Inclusion Criteria:

- age >= 18 years

- recurrence of a histologically confirmed glioblastoma or gliosarcoma

- at least 6 months after first surgery

- at least 6 months after first radiotherapy

- interdisciplinary recommendation for reirradiation

- karnofsky performance status >= 60, ECOG <= 2

- creatinine <= 2,0 mg/dl, urea <= 100 mg/dl

- ALAT, ASAT <= 7x upper normal limit

Exclusion Criteria:

- bowel obstruction, subileus

- insulin-dependent diabetes

- decompensated heart failure (NYHA > 2)

- myocardial infarction within the last 6 months, symptomatic atrial fibrillation

- severe acute infection

- malnutrition, cachexia

- other medical conditions that might increase the risk of the dietary intervention

- pregnancy

- uncontrolled thyroid function

- pancreatic insufficiency

- dementia or other clinically relevant alterations of the mental status which could
impair the ability of the patient to apply to the diet or understand the informed
consent of the study