This is a prospective, phase II study of women with localized breast cancer receiving non-operative management with SABR delivered to the primary tumor. Patients will be screened that are age 65 and over with invasive breast cancer. Those eligible for the trial will have ER-positive and HER2-negative disease measuring ≤5 cm with negative clinical lymph nodes. Patients that were previously on primary ET, with either stable or progressive disease, will be eligible for this study if other inclusion criteria are met. Participants will receive SABR targeting the tumor consisting of 5 total treatments. Participants will then follow-up as per protocol with quality-of-life assessments and treatment-related toxicity evaluation.
