Radiation Doses and Fractionation Schedules in Non-low Risk Ductal Carcinoma In Situ (DCIS) of the Breast

Inclusion Criteria: Patients must fulfill all of the following criteria for admission to study: * Women ≥ 18 years. * Histologically proven DCIS of the breast without an invasive component. * Bilateral mammograms performed within 6 months prior to randomization. * Clinically node-negative. * Treated by breast conserving surgery (primary excision or re-excision) with complete microscopic excision and clear radial margins of ≥1 mm\* (\*Patients with superficial or deep resection margin of \<1 mm are eligible if surgery has removed all of the intervening breast tissue from the subcutaneous tissue to the pectoralis fascia). * Women who are at high risk of local recurrence due to: * Age \< 50 years; OR * Age ≥ 50 years plus at least one of the following: * Symptomatic presentation * Palpable tumour * Multifocal disease * Microscopic tumour size ≥ 1.5 cm in maximum dimension * Intermediate or high nuclear grade * Central necrosis * Comedo histology * Radial\* surgical resection margin \< 10 mm. (\*Patients with superficial or deep resection margin of \< 10 mm are eligible if surgery has not removed all of the intervening breast tissue from the subcutaneous tissue to the pectoralis fascia.) * Assessed by surgeon and radiation oncologist to be suitable for breast conserving therapy including whole breast RT. * Ability to tolerate protocol treatment. * Protocol RT should preferably commence within 8 weeks but must commence no later than 12 weeks from the last surgical procedure. * ECOG performance status 0, 1 or 2. * Patient's life expectancy \> 5 years. * Availability for long-term follow-up. * Written informed consent. Exclusion Criteria: Patients who fulfill any of the following criteria are not eligible for admission to study: * Multicentric disease or extensive microcalcifications that could not be completely excised by breast conserving surgery with radial margins of ≥1 mm\*. \*Patients with superficial and/or deep margin of \<1 mm are eligible if surgery has removed all of the intervening breast tissue from the subcutaneous tissue to the pectoralis fascia. * Presence of tumour cells in lymph nodes detected using H\&E or immunohistochemical examination (if lymph node biopsy or dissection has been performed). * Locally recurrent breast cancer. * Previous DCIS or invasive cancer of the contralateral breast. * Bilateral DCIS of the breasts * Synchronous invasive carcinoma of the contralateral breast * Other concurrent or previous malignancies except: * Non-melanomatous skin cancer; * Carcinoma in situ of the cervix or endometrium; and * Invasive carcinoma of the cervix, endometrium, colon, thyroid and melanoma treated at least five years prior to study admission without disease recurrence. * Serious non-malignant disease that precludes definitive surgical or radiation treatment (e.g., scleroderma, systemic lupus erythematosus, cardiovascular/pulmonary/renal disease). * ECOG performance status ≥ 3. * Women who are pregnant or lactating.