The trial is a one-arm single-center, non-randomized observational (biomarker) study. After resection or biopsy, patients with newly diagnosed glioblastoma multiforme will receive a [11C]MET-PET/MRI scan before start of concurrent radiochemotherapy (~1-4 weeks after surgery). A fusion with the planning CT for irradiation treatment planning will be performed. Postoperative radiotherapy will be applied in daily fractions of 2 Gy given 5 days per week for 6 weeks, for a total dose of 60 Gy (50 Gy and a boost of 10 Gy), with concomitant administration of daily Temozolomide (75 mg/m2 p.o.) from the 1st day to the last day of radiotherapy. Follow up with [11C]MET-PET/MRI will be performed every 3 months until recurrence or until death of the patient.