Single Fraction Accelerated Partial Breast Irradiation vs. Five Fraction Accelerated Partial Breast Irradiation 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 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 surg…