Fibromatosis of the breast parenchyma with a benign-like nodular appearance

We report the clinical, sonographic and pathologic features of a rare case of primary fibromatosis of the breast parenchyma in a 29-year-old female. Unlike most cases reported in the literature, this tumor presented as an oval-shaped nodular mass lacking the typical infiltrating (finger-like) margins both at ultrasonography and macroscopic examination. Frozen section diagnosis revealed a spindle cell proliferation entrapping mammary ducts and lobules and, thus, the provisional diagnosis of “fibromatosis-like spindle cell proliferation” was rendered, but the possibility of a spindle cell metaplastic carcinoma could not be completely ruled out. The final diagnosis was achieved in formalin-fixed tissues by a combined morphological and immunohistochemical study. Tumor was composed of long, interlacing fascicles of bland-looking spindle-shaped cells embedded in a variably fibrous stroma. Only focally tumor margins were of infiltrative type. Immunohistochemically neoplastic cells were stained diffusely with vimentin, -smooth muscle actin and -catenin. Awareness that fibromatosis of the breast may exhibit an unusual benign-like nodular appearance is crucial to avoid an underrecognition of this locally aggressive tumor.