Mammary epithelioid myofibroblastoma mimicking invasive carcinoma: case report and literature review

Myofibroblastoma (MFB) of the breast is a rare benign neoplasm, which exhibits several morphologic variants and presents diagnostic dilemmas for pathologists. Here, we describe a case of a 42-year-old female patient diagnosed as epithelioid MFB. This painless tumor was well-circumscribed and found in the left breast for three months. Histologically, this tumor was predominantly composed of epithelioid cells, which arranged as single cells or small clusters, and formed a cellular nodule. Tumor stroma was collagenized, with scattered myxoid areas. This case was misinterpreted as invasive lobular carcinoma in the original diagnosis. Immunohistochemical profile demonstrated positivity for desmin, SMA, calponin, CD34 and hormone receptors, whereas pan-CK, CK7, CK8, CK34bE12, CK5/6, EMA, p63 and S-100 were negative, confirming the diagnosis of epithelioid MFB. Awareness of this unusual variant and careful integration of clinicopathologic findings would be critical to diagnosis this challenging lesion and avoid potential diagnostic pitfalls.