Breast-conserving surgery followed by re-irradiation with partial breast irradiation (rPBI) has recently been found to be a safe alternative to mastectomy for women who have undergone prior whole breast radiation. By reducing the volume of tissue receiving radiation, rPBI has been associated with less toxicity and improved cosmetic outcomes. For many women with early-stage breast cancer, shorter 1-week (5-fraction) courses of breast radiation (ultra-fractionation) have been found to be equivalent to longer fractionation schedules in the upfront treatment setting. These 1-week schedules are more convenient for patients, with fewer treatments and shorter overall treatment time. The investigators hypothesize that a 1-week ultra-hypofractionated rPBI regimen following breast-conserving surgery (BCS) for local recurrence or new primary breast cancer in the previously irradiated breast (LR) will be associated with acceptable toxicity at 1 year (\<13% grade \>3 toxicity).