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): 40.0-999.0,  Breast Cancer . Female patient with a new histological diagnosis of DCIS only. OR 1b. Female patient with a new histological diagnosis of invasive carcinoma of the breast and 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 micrometastasis <= 0.2mm or positive cells only identified by IHC as determined either by: (i) sentinel node biopsy (ii) axillary node dissection or (iii) clinical exam for patients with DCIS only  Age < 40 years. 2. A known deleterious mutation in BRCA 1 and/or BRCA 2. 3. Tumour size > 3 cm in greatest diameter on pathological examination (including both the invasive and non-invasive component). 4. Tumour histology limited to lobular carcinoma only. 5. History of cancer Patients with another active malignancy or malignancy treated < 5 years prior to randomization are excluded Patients with a prior diagnosis of invasive or non-invasive breast cancer in either breast are excluded regardless of disease free interval. Patients with concurrent invasive or non-invasive contralateral breast cancer are also excluded Patients with prior or concurrent basal cell or squamous cell skin cancers are eligible for the trial. 6. More than one primary tumour in different quadrants of the same breast. 7. Previous irradiation to the ipsilateral breast that would preclude whole breast irradiation. 8. Presence of an ipsilateral breast implant or pacemaker. 9. Serious non-malignant disease (e.g. cardiovascular, pulmonary, systemic lupus erythematosus (SLE), scleroderma) which would preclude definitive radiation treatment. 10. Estrogen receptor status (ER) not known. 11. For patients not treated with adjuvant chemotherapy: unable to commence radiation therapy within 12 weeks of the last surgical procedure on the breast. 12. For patients treated with adjuvant chemotherapy: unable to commence within 8 weeks of the last dose of chemotherapy. 13. Currently pregnant or lactating. 14. Psychiatric or addictive disorders which would preclude obtaining informed consent or adherence to protocol. 15. Geographic inaccessibility for follow-up. 16. Inability to localize surgical cavity on CT (i.e., no evidence of surgical clips or seroma). 17. Inability to adequately plan the patient for the experimental technique