Desmoid-Type Fibromatosis Masquerading as Disease Progression in Metastatic Breast Cancer in a Patient on Ribociclib Therapy: A Case Report.

Ribociclib combined with an aromatase inhibitor (AI) is widely used as first-line therapy for hormone receptor (HR)-positive, human epidermal growth factor receptor-2 (HER-2)-negative metastatic breast cancer. Desmoid-type fibromatosis (DTF) is a rare, locally invasive soft-tissue tumor that accounts for only 0.2% of all breast neoplasms and can mimic malignancy upon imaging. A 44-year-old premenopausal woman with HR-positive, HER2-negative bone-only metastatic breast cancer developed a rapidly growing breast mass after 30 cycles of ribociclib with an AI. Imaging suggested cancer progression, necessitating total mastectomy. However, postoperative pathology confirmed DTF, which is a rare occurrence that has not been previously reported. Appropriate therapeutic planning and follow-up for metastatic breast cancer should include histopathological diagnosis, when necessary, beyond clinical and imaging assessments.