BACKGROUND : Locoregional recurrence of breast cancer is challenging for clinicians, due to the various former treatments patients have undergone. However, treatment of the recurrence with systemic therapy and subsequent reirradiation of chest wall is accompanied by increased toxicities, particularly radiation-induced cardiovascular disease. Reirradiation by proton beam therapy (PBT) enables superior preservation of adjacent organs at risk as well as concurrent dose escalation for delivery to the gross tumor. This technology is expected to improve the overall outcome of recurrent breast cancer. METHODS : A 47-year-old female presented with an extensive locoregional recurrence at 10 yr after primary treatment of a luminal A breast cancer. Because of tumor progression despite having undergone bilateral ovarectomy and systemic therapy, the patient was treated with PBT total dose of 64.40 Gy to each gross tumor and 56.00 Gy to the upper mediastinal and retrosternal lymphatics including the entire sternum in 28 fractions. Follow-up computed tomography showed a partial remission, without evidence of newly emerging metastasis. At 19 mo after the PBT, the patient developed a radiation-induced pericardial disease and pleural effusions with clinical burden of dyspnea, which were successfully treated by drainage and corticosteroid. Cytological analysis of the puncture fluid showed no malignancy, and the subsequent computed tomography scan indicated stable disease as well as significantly decreased pericardial and pleural effusions. The patient remains free of progression to date. CONCLUSIONS : PBT was a safe and effective method of reirradiation for locoregionally recurrent breast cancer in our patient.