Study of Stereotactic Radiotherapy for Breast Cancer

Inclusion Criteria: * Female sex * Age \> or = to 50 years of age * Invasive ductal carcinoma * Clinically and radiographically T1 tumor * Clinically node negative * Clearly demarcated tumor on magnetic resonance imaging (MRI), as determined by treating physician (MRI may be done after enrollment if not done prior) * Planning breast conserving surgery including sentinel node biopsy * ≥10% expression of ER and/or PR * HER2- using the current College of American Pathologists guidelines * Post-menopausal * Willing and able to provide informed consent Exclusion Criteria: * Medical conditions that may increase risk for poor cosmetic outcome (i.e. Lupus, rheumatoid arthritis, scleroderma * Pure DCIS without invasive cancer * Patients who have received or will be receiving neoadjuvant systemic therapy, endocrine therapy, or targeted agents * Breast implant in the involved breast unless the implant will be removed prior to initiation of study treatment * Positive pregnancy test * Subjects without placement of a biopsy clip at the diagnostic procedure who are unwilling to undergo clip placement. * Unable to meet dosimetric constraints due to tumor location and/or patient anatomy * Planning mastectomy * Unable to tolerate prone positioning