Breast Fibromatosis in a Patient With a History of Treated Breast Cancer: A Case Report

Desmoid tumors are benign mesenchymal neoplasms that originate from muscular fasciae and aponeuroses. Breast involvement is exceptionally rare, accounting for less than 0.2% of all breast tumors. A 41-year-old woman with a history of right-sided invasive ductal carcinoma (IDC) diagnosed in 2022 underwent breast-conserving surgery (BCS) and axillary lymph node dissection (ALND), followed by adjuvant chemotherapy, radiotherapy, and daily tamoxifen (20 mg). The tumor measured 3.5 cm at its greatest dimension, was grade 2, estrogen receptor (ER)-positive, progesterone receptor (PR)-positive, <i>HER2</i> negative, and had a Ki-67 proliferation index of 25%. Histologic examination revealed a cribriform growth pattern without associated ductal carcinoma in situ (DCIS) or lymphovascular invasion (LVI), and one of nine axillary lymph nodes was positive for metastasis. In 2023, a total abdominal hysterectomy with bilateral salpingo-oophorectomy was performed for ovarian suppression. During routine surveillance in 2024, a new mass was detected at the 2 o'clock position in the right breast. Two core needle biopsies performed over 6 months confirmed fibromatosis. Ongoing tumor enlargement and severe pain, despite radiotherapy, led to a wide local excision. Breast fibromatosis can closely mimic carcinoma both clinically and radiologically, and histologic analysis remains essential for definitive diagnosis. Complete surgical excision with negative margins remains the treatment of choice.