Inclusion Criteria

Patients must have newly diagnosed histologically proven invasive carcinoma of the breast with no evidence of metastases

Patients must have been treated by BCS or mastectomy

Patients treated by BCS or mastectomy and axillary dissection must have 1-3 positive axillary nodes (macrometastases, > 2 mm)

Patients treated by BCS and SLNB alone must have only 1-2 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 <18

Patient must consent to provision of, and investigator(s) must confirm access to and agree to submit to the CCTG Central Tumour Bank, a representative formalin fixed paraffin block of tumour tissue in order that the specific correlative marker assays described in the protocol may be conducted

Patient must consent to provision of samples of blood in order that the specific correlative marker assays described in the protocol may be conducted

 Patients must have had endocrine therapy initiated or planned for ≥ 5 years. Endocrine therapy can be given concurrently or following RT.

Patients may or may not have had adjuvant chemotherapy

RT must be administered within 12 weeks of definitive surgery if the patient is not treated with chemotherapy. If adjuvant chemotherapy is given, RT must begin within 2-8 weeks after the last dose

Women of childbearing potential must have agreed to use an effective contraceptive method. A woman is considered to be of "childbearing potential" if she has had menses at any time in the preceding 12 consecutive months.

Exclusion Criteria

Patients with nodal disease limited to micrometastases (pN1Mi, > 0.2 mm and ≤ 2 mm) or isolated tumour cells (pN0i+ < 0.2 mm)

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 not treated with radiation are eligible.)

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 with pT3 or pT4 disease

Patients who are pregnant

Patients that have had prior ipsilateral chestwall/thoracic radiation

Patients treated with neoadjuvant chemo or endocrine therapy for breast cancer

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)