Primary low-grade fibromyxoid sarcoma of the breast: a rare case report with immunohistochemical and fluorescence in situ hybridization detection.

Low-grade fibromyxoid sarcoma (LGFMS) is a rare tumor with a bland histologic appearance but malignant biological behavior. Primary LGFMS of the breast has not been described in the English-language literature. Here, we report a 58-year-old Chinese female patient who presented with a painless mass in the right breast for more than 30 years. The tumor consists of spindle cells resembling fibroblasts and includes 2 kinds of morphologic change, which are alternating collagenized hypocellular zone and cell-rich myxoid area. There are more arcades of curvilinear blood vessels. The spindle cells are not heteromorphic, and mitotic figures are scarce. Immunostaining shows that tumor cells are positive for vimentin, mucin4, CD99, and Bcl-2, but negative for smooth muscle actin, desmin, S100, CD34, ALK, and myogenin. FUS gene rearrangement is positively detected by fluorescence in situ hybridization. The patient has been followed up for 59 months and is in a favorable condition. This rare location of LGFMS should be noted.