Rationale:Phyllodes tumor of the breast (PTB) is a rare fibroepithelial neoplasm with controversial postoperative management, particularly for borderline and malignant subtypes. Limited large-scale studies exist on optimal radiotherapy strategies and recurrence prevention, necessitating further exploration of multidisciplinary approaches.Patient concerns:A 50-year-old female presented with a rapidly enlarging right breast mass (from 2 to 3 cm to 12 cm within a week), initially detected 10 years prior. Imaging revealed a BI-RADS 4B cystic-solid lesion, and biopsy confirmed a malignant neoplasm with necrosis.Diagnoses:Postmastectomy pathology confirmed a malignant PTB (T4N0M0GX) with stromal overgrowth, adipose tissue invasion, and high Ki-67 (90%+). Immunohistochemistry showed Vimentin+, CK+, and BCL2+, while ER/PR were negative.Interventions:The patient underwent right mastectomy followed by adjuvant radiotherapy targeting the tumor bed (gross tumor volume tumor bed: 60 Gy/25F) and expanded margins (planning tumor volume: 50 Gy/25F). No chemotherapy or endocrine therapy was administered due to limited evidence of efficacy.Outcomes:Postoperative recovery was uneventful, with no local recurrence observed during follow-up. Radiotherapy achieved optimal local control, aligning with studies suggesting reduced recurrence in margin-challenged cases.Lessons:This case provides critical clinical evidence supporting adjuvant radiotherapy as a viable salvage strategy for malignant PTB with close/positive margins (<= 1 cm), particularly when re-excision is contraindicated. By demonstrating 12-month disease-free survival with acceptable toxicity, our findings directly address the NCCN guideline gap regarding postmastectomy radiation for borderline resectable PTB. The documented multidisciplinary decision-making process - incorporating tumor biology (high Ki-67), anatomical constraints, and patient preferences - offers a replicable framework for similar cases while underscoring the urgent need for consensus protocols through prospective registry studies.