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 Cancer Women who satisfy all of the following conditions are the only patients who will be eligible for this study: 1. Patient must have invasive ductal, medullary, papillary, colloid (mucinous), or tubular histologies. 2. AJCC Stage I or IIA (T1N0 or T2N0) histologically confirmed invasive carcinoma of the breast with a primary lesion (less than/equal to 3 cm)by MRI or ultrasound 3. Clinically negative axillary lymph nodes. Standard routine imaging assessment of the axilla is performed by either ultrasound or MRI. If lymph nodes are suspicious, a biopsy is required. An involved axilla is purposely excluded. 4. Unifocal breast cancer (single focus which can be encompassed by one lumpectomy). 5. Patients must have an estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth receptor 2 (H2N) analyses performed on the primary tumor prior to enrollment. 6. Based on pre-treatment planning CT scan, the ratio of the PTV to the reference breast volume should not exceed 25%. 7. Patients must be greater than/equal to 18 years of age. 8. Pretreatment evaluations required for bilateral mammogram, histologic confirmation of malignancy, and physical exam. 9. Signed study-specific informed consent form prior to study entry. 10. Women of childbearing potential must use an effective contraceptive method such as condom/diaphragm and spermicidal foam, intrauterine device (IUD), or prescription birth control pills  Men are not eligible for this study. Women with one or more of the following conditions also are ineligible for this study: 1. Evidence of suspicious microcalcifications which are separate from the known lesion unless pathologically confirmed to be benign. 2. Patient with distant metastases. 3. Patients with invasive lobular or extensive in-situ lobular carcinoma or pure ductal carcinoma in-situ or non-epithelial breast malignancies such as sarcoma or lymphoma. 4. Patients with proven multicentric carcinoma (tumors in different quadrants of the breast or tumor separated by at least 4 cm) or with other clinically or radiographically suspicious areas in the ipsilateral breast unless confirmed to be negative for malignancy by biopsy. 5. Patient whose tumor is not visible on radiation treatment planning CT scan 6. Palpable or radiographically suspicious ipsilateral axillary, supraclavicular, infraclavicular or internal mammary nodes, unless there is histologic confirmation that these nodes are negative for tumor. 7. Prior hormonal or non-hormonal therapy or radiation therapy for the current breast cancer. 8. Patients with Paget's disease of the nipple. 9. Patients with skin involvement, regardless of tumor size. 10. Patients with a breast technically unsatisfactory for radiation therapy. 11. Patients with collagenous diseases, specifically systemic lupus erythematosis, scleroderma, or dermatomyositis. 12. Patients with co-existing medical conditions with life expectancy less than 2 years. 13. Patients with psychiatric or addictive disorders that would preclude obtaining informed consent. 14. Patients who are pregnant or lactating due to potential exposure of the fetus to RT and unknown effects of RT to lactating females