This study compares the effectiveness of treatment with the drug anastrozole before and during adjuvant radiotherapy compared with anastrozole therapy delayed until after radiotherapy. This study is for woman who have had a mastectomy or lumpectomy for breast cancer, and all planned surgery is now complete and radiotherapy is recommended. Participants will be divided into two groups. One group will receive anastrozole orally before and during radiotherapy. The other group will receive standard treatment, where anastrozole and subsequent anti-oestrogen therapy is delayed until after radiotherapy. Participants will be monitored for 10 years after radiotherapy. The study aims to determine if commencement of anastrozole before radiotherapy results in improved local control of the disease.