Mammary myofibroblastoma: an update with emphasis on the most diagnostically challenging variants

Myofibroblastoma (MFB) is a rare benign mesenchymal tumor which usually occurs in the breast parenchyma of both females and males. Although this tumor is typically composed of bland-looking spindle-shaped cells arranged in short fascicles interrupted by keloidal-like collagen fibers, several variations on this basic morphological theme do exist. With the advent of mammographic screening, an increased number of mammary MFBs are being detected and pathologists should be aware of the wide morphological and immunohistochemical spectrum exhibited by this unusual tumor. This review focuses on the most diagnostically challenging variants of mammary MFB, which could represent potential diagnostic pitfalls of malignancy, especially when evaluating needle core biopsies. In this regard the following variants of MFB, including the most recently recognized, will be presented: myxoid MFB, lipomatous MFB, epithelioid cell MFB, deciduoid cell MFB, epithelioid cell MFB with multinodular growth pattern, palisaded/schwannian-like MFB and MFB with extensive myxo-edematous stromal changes. Histological illustrations along with differential diagnostic problems for each single variant of MFB will be provided to offer helpful suggestions for a correct diagnostic approach in daily practice.