This study focuses on children and young people with high-grade brain tumours (gliomas) that have specific problems in how they repair DNA. It is testing a new treatment approach using an immunotherapy drug called nivolumab to help the body’s immune system recognise and attack the cancer earlier, before radiation is given to the child. Because the study is small and exploratory, its main goal is to assess how long children are treated without radiation and whether this approach is safe and shows early signs of effectiveness. Researchers hope that delaying or reducing the need for radiation treatment could improve long-term outcomes and quality of life for these patients.