Case Report: Fibromatosis-like metaplastic breast cancer: a case with detection of three genetic alterations, accompanied by local recurrence, lymph node metastasis, and distant metastasis.

Fibromatosis-like metaplastic carcinoma (FLMC) is defined as a rare histologic subtype of metaplastic carcinoma within the 2019 World Health Organization (WHO) classification framework for breast tumors. We present a case of FLMC demonstrating a notably atypical clinical presentation. A 73-year-old woman presented with a self-detected breast mass. Radiological and pathological evaluations both indicated potential malignancy. Histological examination showed a lesion composed predominantly of spindle cells and displaying an infiltrative growth pattern. Immunohistochemical staining confirmed co-expression of cytokeratin and p63 within the spindle cell component, supporting a diagnosis of FLMC. During follow-up, the patient developed local recurrence along with lymph node and distant metastases, leading to subsequent next-generation sequencing (NGS) to investigate underlying molecular alterations. Through NGS, we identified a deletion mutation in PIK3R1, a frameshift mutation in RUNX1, and MYC gene amplification. These findings provide preliminary molecular clues for the relatively aggressive biological behavior of FLMC.