Preoperative Irradiation for Stage I Breast Cancer

Inclusion Criteria: 1. Pathologically confirmed diagnosis of breast cancer within 90 days from breast biopsy. NOTE: The day of biopsy is Day "0" 2. Patient biologically of the female sex 3. Must meet ALL the following criteria: 3.1. Age ≥ 50 years 3.2. Clinical size ≤ 3 cm (invasive) or ≤ 4 cm (DCIS) based on results of dedicated breast imaging. 3.3. All invasive subtypes and DCIS 3.4. ER positive, HER2/neu negative 3.5. No LVSI 3.6. Clinically negative lymph nodes based on results of dedicated breast imaging 4. The gross tumor should be \>1cm from the chest wall and the skin surface 5. Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 2 within 90 days prior to registration 6. Patients of child-bearing age should have either a negative urine or serum pregnancy test within 14 days of study entry. 7. Patients of child-bearing age should be non-pregnant and non-lactating. They should use a medically acceptable form of contraception during RT. 8. Patients must have signed this study's informed consent prior to study entry. Exclusion Criteria: 1. Clinically staged breast cancer that does not meet all of the criteria delineated in Inclusion criteria 3. 2. Prior invasive non-breast malignancy (except non-melanomatous skin cancer, carcinoma in situ of the cervix) unless disease free for a minimum of 5 years prior to study entry 3. Prior invasive non-breast malignancy (except non-melanomatous skin cancer, carcinoma in situ of the cervix, melanoma in situ) unless disease free for a minimum of 5 years prior to study entry 4. Non-epithelial breast malignancies such as sarcoma or lymphoma. 5. Two or more documented breast cancers within the index breast or bilateral breast cancer. 6. Suspicious unresected microcalcifications, densities, or palpable abnormalities (in the ipsilateral or contralateral breast) unless biopsied and found to be benign. 7. Paget's disease of the nipple. 8. Male breast cancer. 9. Evidence of distant metastases. 10. Clinical regional lymph node involvement. 11. Prior RT to the region of the breast that would result in overlap of RT fields. 12. Intention to administer concurrent chemotherapy for current breast cancer. 13. Severe, active co-morbidity, defined as follows: 13.1. Unstable angina and/or congestive heart failure requiring hospitalization within the last 6 months 13.2. Transmural myocardial infarction within the last 6 months 13.3. Acute bacterial or fungal infection requiring intravenous antibiotics at the time of registration 13.4. Chronic Obstructive Pulmonary Disease exacerbation or other respiratory illness requiring hospitalization or precluding study therapy within 30 days before registration 13.5. Hepatic insufficiency resulting in clinical jaundice and/or coagulation defects; note, however, that laboratory tests for liver function and coagulation parameters are not required for entry into this protocol 13.6. Acquired Immune Deficiency Syndrome (AIDS) based upon current CDC definition; note, however, that HIV testing is not required for entry into this protocol. The need to exclude patients with AIDS from this protocol is necessary because the treatments involved in this protocol may be significantly immunosuppressive. Protocol-specific requirements may also exclude immunocompromised patients. 14. Active connective tissue disorders/collagen vascular disease, specifically dermatomyositis with a CPK level above normal or with an active skin rash, systemic lupus erythematosis, or scleroderma. 15. Medical, psychiatric or other condition that would prevent the patient from receiving the protocol therapy or providing informed consent. 16. Unable to delineate tumor on pre-RT MRI and CT simulation scan.