In this clinical trial the investigators combine radiation therapy with a low-carbohydrate diet, in patients with recurrent brain tumours. In addition, subjects will receive medication with metformin, a drug usually used to treat diabetes. Metformin inhibits glucose metabolism within cancer cells, and additionally has reported intrinsic anti-cancer activity. Subjects will undergo advanced imaging and hormonal studies before, during and after the trial in order to obtain maximal translational-scientific impact.