English

Abstract

Introduction 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 tumour presented 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. Tumour was composed of long, interlacing fascicles of bland-looking spindleshaped cells embedded in a variably fibrous stroma. Only focally tumour margins were of infiltrative type. Immunohistochemically, neoplastic cells were stained diffusely with vimentin, ಉ-smooth muscle actin and β-catenin. Awareness that fibromatosis of the breast may exhibit an unusual benign-like nodular appearance is crucial to avoid underrecognition of this locally aggressive tumour. Conclusion We believe that a correct diagnosis of breast fibromatosis, even on frozen sections, is primarily dependent on awareness by pathologists that this tumour can rarely arise in this unusual site.