Low-Grade Fibromatosis-Like Spindle Cell Carcinoma of the Breast: a Mimicker of Benign Mesenchymal Tumors

We described a case of low-grade fibromatosis-like spindle cell carcinoma (FLSCC) of the breast of a 49-year-old woman, which was diagnosed by excisional biopsy. The histopathology was characterized by bland neoplastic spindle cells with mild or no nuclear atypia, mimicking a benign mesenchymal tumor. An immunopanel of markers including pancytokeratin, p63, smooth muscle actin (SMA), CD10, and β-catenin was performed to establish the diagnosis and exclude other mimickers. The patient subsequently received modified radical mastectomy and was alive without disease for 10-year follow-up. Low-grade fibromatosis-like spindle cell carcinoma is a rare subtype of metaplastic carcinomas of the breast and typically occurs as palpable breast mass in an older woman. It can be histologically confused with nodular fasciits, desmoids-type fibromatosis, and inflammatory myofibroblastic tumor. Immunohistochemical study was essential for a correct diagnosis. It behaves as a locally aggressive disease but with a low potential of regional lymph node or distant metastasis.