Epithelioid stromal cells in lymphocytic mastitis--a source of confusion with invasive carcinoma.

: Fourteen examples of lymphocytic mastitis (LM) with epithelioid stromal cells characterized by formation of a palpable, hard breast mass composed of a predominantly lobulocentric lymphocytic infiltrate, stromal fibrosis, and an unusual stromal infiltrate of epithelioid cells are presented. The epithelioid cells were so prominent and abundant that the possibility of an infiltrating carcinoma was raised in three cases, and a fourth case was misinterpreted as a granular cell tumor. Interestingly, eight of the 12 women and the only male patient had long standing, insulin dependent diabetes mellitus (IDDM), whereas two had IDDM and hypothyroidism, one had hypothyroidism alone, and one had systemic lupus erythematosus. Contrary to the conclusions in a recent report, our findings indicate that these mammary changes are not exclusive to patients with IDDM, and may also occur in nondiabetic patients particularly those with autoimmune disorders. The morphologic features of the epithelioid stromal cells which have been mistaken for infiltrating carcinoma are emphasized; immunohistochemical and ultrastructural findings favoring their myofibroblastic nature are presented.