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