This is a study aimed at testing a commonly available and inexpensive chemical (hydrogen peroxide) for safety and activity in sensitising large cancerous lumps in the breast to a standard course of radiotherapy in women with poorly controlled symptoms. Laboratory research and initial clinical trials in Japan suggest that 4 to 6 injections of a radiation sensitiser ('KORTUC') based on very dilute (0.5%) hydrogen peroxide injected into cancers under local anaesthetic twice a week during radiotherapy greatly increases the effectiveness of standard doses of radiotherapy alone. The side effects are limited to mild/moderate discomfort at the injection site for up to 24 hours reported by Japanese breast cancer patients in whom this treatment has been tested. Complete tumour shrinkage in 70/71 (98%) primary breast cancers up to 5 cm diameter have been reported by Japanese collaborators.