Epithelioid-cell myofibroblastoma of the breast: expanding the morphologic spectrum.

Myofibroblastoma (MFB) of the breast is an uncommon benign tumor, which exhibits a wide variety of cytomorphologic features and architectural patterns. Epithelioid-cell MFB is a rare morphologic variant exclusively or predominantly (>50%) composed of cells with epithelioid morphology, which can represent a potential diagnostic pitfall. This study describes the clinicopathologic features of 4 cases of epithelioid-cell MFB, including core biopsy-based morphology in 2 cases. Characteristic histologic features included the presence of well-circumscribed margins; epithelioid cells variably admixed with a minority (10% to 40%) of round, polygonal, and spindle-shaped cells; low mitotic activity (0 to 2 mitoses/10 high-power fields); and a mild-to-moderate degree of nuclear pleomorphism. The following growth patterns were observed: alveolar, single-cell, single-file, solid, and fascicular growth patterns. In 1 case, neoplastic cells exhibited an unusual cellular arrangement, with the formation of neural-like structures resembling small peripheral nerves. All but 1 case contained an intratumoral mature fatty component that variably accounted from 10% to 40% of the entire tumor. The close admixture of atypical epithelioid cells with intratumoral adipocytes imparted a pseudo-infiltrative appearance, particularly in 1 case. This study emphasizes that epithelioid-cell MFB may be a diagnostic challenge, especially when evaluating needle core biopsies. Immunohistochemistry, revealing a variable coexpression of vimentin, desmin, alpha-smooth muscle actin, CD34, bcl-2 protein, CD99, CD10, estrogen/progesterone/androgen receptors, is crucial in confirming the diagnosis. Awareness of epithelioid-cell MFB is important to avoid a misdiagnosis of malignancy.