotype.