Metastatic malignant phyllodes tumor of the breast with rapid progression and subsequent response to immunotherapy: a case report.

Background: Malignant phyllodes tumors (MPTs) of the breast are rare fibroepithelial neoplasms characterized by aggressive clinical behavior and limited evidence guiding systemic therapy in the metastatic setting. Immune checkpoint inhibitors (ICIs) have demonstrated activity in several sarcoma subtypes; however, their role in MPTs of the breast remains poorly defined. Case presentation: A 53-year-old female patient underwent modified radical mastectomy for a rapidly enlarging left breast mass. Pathology revealed a 16.0 × 15.0 × 9.5 cm malignant phyllodes tumor with lymphovascular invasion (pT4N0). Despite the presence of an 8-mm pulmonary nodule at baseline, multiple bilateral lung metastases developed shortly after surgery. Following eight cycles of ifosfamide plus epirubicin chemotherapy, tumor burden was reduced by more than 90%. The patient subsequently received anlotinib as maintenance therapy; four months later, new hepatic and colorectal metastases emerged while the pulmonary lesions remained stable. Second-line sintilimab-based combination therapy achieved a marked partial response, with significant regression of the abdominal lesions, liquefactive necrosis of the hepatic metastases, and complete relief of abdominal pain. Conclusion: This case illustrates the diagnostic and therapeutic complexity of metastatic malignant phyllodes tumors. Although first-line chemotherapy produced marked pulmonary tumor regression, subsequent anlotinib maintenance therapy did not prevent disease progression. In this PD-L1-positive tumor (CPS 10), sintilimab-based combination therapy yielded a marked clinical and radiographic response, supporting further investigation of ICI-based therapy in selected patients with metastatic MPT.