Patient description: The patient is a 42 year-old postmenopausal woman who had a screening sonogram which revealed an abnormality in the right breast. She had no palpable masses on breast exam. Core biopsy was done and revealed a 1.8 cm infiltrating ductal breast carcinoma in the left upper outer quadrant. Lumpectomy was done and the surgical margins were clear. The tumor was HER2-positive and ER/PR negative. Axillary sampling revealed 1 positive lymph node out of 12 sampled. CXR was unremarkable. She is using “well women” multivitamins daily and no other medication. She smokes frequently and consumes alcohol occasionally. She is in a relation with only one partner and has a history of 3 pregnancies and live births. She breastfed all three children.

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  Breast Neoplasms Histologic Documentation Patients will have histologically confirmed Unicentric Stage I (T1 N0 M0) invasive ductal breast cancer Histologically negative tumor margin 2 mm or more from any inked edges, or no tumor in a re-excision specimen or final shaved specimen Tubular, mucinous and medullary variant histologies of infiltrating ductal carcinoma are permitted Low-grade DCIS (I and II) of 2 cm or less with histologically negative margins of at least 2 mm margins (or a negative re-excision) are permitted Women age 70 years or older with T1 invasive ductal carcinoma which are estrogen-receptor positive (ER+) with clinically negative axillary nodes who do not undergo surgical lymph node evaluation are also eligible if patient will take hormonal therapy Patients with T1N0 (i+) tumors on sentinel lymph node mapping or dissection (i.e. is tumor deposit is 0.2mm or less, regardless of whether the deposit is detected by IHC or H&E staining) will also be eligible, provided that completion axillary dissection has been performed to confirm N0 status In the case where invasive cancer is present, the invasive cancer's pathology will be used regardless if DCIS is also present. 2. Prior Treatment: Patient may have been treated with adjuvant chemotherapy. Patients may be on adjuvant hormonal therapy or begin hormonal therapy following XRT at the discretion of the medical oncologist. Radiation therapy should begin within 12 weeks from definitive surgical procedure 6 weeks after chemotherapy, if chemotherapy given first  The following guidelines are to assist physicians in selecting patients for whom protocol therapy is safe and appropriate. Physicians should recognize that the following may seriously increase the risk to the patient entering this protocol. Patients who meet the following should not be entered in this study: 1a Multicentric IDC of the breast defined as discontiguous tumors separated by at least 5 cm of uninvolved tissue; alternatively, discontiguous tumors that are clinically or mammographically within separate breast quadrants or subareolar central region. 1. b Multifocal IDC of the breast, defined as discontiguous discrete foci of invasive carcinoma, separated by uninvolved intervening tissue, but within an overall span of 5cm, or within the same breast quadrant or subareolar central region. 2. Tumor > 2.0 cm, nodal involvement on H&E staining, or metastatic involvement 3. Histological evidence of: 1. Lymphovascular invasion: as defined by a tumor embolus present in an endothelial-lined space; cases with tumor emboli present in a space not lined by endothelial cells but otherwise very suspicious for an angiolymphatic space were also considered ineligible. 2. EIC (Extensive Intraductal Component): defined as the presence of intraductal carcinoma both within the primary infiltrating ductal tumor (comprising at least 25% of the tumor area) and intraductal carcinoma present clearly beyond the edges of the invasive tumor, or as a predominantly intraductal tumor with one or more areas of focal invasion 7, 55. 3. Invasive Lobular Carcinoma 4. Infiltrating carcinoma with mixed ductal and lobular features: cases with ambiguous or mixed histologic features that showed positive E-cadherin staining throughout the tumor by immunohistochemistry were classified as ductal type and considered eligible 56, 57. 5. Infiltrating papillary carcinoma 6. Margins: In-situ or invasive carcinoma present less than 2 mm from the inked resection margin. 4. History of cosmetic or reconstructive breast surgery 5. Psychiatric illness which would prevent the patient from giving informed consent. Medical condition such as uncontrolled infection (including HIV), uncontrolled diabetes mellitus or connective tissue diseases (lupus, systemic sclerosis or other collagen vascular diseases) which, in the opinion of the treating physician, would make this protocol unreasonably hazardous for the patient. 6. Patients with a "currently active" second malignancy other than non-melanoma skin cancers. Patients are not considered to have a "currently active" malignancy if they have completed therapy and considered by their physician to be at less than 5% risk of relapse within three years. 7. Patients with diffuse (> 1 quadrant or > 5cm) suspicious microcalcifications 8. Women who are pregnant