KORTUC Phase II - Intra-tumoural Radiation Sensitizer in Patients With Locally Advanced/Recurrent Breast Cancer

This study is testing a hydrogen peroxide-based injection given directly into locally advanced or recurrent breast tumors to make them more sensitive to radiation therapy. The goal is to boost how well radiation works against tumors that are difficult to treat surgically, reducing the chance they grow back in the same area. **You may be eligible if...** - You are 18 years or older - You have locally advanced breast cancer (primary) or breast cancer that has come back locally, with or without stable spread to other sites - Your cancer requires high-dose radiation for long-term local control - You are in good enough health to receive radiation therapy **You may NOT be eligible if...** - Your cancer has spread widely and is progressing quickly - You have had prior radiation to the same breast area at doses that would not allow further treatment - You have serious medical conditions that make radiation or injection unsafe - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.