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 Adenocarcinoma Ductal Breast Carcinoma In Situ Invasive Breast Carcinoma Stage 0 Breast Cancer AJCC v6 and v7 Stage I Breast Cancer AJCC v7 Stage IA Breast Cancer AJCC v7 Stage IB Breast Cancer AJCC v7 Stage II Breast Cancer AJCC v6 and v7 Stage IIA Breast Cancer AJCC v6 and v7 Stage IIB Breast Cancer AJCC v6 and v7 The patient must consent to be in the study and must have signed an approved consent form conforming with federal and institutional guidelines The patient must have stage 0, I, or II breast cancer; if stage II, the tumor size must be 3 cm or less On histological examination, the tumor must be ductal carcinoma in situ (DCIS) or invasive adenocarcinoma of the breast Surgical treatment of the breast must have been lumpectomy; the margins of the resected specimen must be histologically free of tumor (DCIS and invasive); reexcision of surgical margins is permitted Gross disease must be unifocal with pathologic (invasive and/or DCIS) tumor size 3 cm or less; (patients with microscopic multifocality are eligible as long as total pathologic tumor size is 3 cm or less) Patients with invasive breast cancer are required to have axillary staging which can sentinel node biopsy alone (if sentinel node is negative), sentinel node biopsy followed by axillary dissection or sampling with a minimum total of 6 axillary nodes (if sentinel node is positive), or axillary dissection alone (with a minimum of 6 axillary nodes); axillary staging is not required for patients with DCIS The target lumpectomy cavity must be clearly delineated and the target lumpectomy cavity/whole breast reference volume must be less than or equal to 30% based on the postoperative/pre-enrollment computed tomography (CT) scan Patients are eligible if, based on the postoperative CT scan, partial breast irradiation (PBI) is judged to be technically deliverable Patients with a history of non-breast malignancies are eligible if they have been disease-free for 5 or more years prior to enrollment and are deemed by their physician to be at low risk for recurrence; patients with the following cancers are eligible if diagnosed and treated within the past 5 years; carcinoma in situ of the cervix, carcinoma in situ of the colon, melanoma in situ, and basal cell and squamous cell carcinoma of the skin  Men are not eligible for this study T2 (> 3.0 cm), T3, stage III, or stage IV breast cancer More than 3 histologically positive axillary nodes Axillary nodes with definite evidence of microscopic or macroscopic extracapsular extension Palpable or radiographically suspicious ipsilateral or contralateral axillary, supraclavicular, infraclavicular, or internal mammary nodes at time of enrollment unless there is histologic confirmation that these nodes are negative for tumor Suspicious microcalcifications, or densities (in the ipsilateral or contralateral breast as documented on mammogram or breast ultrasound) unless biopsied and found to be benign Non-epithelial breast malignancies such as sarcoma or lymphoma Proven multicentric carcinoma (invasive cancer or DCIS) in more than one quadrant or separated by 4 or more centimeters Paget's disease of the nipple Synchronous bilateral invasive or non-invasive breast cancer