Patient description: The patient is a 42 year-old postmenopausal woman who had a screening sonogram which revealed an abnormality in the right breast. She had no palpable masses on breast exam. Core biopsy was done and revealed a 1.8 cm infiltrating ductal breast carcinoma in the left upper outer quadrant. Lumpectomy was done and the surgical margins were clear. The tumor was HER2-positive and ER/PR negative. Axillary sampling revealed 1 positive lymph node out of 12 sampled. CXR was unremarkable. She is using “well women” multivitamins daily and no other medication. She smokes frequently and consumes alcohol occasionally. She is in a relation with only one partner and has a history of 3 pregnancies and live births. She breastfed all three children.

Clinical trial eligibility criteria: eligible ages (years): 45.0-999.0,  Invasive Ductal Breast Carcinoma Invasive Ductal Carcinoma, Breast DCIS DCIS Grade 1 DCIS Grade 2 Breast Cancer Female Age >/= 45 years Unicentric pathological stage I (pT1 or T2 pN0_M0) invasive ductal breast cancer or DCIS measuring <3 cm in longest diameter on pathology and/or mammogram that is histologically confirmed. If T2, the tumor must be less than 3cm in longest diameter. Note: Women ≥ 70 years or older with T1 invasive ductal carcinoma who are estrogen- receptor positive (ER+) with clinically negative axillary nodes, and do not undergo surgical lymph node mapping or dissection (i.e., if tumor deposit is 0.2 mm or less, regardless of whether the deposit is detected by immunohistochemistry or hematoxylin and eosin staining) will also be eligible Histologically negative tumor margin or no tumor in a re-excision specimen on final shaved specimen ECOG Performance Status of 0 or 1 Clips must be placed in the lumpectomy cavity at the time of final excision in order to aid in the delineation of the tumor cavity at the time of simulation and radiation delivery Negative serum pregnancy test within 14 days prior to study treatment if a woman has child-bearing potential. Subjects of child bearing potential are those who have not been surgically sterilized or have not been free from menses for > 1 year Written informed consent obtained from subject and ability for subject or comply with the requirements of the study Female subjects of childbearing potential should be willing to use 2 methods or birth control or be surgically sterile or abstain from heterosexual activity for the duration of study participation. Should a woman become pregnant while participating on study, she should inform the treating physician immediately. for intermediate risk substudy Post-NAC cohort patients: clinical T1 or T2 (less than or equal to 5cm in longest diameter on available imaging) and clinical N0 (on exam and imaging) with pathological pT0 or pTis and pN0 (any regimen of neoadjuvant chemotherapy agents is allowed) OR  Patients with distant metastasis Patients who are pregnant or breastfeeding Patients with diffuse (>1 quadrant or >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 <5% risk of relapse within 3 years