Locally Aggressive Breast Desmoid Fibromatosis in a Young Woman With Multiple Fibroadenomas.

Desmoid fibromatosis is a rare spindle cell neoplasm with fibroblastic/myofibroblastic differentiation, accounting for less than 0.2% of all breast tumors. It affects predominantly women and although can be locally aggressive, it has no metastatic potential. Diagnosis requires tissue sampling, correlation with imaging and clinical findings, and the use of ancillary studies. Breast fibromatosis typically exhibits nuclear β-catenin positivity, with variable expression of smooth muscle actin (SMA), desmin, and calponin, but is generally negative for h-caldesmon-a marker traditionally considered helpful in distinguishing fibromatosis from tumors with true smooth muscle differentiation. A 22-year-old woman presented with a left breast mass initially diagnosed as fibroadenoma on biopsy. Surgical excision revealed bland spindle cells within a collagenous stroma and infiltrative borders. Immunohistochemistry demonstrated positivity for h-caldesmon, nuclear β-catenin, and calponin. Molecular analysis confirmed an activating CTNNB1 mutation, establishing the diagnosis of fibromatosis.