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): 50.0-999.0,  Breast Neoplasms The patient must consent to be in the study and must have a signed an approved consent form conforming with institutional guidelines Patient must be > 50 years old The patient should have a life expectancy of at least two years The patient must have stage 0 , I , or II breast cancer. If stage IIA, the tumor size must be 3 cm or less and the patients must be node negative On histological examination, the tumor must be 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 (>2mm, DCIS and invasive). Re-excision 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 negative), sentinel node biopsy followed by axillary dissection or sampling with a minimum total of 6 axillary nodes or axillary dissection alone (with a minimum of 6 axillary nodes). (Axillary staging is NOT required for patients with DCIS.) The patient must have the MammoSite catheter placed within 4 weeks or 28 days of the final surgery for their breast cancer (lumpectomy, re-excision of margins, or axillary staging procedure). Placement of a spacer for the MammoSite catheter is permitted at their final surgery Patients with a history of non-ipsilateral breast malignancies are eligible if they have been disease-free for 2 or more years prior to randomization. Patients with the following cancers are eligible even if diagnosed and treated within the past 2 years: carcinoma in situ of the cervix, colon, melanoma in situ, and basal cell and squamous cell carcinoma of the skin. Patients with a prior diagnosis of ipsilateral breast cancer are ineligible  Men are not eligible for this study T2 (>3.0 cm), T3, node positive, stage III or IV breast cancer Any positive axillary nodes Palpable or radiographically suspicious ipsilateral or contralateral axillary, supraclavicular, infraclavicular or internal mammary nodes, unless biopsy proven to be negative for tumor Suspicious microcalcifications, densities or palpable abnormalities in either breast unless biopsy proven to be benign Non-epithelial breast malignancies such as sarcoma or lymphoma Proven multicentric carcinoma in more than one quadrant or separated by more than 3 centimeters Paget's disease of the nipple History of invasive breast cancer or DCIS in the same breast Surgical margins that cannot be microscopically assessed or are less then 2 mm