Methods: A single-arm, phase II, non-inferiority, prospective study will be conducted. 274 women with newly diagnosed, invasive or in-situ ductal carcinoma treated with BCS and sentinel lymph node biopsy (or axillary dissection) who are candidates for WBI alone, will be treated. Subjects will be age 50 years and older, with tumors less than 3cm diameter, with negative margins and nodes and with excellent or good baseline cosmetic outcome following BCS. Patients with extensive ductal carcinoma in-situ, BRCA mutations, Grade 3 cancers with lymphatic or vascular invasion, or lobular carcinoma will be excluded. These selection criteria are similar to the Canadian RAPID trial. In the first phase of the study, 150 patients will be recruited and interim analyses will be conducted to rule out unacceptable toxicity at 2 years.