This study will test the safety of partial breast RT using 27Gy in 5 daily fractions which is expected to be equally tolerated as standard whole breast irradiation (WBI) based on radiobiologic modeling of the fibrosis response of normal tissues but be more convenient and less costly than 3-6 weeks of standard WBI. 274 women will be treated. Rates of fibrosis and cosmesis at 2 years will be compared to data already collected using standard WBI in the Canadian RAPID trial.