This study reports 3-year outcomes of the Proton Collaborative Group phase II trial (PCG BRE007-12) that recruited women ≥ 50 years with node-negative, estrogen receptor (ER)-positive, ≤3cm, invasive ductal carcinoma (IDC) or ductal carcinoma in situ undergoing breast conserving surgery followed by partial breast irradiation (PBI) with a total dose 40 Gy(RBE) in 10 daily fractions. For the 38 evaluable patients, treatments were delivered in ≥ 2 fields predominantly with uniform scanning PT (n = 37). At 35-month median follow-up, all patients were alive, and none had local, regional or distant disease progression. One patient developed an ER-negative contralateral IDC. Seven grade 2 adverse events occurred; no radiotherapy-related grade ≥ 3 toxicities occurred. There was no meaningful change in patient-reported quality of life. Median heart volume receiving 5 Gy (V5Gy), lung V20Gy, and lung V10Gy were 0 %, 0 % and 0.19 %, respectively. These findings support PT as a safe and effective PBI delivery option.