127 patients received unilateral conventionally fractionated postmastectomy IMPT. Median follow-up was 4.1 years. Five-year LRC was 98.4% and OS was 87.9%. Acute grade 2 and 3 dermatitis was seen in 45% and 4% of patients. Three patients (2%) experienced acute grade 3 infection, all of whom had breast reconstruction. Three late grade 3 adverse events occurred: morphea (N=1), infection (N=1), and seroma (N=1). There were no cardiac or pulmonary adverse events. Among the 73 patients at risk for postmastectomy RT associated reconstruction complications, seven (10%) experienced reconstruction failure.