A Randomized Phase III Clinical Trial Evaluating Post-Mastectomy Chestwall and Regional Nodal XRT and Post-Lumpectomy Regional Nodal XRT in Patients With Positive Axillary Nodes Before Neoadjuvant Chemotherapy Who Convert to Pathologically Negative Axillary Nodes After Neoadjuvant Chemotherapy

• 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