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,  Ductal Breast Carcinoma In Situ Invasive Breast Carcinoma Stage 0 Breast Cancer Stage I Breast Cancer Stage IA Breast Cancer Stage IB Breast Cancer Stage II Breast Cancer Stage IIA Breast Cancer Stage IIB Breast Cancer Histologically confirmed invasive carcinoma and/or ductal carcinoma in situ (DCIS) of the breast Final pathologic Tis, T1-T3, all must be N0 and M0 status Negative inked histologic margins from lumpectomy, with the exception of a focus of positive margin at the pectoralis fascia Radiation oncologist does not plan to treat regional lymph nodes beyond standard whole breast tangent fields Lumpectomy with negative lymph node on surgical evaluation (Isolated tumor cells in lymph nodes will be permitted). Patients with invasive carcinoma ≥ 70 yrs and with ER+ positive tumor ≤ 2.0cm may enroll without surgical lymph node evaluation, per section 5.1. Patients with Ductal Carcinoma In Situ (DCIS) of the breast only may enroll without surgical lymph node evaluation Negative serum or urine beta-human chorionic gonadotropin (HCG) in women of child-bearing potential =< 7 days prior to registration A female of childbearing potential is a sexually mature female who has not undergone a hysterectomy or bilateral oophorectomy and has not been naturally postmenopausal for at least 12 consecutive months Women of child-bearing potential must agree to utilize a form of birth control or agree to undergo sexual abstinence during radiation therapy Eastern Cooperative Oncology Group (ECOG) (Zubrod) performance status 0-1 Able to provide informed consent and willing to sign an approved consent form that conforms to federal and institutional guidelines  Prior radiation therapy to the chest, neck or axilla Prior history of ipsilateral breast cancer (invasive disease or DCIS); lobular carcinoma in situ (LCIS) and benign breast disease is allowed History of prior or concurrent contralateral invasive breast cancer; benign breast disease, LCIS or DCIS of contralateral breast is allowed Active collagen vascular diseases, such as: systemic lupus erythematous, scleroderma, or dermatomyositis Significant post lumpectomy complications requiring an unplanned re-operation or admission for intravenous (IV) antibiotics; re-operation for margins evaluation or nodal evaluation is acceptable Co-existing medical conditions with life expectancy < 5 years Other malignancy within 5 years of registration with the exception of basal cell or squamous cell carcinoma of the skin treated with local resection only or carcinoma in situ of the cervix Neoadjuvant chemotherapy or adjuvant chemotherapy delivered before radiation Neuroendocrine carcinoma or sarcoma histology Concurrent radiation sensitizing medications concurrent with radiation, per treatment physician