Epithelioid mammary myofibroblastoma mimicking invasive lobular carcinoma

A case of a 53-year-old woman with the epithelioid variant of mammary myofibroblastoma, which was initially misinterpreted as invasive lobular carcinoma, is presented. A needle core biopsy of the 1.6 cm mass showed interlacing bundles of epithelioid myofibroblasts amid dense fibrous tissue associated with lobular carcinoma in situ of the classical type. Most epithelioid cells showed nuclear atypia, and a few exhibited signet-ring cytology. Immunoreactivity for estrogen and progesterone receptors further compounded the deception, and the neoplasm was misinterpreted as invasive lobular carcinoma. Excisional biopsy showed a circumscribed stromal tumor with foci suspicious for invasive lobular carcinoma. The latter was excluded by cytokeratin negativity throughout the tumor. The overall histopathological appearance and immunostaining pattern was confirmatory of myofibroblastoma. This case report emphasizes the potential for mistaking epithelioid myofibroblastoma for invasive lobular carcinoma--particularly in the setting of limited sampling, hormone-receptor immunoreactivity of the lesional cells, and synchronous lobular carcinoma in situ.