Patients older than 60 years account for half of cases of Primary Central Nervous System Lymphoma (PCNSL) and have a poorer outcome. No prognostic or predictive factors exist for survival after initial remission. 18F-2-fluoro-2-deoxy-D-glucose fluorodeoxyglucose - Positron Emission Tomography ([18F]FDG-PET) plays a key role in grading and therapy monitoring of systemic diffuse large B-cell type.

LOCALYZE is an ancillary Positron Emission Tomography / Magnetic Resonance (PET/MR) clinical study from BLOCAGE 01 (Pr Hoang-Xuan). The aim is to evaluate the usefulness of [18F]FDG-PET to monitor treatment response in PCNSL older than 60 years (n=56), in complement to multiparametric Magnetic Resonance Imaging (MRI).

Hypothesis The investigators assume that the development of new imaging biomarker extracted from PET imaging and multiparametric MRI, could improve the assessment of treatment response in PCNSL.

[18F]-FDG PET/MR will be performed in the Department of Nuclear Medicine - Pitié-Salpêtrière Hospital:

• prior to initiation of chemotherapy (rituximab, methotrexate, procarbazine, vincristine and cytarabine) (PET#1),
• after two chemotherapy cycles (PET#2),
• at the end of the first-line chemotherapy regimens (PET#3).

Inclusion Criteria:

• Newly diagnosed primary cerebral lymphoma
• Age ≥60 years
• Pathology proven diagnosis or positive cytology of the Cerebrospinal Fluid (CSF) or vitreous
• Karnofsky Performance Status ≥40
• No evidence of systemic Non-Hodgkin lymphomas (NHL) (body CT scan, bone marrow biopsy)
• Adequate haematological, renal and hepatic function
• Calculated creatinine clearance > 40 ml/min

Exclusion Criteria:

• Uncontrolled diabetes with fasting glycaemia > 200 mg/dL
• Sensitivity to active substance in [18F]-FDG
• Calculated creatinine clearance < 40 ml/min
• No contraindication to MRI (magnetic resonance imaging)