Mammary myofibroblastoma with extensive myxoedematous stromal changes: a potential diagnostic pitfall.

Myofibroblastoma (MFB) of the breast is a relatively rare, benign stromal tumor arising in the breast of both males and females. Several morphological variants have been recognized in the last two decades, including infiltrating, cellular, fibrous/collagenized, epithelioid/deciduoid-like cell, lipomatous and myxoid variants. Myxoid MFB is an extremely rare variant which can represent a potential diagnostic pitfall, posing differential diagnostic problems with other myxoid lesions occurring primarily in the breast parenchyma. We first report an unusual case of mammary MFB showing so extensive myxoedematous stromal changes that obscured the underlying neoplastic cells, rendering difficult its recognition as MFB. Tumor was composed of a few bland-looking spindled, stellate cells embedded in an abundant myxoedematous stroma containing thin and thick collagen fibers and numerous blood vessels with fibrin and foamy histiocytes deposits in their walls. Identification of a small-sized cellular area, with morphological and immunohistochemical features consistent with mammary-type MFB, was crucial for a correct diagnostic interpretation of the lesion. These unusual stromal and vascular features are likely morphological changes to which undergo long-standing mammary MFBs in response to ischemic, traumatic or inflammatory stimuli. Differential diagnostic problems with benign and malignant myxoid lesions of the breast parenchyma are discussed. The present case, for which the term "mammary MFB with extensive myxoedematous stromal changes" is proposed, contributes to widen the morphological spectrum of this unusual mammary tumor, suggesting that when pathologist is dealing with a hypocellular myxoedematous lesion of the breast, a MFB should be ruled out.