Three Fraction Accelerated Partial Breast Irradiation as the Sole Method of Radiation Therapy for Low-risk Stage 0 and I Breast Carcinoma

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). * Neoadjuvant hormone therapy, chemotherapy, or biologic therapy is not allowed prior to TRI-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 TRI-APBI. * Good candidate for treatment per protocol in the judgment of the PI and/or treating physician following simulation. * Postmenopausal status. * Age …