FET-PET-Guided Management of Pseudoprogression in Glioblastoma

The goal of this diagnostic randomised clinical trial is to determine, in glioblastoma patients with diagnostic uncertainty between pseudoprogression and tumor progression on follow-up MRI after chemoradiation, the added value of a direct \[¹⁸F\] FET-PET scan for clinical management. The main questions it aims to answer are: * Does the clinical management guided by an additional FET-PET scan leads to fewer unnecessary interventions, compared with management based on MRI only? * Does the clinical management guided by an additional FET-PET scan leads to better health-related quality of life after 12 weeks, compared with management based on MRI only? * Does the clinical management guided by an additional FET-PET scan leads to reduced net healthcare costs, compared with management based on MRI only? Researchers will compare the investigational arm, where clinical management is based on the index MRI scan and an additional FET-PET scan, with the control arm, where clinical management is based solely on the index MRI scan, to investigate the added value of the FET PET scan for clinical management. Participants in the investigational arm will undergo the FET PET scan. All participants will complete health-related quality of life questionnaires at four different timepoints.