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 as assessed by axillary ultrasound, axillary sampling, or
             axillary sentinel node procedure.

          -  Negative histologic margins of partial mastectomy or re-excision specimen. The
             posterior margin is always considered widely negative if the partial mastectomy
             extended to the pectoralis fascia and there is no tumor on ink. 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.[42]

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

          -  Neoadjuvant hormone therapy, chemotherapy, or biologic therapy is not allowed prior to
             APBI, but adjuvant hormone therapy may have been started after surgery. Planned
             chemotherapy or biologic therapy must not start for at least 4 weeks after the
             completion of APBI.

          -  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.

          -  All radiation therapy must be planned for delivery at BJH or a BJH/Siteman satellite
             location with the following stipulations: F_APBI may be delivered at any Siteman
             location. S_APBI treatment must occur at the main Siteman location at BJH and will be
             delivered on a Viewray Unit, the Varian Edge unit, or the Varian Halcyon unit. Pre and
             post treatment care is allowed at any Siteman center.

        Exclusion Criteria:

          -  Presence of distant metastases.

          -  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.

          -  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.

          -  Paget's disease of the nipple.

          -  Skin involvement, regardless of tumor size.

          -  Unsatisfactory breast for APBI 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 APBI is technically problematic.

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

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

          -  Time between final definitive breast surgical procedures to APBI simulation is greater
             than 8 weeks.