Safety Study of the Hypo-fractionated (Large Doses) Radiation Therapy in Post-menopausal Women With Breast Cancers

Inclusion Criteria: * Post-menopausal women (at least 2 years without menstrual period): hysterectomized patients need follicle-stimulating hormone (FSH) confirmation of post-menopausal status. * Original tumor non-palpable (mammographically detected). * Small primary tumor (pT1) breast cancer, excised with negative margins (defined as at least a 5 mm margin). * N0 or sentinel node negative or N0 clinically if the tumor is \<1 cm in size. * Patient offered six weeks of post-segmental mastectomy conventional radiation therapy and declined. * Prescribed antihormonal therapy as part of their management. Exclusion Criteria: * Previous radiation therapy to the ipsilateral breast. * Presence of a proportion of ductal carcinoma in situ (DCIS) in the pathology specimen which is compatible with extensive intraductal component (EIC). * Women incapable of providing their own consent. Mental status will be assessed by the Principal Investigator using the Radiation Therapy Oncology Group (RTOG) Mini-Mental Status Examination. * Women with a diagnosis of multifocal breast cancer.