Mammary stromal-epithelial lesions with myofibroblastic stroma and HMGA2 overexpression linked smooth muscle differentiation, a case series from single institution

Fibroadenoma and phyllodes tumor are the prototypical mammary fibroepithelial lesions (FELs). Recently, a subset of FELs, identified as stromal-epithelial lesion (SEL) with myofibroblastic stroma have been labelled as myofibroepithelial nodule (MFN). The MFN stromal cells are diffusely positive for SMA immunostaining and frequently show unusual histological features including irregular borders. There is limited literature on FELs with myofibroblastic or smooth muscle stroma. The etiology of the variation in the FEL stromal histology and its clinical significance is unknown. In this short report we describe clinicopathologic features of six FELs with myofibroblastic and/or smooth muscle stroma. We also report immunohistochemical overexpression of HMGA2 in 2 FELs that contained stromal smooth muscle differentiation suggesting a link to mammary myoid hamartoma. On limited follow up all the 6 FELs with myofibroblastic or smooth muscle stroma had benign outcome. The HMGA2 overexpressing FEL with smooth muscle stroma and myoid hamartoma of the breast show overlapping etiology, and histological features.