Breast cancer is the most common malignancy in women. Breast-conserving surgery (BCS) with adjuvant whole-breast irradiation (WBI) is currently the standard of care in the oncological treatment of primary breast cancer and offers an equivalent alternative to mastectomy. The primary aim of adjuvant radiotherapy (RT) is to improve local control and thus improve overall survival and breast cancer-specific mortality. However, nodal-negative women have a 10-year risk for local recurrences after RT in up to 15.6%. The management of ipsilateral breast cancer recurrence depends on the extent of tumor disease and staging results at the time of recurrence and mastectomy is currently the standard of care for previously irradiated patients. The application of particle therapy using proton beam therapy (PBT) represents an innovative radiotherapeutic technique for breast cancer patients. This trial will be conducted as a prospective single-arm phase II study in 20 patients with histologically proven invasive breast cancer recurrences with negative margins after repeat BCS and with an indication for local re-irradiation. Required time interval will be 1 year after previous RT to the ipsilateral breast. Patients will receive partial breast re-RT with proton beam therapy in 15 once daily fractions up to a total dose of 40.05 GyRBE. The primary endpoint is defined as the cumulative overall occurrence of acute / subacute skin toxicity grade ≥3 within 6 months after the start of re-RT.