Preoperative Stereotactic Ablative Body Radiotherapy (SABR) for Early-Stage Breast Cancer

Inclusion Criteria: * Understand and voluntarily sign an informed consent document prior to conducting any study related assessments or procedures. * Histologically proven invasive adenocarcinoma of breast. * Must have marker clip indicating location of target tumor in breast. * Unifocal tumor less than or equal to 2 cm based on contrast-enhanced prone-breast MRI. * Must be clinically and radiographically node negative (N0) to participate on this protocol. Clinically suspicious regional nodes by imaging or physical exam require biopsy evaluation to exclude disease involvement. * Appropriate candidate for breast-conserving surgery based on multi-disciplinary assessment. * Females age ≥ 50 years. * Able to tolerate prone body positioning during radiation therapy. * No prior ipsilateral-breast or thoracic radiotherapy. * As defined on MRI, target lesion must be at least 10 mm distance from skin (defined as volume encompassing first 3 mm from breast surface). * Must be estrogen receptor (ER) positive. * Must be negative for Her-2 amplification. (Either 1+ on semi-quantitative evaluation of immunostain or negative by fluorescent in-situ hybridization). * No implanted hardware or other material that would prohibit appropriate treatment planning or treatment delivery in the investigator's opinion. * No history of an invasive malignancy (other than this breast cancer, or non-metastatic basal or squamous skin cancers) in the last 5 years. * Must not have received nor be planned for neoadjuvant chemotherapy prior to SABR or surgery. * ECOG performance status less than 2. * Females of childbearing potential must have a negative urine pregnancy test prior to simulation and within seven days of SABR start. Exclusion Criteria: * Have invasive lobular carcinoma. * Have a Tumor \> 2 cm as measured on prone contrast-enhanced breast MRI. * Have presence of histologically proven lymph node disease. * Are not a candidate for breast conserving surgery. * Have had prior ipsilateral-breast or thoracic radiotherapy. * History of scleroderma or lupus erythematosus with either cutaneous manifestation or requiring active treatment. * An MRI defined target tumor that is within 10 mm of skin (defined as volume encompassing first 3 mm from skin surface). * Have amplification of Her-2 (Either 3+ by semi-quantitative immunostain or positive by Fluorescent in-situ hybridization (FISH)). * Have implanted hardware or other material that would prohibit appropriate treatment planning or treatment delivery in the investigator's opinion. * History of an invasive malignancy (other than this breast cancer, or non-metastatic basal or squamous skin cancers) in the last 5 years. * Have received or plan to receive neoadjuvant chemotherapy either before radiotherapy or before surgery. * A known carrier of BRCA1 or BRCA2 gene mutation. * Pregnant or unwilling to undergo pregnancy screening.