The main aim of the study is to compare the sensitivity and specificity of a combined PET/MRI examination with the clinical routine MRI examination given to these patients today. It will be investigated whether PET (18F-FDG and 18F-fluciclovine) can provide additional prognostic value at baseline and in response assessment compared to MRI and established pre-treatment prognostic scores in PCNSL, and evaluate which PET/MRI parameters that are best suited as an imaging biomarker for progression-free survival.