Increased glycolysis and reduced oxidative phosphorylation is a characteristic property of many tumors. A change of nutrition by limiting carbohydrates and increasing the proportion of fatty acids and proteins can lead to ketogenic metabolism and might limit energy production in tumor cells and therefore inhibit tumor growth. Standard treatment for glioblastoma includes resection, irradiation and temozolomide chemotherapy. If there is tumor recurrence, no standard therapy is established. Therapeutic options in this situation include resection, irradiation or another chemotherapy. However, some patients cannot be treated in this situation, because none of the available treatment options seems reasonable or applicable, for example if no additional chemotherapy can be started at the time of recurrence due to myelosuppression. The pilot study examines whether in this situation a ketogenic diet can be applied to the patients and may inhibit tumor growth and influence the quality of life of the patients.