The invention relates to the use of biodegradable microspheres that release a radiosensitizing anticancer agent for producing a medicament to be used simultaneously with, separately from or spread over time with a radiotherapy, for treating glioblastoma. The use of said biodegradable microspheres according to the invention results in a patient survival time of at least 90 weeks, a therapeutically effective concentration being maintained in the parenchymatous area throughout this time. The microspheres used preferably contain 5-fluorouracile of the tumor, by intratissular injection. The radiotherapy targeting the tumorous mass is dosed at 60 Gy over approximately 6 weeks. The invention also relates to a method for producing the biodegradable microspheres by emulsion-extraction, and to a suspension containing the biodegradable microspheres obtained using this method.