Fibromatosis-like carcinoma-an unusual phenotype of a metaplastic breast tumor associated with a micropapilloma

BACKGROUND: Fibromatosis-like metaplastic carcinoma is a newly described metaplastic breast tumor, literature on which is still evolving. CASE PRESENTATION: A 77-year-old lady presented with a 2 × 2 cm mass with irregular margins in the upper and outer quadrant of left breast. Fine needle aspiration cytology (FNAC) from the lump was inconclusive. A lumpectomy was performed and sent for frozen section, which revealed presence of spindle cells showing mild atypia in a sclerotic stroma. The tumor cells revealed prominent infiltration into the adjacent fat. A differential diagnosis of a low-grade sarcoma vs. a metaplastic carcinoma, favoring the former, was offered. Final histology sections revealed an infiltrating tumor with predominant spindle cells in a collagenous background, simulating a fibromatosis. Adjacent to the tumor were foci of benign ductal hyperplasia and a micropapilloma. Immunohistochemistry (IHC) showed diffuse co-expression of epithelial markers i.e. cytokeratins (CK, HMWCK, CK7) and EMA along with a mesenchymal marker i.e. vimentin in the tumor cells. Myoepithelial markers (SMA and p63) showed focal positivity. A diagnosis of a low-grade fibromatosis-like carcinoma breast associated with a micropapilloma was formed. CONCLUSION: Fibromatosis-like carcinoma is a rare form of a metaplastic breast tumor. This diagnosis requires an index of suspicion while dealing with spindle cell breast tumors. The importance of making this diagnosis to facilitate an intra operative surgical planning is marred by diagnostic difficulties. In such cases, IHC is imperative in forming an objective diagnosis.