ASHBY: Accelerated Super-Hypofractionated Breast Brachytherapy

Inclusion Criteria: * Female * New diagnosis of ductal carcinoma in situ (DCIS) and/or invasive breast carcinoma per histologic evaluation * Age 45-79 at diagnosis * Previous lumpectomy with surgical margins histologically free of invasive tumor and DCIS as determined by the pathologist. * T stage of Tis, T1, or T2. * T2 tumors must be ≤3 cm in maximum diameter * If the tumor is human epidermal growth factor receptor 2 (HER2)-positive, the patient must receive HER2-directed therapy. * For patients with invasive breast cancer, an axillary staging procedure must be performed (either sentinel node biopsy alone or axillary dissection \[with a minimum of 6 axillary nodes removed\]) and the axillary node\[s\] must be pathologically negative. * Note: N0(i+) is not an exclusion criterion. * Note: Patients meeting all of the following criteria are not required to undergo the axillary staging procedure: * ≥70 years of age * estrogen receptor, Her2 = human epidermal growth factor receptor 2, PR = progesterone receptor negative (ER+PR+HER2-) * G1-2 * Tumor ≤2 cm in size * Agrees to comply with aromatase inhibitor recommendation * Ability to understand and the willingness to sign a written informed consent document in English Exclusion Criteria: * Pregnant or breastfeeding * Active collagen-vascular disease * Paget's disease of the breast * History of DCIS or invasive breast cancer prior to the current diagnosis * Prior breast or thoracic radiation therapy (RT) for any condition * Multicentric carcinoma (DCIS or invasive) * Synchronous bilateral invasive or non-invasive breast cancer * Surgical margins that cannot be microscopically assessed or that are positive * Excision cavity that cannot be clearly delineated per the treating investigator * Any of the dosimetric treatment criteria defined in Section 6.1 have not been met. Patients who become ineligible due to inability to meet dosimetric criteria should not receive treatment as - defined in this protocol and will come off the study. Any subsequent adjuvant radiation will be delivered at the discretion of the treating physician * Medical, psychological, or social condition that, in the opinion of the investigator, may increase the patient's risk or limit the patient's adherence with study requirements