Inclusion Criteria:

          -  AJCC 7th Edition stage 0 or I (TisN0 ≤ 2 cm or T1N0) histologically confirmed
             carcinoma of the breast, treated with partial mastectomy. Axillary sampling is
             required only for cases of invasive cancers. Tumor size is determined by the
             pathologist. Clinical size may be used if the pathologic size is indeterminate.
             Patients with invasive cancer must have no positive axillary lymph nodes with at least
             6 axillary lymph nodes sampled or a negative sentinel node.

          -  Negative histologic margins of partial mastectomy or re-excision specimen. Margins
             generally are positive if there is invasive or noninvasive tumor at the inked
             resection margin, close but negative if the tumor is within 2 mm of the inked margin
             and negative if the tumor is at least 2 mm away from the inked edge.

          -  Invasive ductal, lobular, medullary, papillary, colloid (mucinous), tubular
             histologies, or mixed histologies (lesions ≤ 2 cm) that are estrogen or progesterone
             receptor positive and do not exhibit HER2/neu gene amplification OR ductal carcinoma
             in situ (lesions ≤ 2 cm).

          -  Systemic therapy, if planned, must be adjuvant in nature and not be scheduled to begin
             for at least 4 weeks after completion of HG-PBI.

          -  Good candidate for treatment per protocol in the judgment of the PI and/or treating
             physician following simulation.

          -  Postmenopausal status.

          -  Age ≥ 50 years at diagnosis.

          -  Able to understand and willing to sign IRB-approved written informed consent document.

          -  English speaker.

        Exclusion Criteria:

          -  Presence of distant metastases.

          -  In situ lobular carcinoma or nonepithelial breast malignancies such as sarcoma or
             lymphoma.

          -  Proven multicentric carcinoma (tumors in different quadrants of the breast, or tumors
             separated by at least 4 cm) with other clinically or radiographically suspicious areas
             in the ipsilateral breast unless confirmed to be negative for malignancy by biopsy.

          -  Premenopausal status.

          -  Histologically confirmed positive axillary nodes in the ipsilateral axilla. Palpable
             or radiographically suspicious contralateral axillary, supraclavicular,
             infraclavicular, or internal mammary nodes, unless there is histologic confirmation
             that these nodes are negative for tumor.

          -  Prior non-hormonal therapy for the present breast cancer, including radiation therapy
             or chemotherapy.

          -  Diagnosis of systemic lupus erythematosis, scleroderma, or dermatomyositis.

          -  Diagnosis of a coexisting medical condition which limits life expectancy to < 2 years.

          -  Diagnosis of psychiatric or addictive disorders that would preclude obtaining informed
             consent.

          -  History of other malignancy ≤ 5 years previous with the exception of basal cell or
             squamous cell carcinoma of the skin which were treated with local resection only or
             carcinoma in situ of the cervix.

          -  Paget's disease of the nipple.

          -  Skin involvement, regardless of tumor size.

          -  Unsatisfactory breast for HG-PBI as determined by the treating physician. For example,
             if there is little breast tissue remaining between the skin and pectoralis muscle
             after surgery, treatment with HG-PBI is technically problematic.

          -  Partial mastectomy so extensive that the cosmetic result is fair or poor prior to
             HG-PBI as determined by the treating physician.

          -  Surgical margins which cannot be microscopically assessed or are positive at
             pathological evaluation.

          -  Time between final definitive breast procedure to HG-PBI simulation is greater than 8
             weeks.

        Inclusion of Women and Minorities

        -Women and members of all races and ethnic groups are eligible for this trial. Because
        breast cancer occurs rarely in men, men will not be recruited for participation.