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