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.

Inclusion Criteria:

1. Written informed consent obtained.
2. Female aged ≥18 years.
3. Pathologically confirmed breast cancer after curative surgery (breast-conserving or mastectomy) and indicated for postoperative adjuvant radiotherapy.
4. Scheduled to receive moderate or ultra-hypofractionated proton therapy.
5. KPS ≥80, expected survival >5 years.
6. Well-healed surgical incision without infection.
7. Fertile females must use contraception for ≥1 month before screening and throughout the study.

Exclusion Criteria:

1. Pregnant or lactating female.
2. Severe non-neoplastic medical comorbidities precluding radiotherapy.
3. Inability to cooperate or comply with offline PET/CT examination.

Female patients aged 18 years or older with pathologically confirmed breast cancer after curative surgery, who are scheduled to receive moderate or ultra-hypofractionated proton radiotherapy at Ruijin Hospital, Shanghai Jiao Tong University School of Medicine. Patients must have a KPS score ≥80, expected survival >5 years, and provide written informed consent.