Inclusion Criteria:

   - The patient must have signed and dated an Institutional Review Board (IRB)-approved
   consent form that conforms to federal and institutional guidelines

   - The patient must have an Eastern Cooperative Oncology Group (ECOG) performance status
   of 0 or 1

   - Patient must have clinically T1-3, N1 breast cancer at the time of diagnosis (before
   neoadjuvant therapy); clinical axillary nodal involvement can be assessed by
   palpation, ultrasound, CT scan, magnetic resonance imaging (MRI), positron emission
   tomography (PET) scan, or PET/CT scan

   - Patient must have had pathologic confirmation of axillary nodal involvement at
   presentation (before neoadjuvant therapy) based on either a positive fine needle
   aspirate (FNA) (demonstrating malignant cells) or positive core needle biopsy
   (demonstrating invasive adenocarcinoma); the FNA or core needle biopsy can be
   performed either by palpation or by image guidance; documentation of axillary nodal
   positivity by sentinel node biopsy (before neoadjuvant therapy) is not permitted

   - Patients must have had estrogen receptor (ER) analysis performed on the primary breast
   tumor before neoadjuvant therapy according to current American Society of Clinical
   Oncology (ASCO)/College of American Pathologists (CAP) guideline recommendations for
   hormone receptor testing; if negative for ER, assessment of progesterone receptor
   (PgR) must also be performed according to current ASCO/CAP guideline recommendations
   for hormone receptor testing (http://www.asco.org)

   - Patients must have had HER2 testing performed on the primary breast tumor before
   neoadjuvant chemotherapy according to the current ASCO/CAP guideline recommendations
   for human epidermal growth factor receptor 2 testing in Breast Cancer
   (http://www.asco.org); patients who have a primary tumor that is either HER2-positive
   or HER2-negative are eligible

   - Patient must have completed a minimum of 8 weeks of standard neoadjuvant chemotherapy
   consisting of an anthracycline and/or taxane-based regimen

   - For patients who receive adjuvant chemotherapy after surgery, a maximum of 12 weeks of
   intended chemotherapy may be administered but must be completed before randomization;
   (if treatment delays occur, chemotherapy must be completed within 14 weeks); the dose
   and schedule of the adjuvant chemotherapy are at the investigator's discretion; Note:
   It is preferred that all intended chemotherapy be administered in the neoadjuvant
   setting

   - Patients with HER2-positive tumors must have received neoadjuvant anti-HER2 therapy
   (either with all or with a portion of the neoadjuvant chemotherapy regimen), unless
   medically contraindicated

   - At the time of definitive surgery, all removed axillary nodes must be histologically
   free from cancer; acceptable procedures for assessment of axillary nodal status at the
   time of surgery include:

      - Axillary node dissection

      - Sentinel node biopsy alone or

      - Sentinel node biopsy followed by axillary node dissection

      - Note: Patients are eligible whether there is residual invasive carcinoma in the
      surgical breast specimen or whether there is evidence of pathologic complete
      response; patients who are found to be pathologically node-positive at the time
      of surgery, based on sentinel node biopsy alone, are candidates for A011202, a
      study developed by the Alliance in Oncology, an NCI Cooperative Group; if A011202
      is open at the investigator's institution, patients should be approached about
      participating in the A011202 study

   - Patients with pathologic staging of ypN0(i+) or ypN0(mol+) are eligible (Note:
   Postneoadjuvant therapy is designated with a "yp" prefix.)

   - Patient who have undergone either a total mastectomy or a lumpectomy are eligible

   - For patients who undergo lumpectomy, the margins of the resected specimen or
   re-excision must be histologically free of invasive tumor and DCIS as determined by
   the local pathologist; additional operative procedures may be performed to obtain
   clear margins; if tumor is still present at the resected margin after re-excision(s),
   the patient must undergo total mastectomy to be eligible; (patients with margins
   positive for lobular carcinoma in situ [LCIS] are eligible without additional
   resection)

   - For patients who undergo mastectomy, the margins must be histologically free of
   residual (microscopic or gross) tumor

   - The interval between the last surgery for breast cancer (including re-excision of
   margins) and randomization must be no more than 70 days; also, if adjuvant
   chemotherapy was administered, the interval between the last chemotherapy treatment
   and randomization must be no more than 70 days

   - The patient must have recovered from surgery with the incision completely healed and
   no signs of infection

   - If adjuvant chemotherapy was administered, chemotherapy-related toxicity that may
   interfere with delivery of radiation therapy should have resolved

Exclusion Criteria:

   - Definitive clinical or radiologic evidence of metastatic disease

   - T4 tumors including inflammatory breast cancer

   - Documentation of axillary nodal positivity before neoadjuvant therapy by sentinel node
   biopsy alone

   - N2 or N3 disease detected clinically or by imaging

   - Patients with histologically positive axillary nodes post neoadjuvant therapy

   - Patients with microscopic positive margins after definitive surgery

   - Synchronous or previous contralateral invasive breast cancer or DCIS; (patients with
   synchronous and/or previous contralateral LCIS are eligible)

   - Any prior history, not including the index cancer, of ipsilateral invasive breast
   cancer or ipsilateral DCIS treated with radiation therapy; (patients with synchronous
   or previous ipsilateral LCIS are eligible)

   - History of non-breast malignancies (except for in situ cancers treated only by local
   excision and basal cell and squamous cell carcinomas of the skin) within 5 years prior
   to randomization

   - Any radiation therapy for the currently diagnosed breast cancer prior to randomization

   - Any continued use of sex hormonal therapy, e.g., birth control pills, ovarian hormone
   replacement therapy; patients are eligible if these medications are discontinued prior
   to randomization

   - Prior breast or thoracic radiation therapy (RT) for any condition

   - Active collagen vascular disease, specifically dermatomyositis with a creatinine
   phosphokinase (CPK) level above normal or with an active skin rash, systemic lupus
   erythematosus, or scleroderma

   - Pregnancy or lactation at the time of study entry; (Note: Pregnancy testing must be
   performed within 2 weeks prior to randomization according to institutional standards
   for women of childbearing potential)

   - Other non-malignant systemic disease that would preclude the patient from receiving
   study treatment or would prevent required follow-up

   - Psychiatric or addictive disorders or other conditions that, in the opinion of the
   investigator, would preclude the patient from meeting the study requirements