Trial of Radiation Fractionation Schedules for Once-a-Day Accelerated Partial Breast Irradiation

Inclusion Criteria: 1. Females with a new histological diagnosis of DCIS or invasive carcinoma of the breast with no evidence of metastatic disease. 2. Treated by BCS with microscopically clear resection margins for invasive and non-invasive disease (or no residual disease on re-excision). 3. Negative axillary node involvement, including positive cells only identified by Immunohistochemistry (IHC) as determined either by sentinel node biopsy or axillary node dissection, or a clinical exam for patients with DCIS only. Exclusion Criteria: 1. Age less than 50 years 2. Known to be Breast Cancer Type 1 Susceptibility Protein (BRCA 1) and/or Breast Cancer Type 2 Susceptibility Protein (BRCA 2) positive 3. Tumour size \>3 cm in greatest diameter on pathological examination (including the invasive and non-invasive components) 4. Tumour invades the skin (i.e. T4 disease) 5. Tumour histology limited to lobular carcinoma only 6. Grade III invasive breast carcinoma or nuclear grade III for patients with DCIS alone 7. Triple negative invasive breast cancer 8. Her2+ve invasive breast cancer that will not receive Herceptin 9. Bilateral invasive malignancy of the breast (synchronous or metachronous) 10. More than one primary tumor in different quadrants of the same breast 11. Presence of an ipsilateral breast implant or pacemaker 12. Status for adjuvant systemic therapy not determined 13. Unable to commence radiation therapy: 1. within 12 weeks of the last surgical procedure on the breast, or 2. within 8 weeks of the last dose of chemotherapy 14. Previous or concurrent malignancies except: 1. Non-melanoma skin cancer 2. Carcinoma in-situ of the cervix 3. Invasive carcinoma of the cervix, endometrium, colon and thyroid, treated 5 years prior to study entry with no evidence of disease 15. Serious non-malignant disease (e.g. cardiovascular, pulmonary, systemic lupus erythematosus (SLE), scleroderma) which would preclude definitive radiation treatment. 16. Current pregnancy or lactating. 17. Psychiatric or addictive disorders which would preclude obtaining informed consent or adherence to the protocol. 18. Geographic inaccessibility for follow-up. 19. Inability to localize surgical cavity on CT (i.e. no evidence of surgical clips or seroma) 20. Inability to adequately plan the patient for the experimental technique.