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 12 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 trastuzumab or
other 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 56 days; also, if adjuvant
chemotherapy was administered, the interval between the last chemotherapy treatment
and randomization must be no more than 56 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