This study investigates the effects of regional radiation therapy in patients with low risk breast cancer. The purpose is to compare the outcomes of those receiving usual care, which includes regional radiation therapy, with those not receiving this type of radiation. Researchers want to determine if skipping regional radiation therapy is as effective at preventing the return of breast cancer.

Participants will be divided into study arms, with some receiving regional radiation therapy and others not. The study will monitor the recurrence of breast cancer to assess the effectiveness of the treatment. Regional radiation therapy involves targeting the lymph nodes around the breast to prevent cancer spread. The study aims to provide insights into the necessity of this treatment in low risk cases.

• Who can participate: Women with newly diagnosed invasive breast cancer, treated by surgery, and meeting specific criteria, such as having 1-3 positive axillary nodes, are eligible. They must have an Oncotype DX recurrence score below 18 and be undergoing endocrine therapy for at least 5 years.
• Study details: Participants may receive regional radiation therapy or no radiation. Blood and tumor samples will be collected for research. A placebo is not used.