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.