2.1 Primary objective The main objective is to evaluate the relative benefit of FET PET in comparison to the MRI in differentiating biologically active tumour tissue from therapy-related changes in paediatric brain tumours after first line therapy (Δ specificityFET PET to specificityMRT) 2.2 Secondary Objectives To assess sensitivity of FET PET in comparison with the sensitivity of MRI (Δ sensitivityFET PET to sensitivityMRT) To assess the positive and negative predictive values (PPV, NPV) of FET PET in comparison with the PPV and NPV of MRI (Δ PPVFET PET to PPVMRT, Δ NPVFET PET to NPVMRT) To evaluate specificity, sensitivity, PPV, and NPV by SUVratio analyses of FET PET data To evaluate the potential of FET PET for non-invasive tumour grading (WHO I/II vs. III/IV) by kinetic studies when histology is available To assess adverse events and toxicity profile