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.
