5 vs 9-day Whole Breast Radiation for Early-Stage Breast Cancer

Inclusion Criteria: * Female participant aged ≥ 18 years. --Participants must have at least one of the following risk factors: * Grade 3 invasive histology * Estrogen receptor positivity less than 5% * Lymphovascular invasion * Invasive margins \<2mm on surgical pathology * DCIS final positive margin * Extensive intraductal component * Age ≤ 50 years * Tumor size \> 2 cm * Histologically confirmed invasive carcinoma or Ductal Carcinoma In Situ (DCIS) of the breast. * Breast cancer stage (AJCC v8) T0-3, N0, M0. T0 N0 is allowed if whole breast radiation is recommended by the treating physician. * Lumpectomy within 84 days of the start of radiation. * ECOG Performance Status ≤ 2, or KPS ≥ 50 * Negative inked histologic margins from lumpectomy, with the exception of a focus of positive margin at the pectoralis fascia. * Negative pregnancy test for participants of childbearing potential, evidence of permanent surgical sterilization, or post-menopausal status. The post-menopausal status will be defined as having been amenorrheic for 12 months without an alternative medical cause. The following age-specific requirements apply: * \< 50 years of age: * Amenorrheic for ≥ 12 months following cessation of exogenous hormonal treatments; and * Luteinizing hormone and follicle-stimulating hormone levels in the post-menopausal range for the institution * ≥ 50 years of age: * Amenorrheic for 12 months or more following cessation of all exogenous hormonal treatments; or * Had radiation-induced menopause with last menses \>1 year ago; or * Had chemotherapy-induced menopause with last menses \>1 year ago * Sexually active participants of childbearing potential must agree to use highly effective method of contraception (defined in Section 5.4.1) during the course of radiation and for 30 days after radiation. * Able to provide informed consent and willing to sign an approved consent form that conforms to federal and institutional guidelines. Exclusion Criteria: * Bilateral breast cancer. * Prior radiation therapy to the chest. * Prior chemotherapy. * Recurrent disease. * Known metastases or node positive. * Major chest surgery which is expected to impact study participation 8 weeks prior to starting study drug. * Prior breast malignancy in either breast. * The diagnosis of any other malignancy which, in the opinion of the Investigator, is likely to negatively impact the subject's safety or ability to participate in the study. * Current evidence of uncontrolled, significant intercurrent illness including, but not limited to, the following conditions: * Cardiovascular disorders: * Congestive heart failure New York Heart Association Class III or IV, unstable angina pectoris, serious cardiac arrhythmias. * Stroke (including transient ischemic attack \[TIA\]), myocardial infarction (MI), or other ischemic events, or thromboembolic event (e.g., deep venous thrombosis, pulmonary embolism) within 3 months before the first dose. * Any other condition that would, in the Investigator's judgment, contraindicate the participant's participation in the clinical study due to safety concerns or compliance with clinical study procedures (e.g., infection/inflammation, intestinal obstruction, unable to swallow, social/ psychological issues, etc.) * Significant post lumpectomy complications requiring an unplanned re-operation for surgical complications or admission for IV antibiotics. Re-operation for margins evaluation or nodal evaluation is acceptable. Draining of a seroma is not considered a complication unless it has become infected. * Breast neuroendocrine carcinoma or sarcoma histology. * Radiation sensitizing disease or condition (e.g. connective tissue disease, li fremani, etc.). * Medical, psychiatric, cognitive, or other conditions that may compromise the participant's ability to understand the participant information, give informed consent, comply with the study protocol or complete the study. * Participants receiving concurrent radiation sensitizing medications or therapies.