Study of 3-Day Partial Breast Radiation Therapy in Women With Breast Cancer

The purpose of this study is to determine if the dose of radiation therapy that is effective in producing a treatment response, delivered over a shorter treatment period, is a safe approach that causes few or mild side effects in women with newly diagnosed breast cancer or DCIS who have had a lumpectomy procedure.

Inclusion Criteria: * Female * Age \>/= 45 years * Unicentric pathological stage I (pT1 or T2 pN0\_M0) invasive ductal breast cancer or DCIS measuring \5cm) suspicious microcalcifications or patients with known multicentric OR multifocal disease. (microscopic multifocal disease that may be unifocal and/or appear multifocal due to sectioning is allowed after review with PI). * Prior radiation therapy to the ipsilateral or contralateral breast or thorax. * Histological evidence of extensive lymphovascular invasion (LVI) * Histological evidence of extensive intraductal component (EIC), defined as the presence of intraductal carcinoma both within the primary infiltrating ductal tumor (comprising at least 25% of the tumo area) and intraductal carcinoma present clearly beyond the edges of the invasive tumor. * Patients are not required to undergo BRCA1 and BRCA2 or other genetic mutation tests in order to enroll on the study. Note: in the event a patient is tested and is found to be a mutation carrier, she would be excluded from the study. It would be an extremely rare/unlikely scenario for patients to be discovered BRCA positive after the completion of PBI, as all patients with risk factors for BRCA mutations (positive family history, Ashkenazi Jewish descent, ER-/PR-/her2-neu-negative receptor status) are usually tested prior to radiation. Should such a situation exist, these patients will be given the option of remaining on the breast conservation paradigm or opting for mastectomy (as is done in this rare scenario is standard of care practice). The patient will be replaced on the trial. * History of cosmetic or reconstructive breast surgery. * Medical condition such as uncontrolled infection (including HIV), uncontrolled diabetes mellitus, or connective tissue diseases (lupus, systemic sclerosis, or other collagen vascular diseases) that, in the opinion of the treating physician, would make this protocol unreasonably hazardous for the patient. * Patients with a "currently active" second malignancy other than non-melanoma skin cancers. Patients are not considered to have "currently active" malignancies if they have completed therapy and are considered by their physicians to be at \