The objective of the BELGICA trial is to evaluate if radiotherapy could be given in a more focused manner in patients with glioblastoma in order to reduce side effects and improve quality of life. The glioblastoma (GBM) is the most common and aggressive tumour originating from the brain, affecting approximately 600 patients per year in Belgium. The treatment consists in surgical resection of the tumour (when feasible), followed by a combination of radiotherapy and chemotherapy. Despite multimodal treatment (surgery, radiotherapy, and chemotherapy), the life expectancy of patients with GBM remains limited, with an average survival of 12-18 months and only 5% of patients surviving more than 5 years. In addition to limited survival, most patients with GBM experience impaired quality of life, both because of the disease and treatments. Radiotherapy is a treatment where radiation is used to kill cancer cells. In GBM, radiotherapy is targeted at the tumour (or tumour bed if the tumour was resected) with a safety margin around it (the Clinical Target Volume or CTV) to account for potential microscopic spread of the tumour. The downside of this safety margin is that a substantial amount of brain tissue is irradiated, which can lead to treatment toxicity. Reducing the CTV margin would enable to decrease the volume of brain being irradiated and could thereby allow to reduce the side effects of brain irradiation. The BELGICA trial (Achieving a BEtter outcome through Limiting the GlIoblastoma Clinical tArget volume) is a national multicentre trial which will evaluate if reducing the irradiation volume in glioblastoma is safe and allows for lowering side effects and improving quality of life.