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.