- Patients must be women with newly diagnosed histologically proven invasive carcinoma of the breast with no evidence of metastases, staged as per site standard of care. - Patients must have been treated by BCS or mastectomy with clear margins of excision. Post-mastectomy positive margins for invasive disease and/or DCIS is not allowed. Multifocal disease (i.e. the presence of two or more foci or breast cancer within the same breast quadrant) and multicentric disease (i.e. the presence of two or more foci of breast cancer in different quadrants of the same breast) are allowed. - Patients with T3N0 disease are eligible. - Patients with disease limited to nodal micrometastases are not eligible - Patients with nodal macrometastases (>2mm) treated by axillary dissection must have 1-3 positive axillary nodes (macrometastases, > 2 mm). - Patients treated by mastectomy and SLNB alone must have only 1 positive axillary node (macrometastases, > 2 mm). - Patients must be ER = 1% and HER2 negative on local testing - Patients must have an Oncotype DX recurrence score

- Patients with nodal disease limited to isolated tumour cells (pN0i+ < 0.2 mm). - Patients with pT3N1 and pT4 disease (Note: patients with T3N0) are eligible. - Any prior history, not including the index cancer, of ipsilateral invasive breast cancer or ipsilateral DCIS treated with radiation therapy. (Patients with synchronous or previous ipsilateral LCIS are eligible.) - Synchronous or previous contralateral invasive breast cancer. (Patients with contralateral DCIS are eligible unless previously treated with radiation.) - History of non-breast malignancies except adequately treated non-melanoma skin cancers, in situ cancers treated by local excision or other cancers curatively treated with no evidence of disease for = 5 years. - Patients who are pregnant. - Patients that have had prior ipsilateral chestwall/thoracic radiation. - Patients treated with chemo or endocrine therapy administered in the neoadjuvant setting for breast cancer. Endocrine therapy exposure 12 weeks or less prior to surgery is permitted. - Patients with serious non-malignant disease (e.g. cardiovascular, scleroderma etc.) which would preclude RT. - Patients with any serious active or co-morbid medical conditions, laboratory abnormality, psychiatric illness, active or uncontrolled infections, or serious illnesses or medical conditions that would prevent the patient from participating or to be managed according to the protocol (according to investigator's decision).