This is a single-arm, single-center, prospective observational study. It uses offline PET/CT to verify and quantify the range accuracy of hypofractionated proton radiotherapy for postoperative breast cancer. The correlation between range error and clinical toxicities (radiation pneumonitis, rib fracture) will be analyzed. A PET signal-proton range correlation model will be established using Monte Carlo simulation to provide evidence for optimizing proton therapy planning.