This study is the first prospective randomized study assessing the patient-reported outcomes and safety outcomes between preoperative and postmastectomy radiotherapy in locally advanced breast cancer patients with immediate reconstruction of deep inferior epigastric perforator(DIEP) flap. Radiotherapy before mastectomy and autologous free-flap breast reconstruction can avoid adverse radiation effects on healthy donor tissues and delays to adjuvant radiotherapy. We aimed to explore the feasibility of preoperative radiotherapy followed by DIEP flap reconstruction in patients with breast cancer requiring mastectomy.