Inclusion Criteria:

          -  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.

        Inclusion Criteria 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

          -  Oncotype RS score of 26 or higher

        OR

          -  PAM50 ROR scored as "HIGH"

        OR

          -  Presence or LVI (focal, limited or "not otherwise specified") in the lumpectomy
             specimen

        OR

          -  Age 40-49 years (ALL OF THE FOLLOWING MUST BE TRUE : 1) no history of prior benign
             breast biopsies, 2) no concomitant or prior atypia in either breast, 3) no concomitant
             or prior LCIS in either breast, 4) no family history of breast cancer in first degree
             relatives)

        OR

          -  Invasive lobular carcinoma

        OR

          -  Women 70 or older with tumors 2.0 cm or less in size and with clinically or
             pathologically negative nodes who decline or are ineligible for hormonal therapy OR
             discontinue hormonal therapy within 3 months of initiating treatment.

        Exclusion Criteria:

          -  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.

          -  Patients who are already enrolled in or planning to enroll in other adjuvant systemic
             therapy protocols for both non-invasive or invasive breast cancer

          -  Expecting to conceive within the projected duration of the trials, starting with
             screening visit through 180 days after the last dose of trial treatment

          -  Concomitant anti-neoplastic treatment is not allowed during protocol treatment and
             should be completed at least 2 weeks prior to commencement of protocol treatment, with
             resolution of associated acute toxicities. Bisphosponates are permitted without
             restriction even during protocol treatment. Note: This does not apply to hormonal
             therapies such as tamoxifen or AIs, which are permitted. This does not apply to
             anti-Her2 therapies such as trastuzumab or TDM-1, which are also permitted.

          -  Ongoing therapy with other investigational agents. Patients may not be receiving any
             other investigational agents.