Single FrAction Interstitial BreaSt BracHytherApy

Assess and compare the toxicity of a 1-day course of accelerated partial breast irradiation (APBI) to that of clinical trials: NSABP-B-39 (NCT00103181), RTOG-0413, TRIUMPH-T (NCT02526498), and ASHBY (NCT06185205)

Incidence of accelerated partial breast irradiation (APBI) related toxicities of interest occurring at grade ≥3 at 1, 3, and 5 years after APBI. The descriptions and grading scales found in the The National Cancer Institutes' (NCI) Common Terminology Criteria for Adverse Events is a descriptive terminology (CTCAE), a list of standardized definitions for adverse events, revised v6.0 will be utilized for adverse (AE) (toxicity) reporting.

Assess the rate of excellent or good cosmesis at 1, 3, and 5 years after a 1-day course of APBI utilizing the Breast Cancer Treatment Outcome Scale (BCTOS-12)

Cosmetic results at 1, 3, and 5 years after APBI, as assessed by The Breast Cancer Treatment Outcome Scale (BCTOS-12), a shortened, validated patient-reported outcome measure designed to assess aesthetic and functional outcomes after breast-conserving surgery. This is scored on a scale of 1-4 ("no difference" to "large difference"), where lower scores indicate better outcomes of satisfaction

Assess the rate of excellent or good cosmesis at 1, 3, and 5 years after a 1-day course of APBI utilizing the breast cancer core checklist (postoperative) (BREAST-Q)

The BREAST-Q radiation-specific module patient-reported outcome measure (PROM) that evaluates patient satisfaction with the cosmetic effects of radiation The Breast-Q is 6 questions and scored on a scale of 1-3 ("not at all" to "a lot") with lower scores indicating better outcomes of satisfaction.

Assess the rate of excellent or good cosmesis after a 1-day course of APBI utilizing the Harvard Cosmesis Scale

The Harvard Cosmesis Scale is a physician-rated form will be used by the radiation oncologist to score cosmesis with the following criteria: * Excellent: treated breast looks essentially the same as the contralateral breast as it relates to radiation effects. This includes patients with grade 1 punctate scarring. * Good: minimal but identifiable radiation effects of the treated breast. This includes patients with grade 2 punctate scarring. * Fair: significant radiation effects readily observable. * Poor: severe sequelae of breast tissue secondary to radiation effect.

Assess the rate of excellent or good cosmesis at 1, 3, and 5 years after a 1-day course of APBI assessed by Digital Photographs

Digital photographs will be taken to visualize any potential changes in breast shape or contour. These will be taken by study staff utilizing the Radiation Oncology Department's designated camera. blindly analyzed by 2-3 radiation oncologists using the following 4-point numerical scale: * 4 (Excellent): the treated breast looks essentially the same as the contralateral breast as it relates to radiation effects. This includes patients with grade 1 punctate scarring. * 3 (Good): minimal but identifiable radiation effects of the treated breast. This includes patients with grade 2 punctate scarring. * 2 (Fair): significant radiation effects readily observable. * 1 (Poor): severe sequelae of breast tissue secondary to radiation effect. Photos will be uploaded to the digital study record. Analysis will be batched as study progresses. Each individual score as well as the average score will be entered into the electronic case report form (eCRF).