Patients with recurrent glioblastoma who are planned to receive a second course of radiation are to be included into this monocentric cohort trial. Due to multiple pre-treatments simultaneous combined positron emission tomography (PET) with O-(2-[18F]fluoroethyl)-l-tyrosine (FET) as well as magnetic resonance imaging (MRI) is used for treatment planning and follow-up imaging as it allows for a better distinction between treatment-related changes and viable tumor tissue.