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): 40.0-999.0,  Breast Cancer Pathologically confirmed ductal carcinoma in situ of the breast or early invasive breast cancer defined as pathologic stage Tis, T1, or T2, N0, N1mic, or N1a (pathologic staging of the axilla is required for all patients with invasive disease but is not required for patients with DCIS only). (Upfront pathologic stage cannot be assigned to patients treated with neoadjuvant chemotherapy. For such patients, the for pathologic stage shall be applied to the initial clinical stage) 2. Treatment with breast conserving surgery. 3. Final surgical margins must be negative, defined as no evidence for ductal carcinoma in situ or invasive breast cancer touching the inked surgical margin. If the invasive or in situ breast cancer approaches within less than 1 mm of the final surgical margin, then a reexcision is strongly encouraged. Lobular carcinoma in situ at the final surgical margin will be disregarded. 4. Age 40 years or older. This age cutoff is justified because breast cancers in women under the age of 40 are known to have a significantly higher risk of IBTR presumably due to underlying biologic differences. 5. Female sex. 6. Attending radiation oncologist declares intention to treat the whole breast only and that a third radiation field to treat regional lymph nodes is not planned (radiation of the undissected level I/II axilla with high tangents is allowed). 7. If the patient has a history of a prior non-breast cancer, all treatment for this cancer must have been completed prior to study registration and the patient must have no evidence of disease for this prior non-breast cancer. 8. Patients must be enrolled on the trial within 12 weeks of the later of two dates: the final breast conserving surgical procedure or administration of the last cycle of cytotoxic chemotherapy  Pathologic or clinical evidence for a stage T3 or T4 breast cancer. 2. Pathologic evidence for involvement of 4 or more axillary lymph nodes, or imaging evidence of involvement of infraclavicular, supraclavicular, or internal mammary lymph nodes. 3. Clinical or pathologic evidence for distant metastases. 4. Any prior diagnosis of invasive or ductal carcinoma in situ breast cancer in either breast. 5. Current diagnosis of bilateral breast cancer. 6. History of therapeutic irradiation to the breast, lower neck, mediastinum or other area in which there could potentially be overlap with the affected breast. 7. Patients not fluent in English or Spanish. (The Informed Consent will be available in these two languages) 8. Patient is pregnant