Single FrAction Interstitial BreaSt BracHytherApy

Inclusion Criteria: * Female * New diagnosis of Ductal Carcinoma in Situ (DCIS) and/or invasive breast carcinoma per histologic evaluation * Age 40-85 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 (HER2)-positive, the patient is planning to receive or has already received 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 (ER)+, progesterone receptor (PR)+/-, HER2- * Grade 1-2 breast cancer * Tumor ≤2 cm in size Note: If patients meet all the following criteria, the decision to avoid sentinel lymph node biopsy can be at the discretion of the breast surgeon: * 50-69 years of age * Grade 1-2 breast cancer * Clinically negative axilla by ultrasound and/or magnetic resonance imaging (MRI) * ER+, PR+, HER2- * Tumor ≤2 cm in size * Agrees to comply with aromatase inhibitor recommendation * Ability to begin brachytherapy within 12 weeks following lumpectomy, re- excision of margins, or completion of chemotherapy * Ability to understand and the willingness to sign a written informed consent document Exclusion Criteria: * Pregnant or breastfeeding * Active collagen-vascular disease * Paget's disease of the breast * Any prior history of DCIS or invasive breast cancer prior to the current diagnosis * Any prior history of breast radiation therapy (RT) or thoracic 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 * Treatment with neoadjuvant chemotherapy * Excision cavity that cannot be clearly delineated or is not amenable to implant per the treating investigator * 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