Conformal Ablative Radiotherapy in Older Women (CARMEN) With Breast Cancer Undergoing Non-Operative Management

Inclusion Criteria: * Willing and able to provide written informed consent obtained to participate in the study and HIPAA authorization for release of personal health information. * Women greater than or equal to age 65 years * Diagnosis of invasive carcinoma of breast undergoing non-operative management * Previously untreated OR previously treated with primary ET or chemotherapy, with either stable or progressive localized disease. * Tumor measuring ≤5 cm (cT1-T2) and clinically node negative * Diagnostic biopsy ER-positive (greater than or equal to 10% ER by immunohistochemistry staining) * Diagnostic biopsy HER2-negative according to ASCO/CAP guidelines (0 or 1+ following IHC staining or proven negative by in-situ hybridization) * Suitable for SABR as deemed by the treating radiation oncologist * Participants with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen are eligible for this trial * Enrollment in another clinical trial is allowed if there would be no interference with interventions on this trial Exclusion Criteria: * Active treatment with systemic chemotherapy for breast cancer. Subjects will need to stop any breast cancer chemotherapy agent before enrollment to be included study. * Multicentric tumor. * Clinical or imaging evidence of distant metastases. * Prior ipsilateral breast or thoracic radiation. * Autoimmune conditions with associated radiation risks.