Brain tumors are the most common solid tumors in children and are the second most common cause of cancer-related death among this population. Posterior brain fossa tumors represent about 50% of children brain tumors.

Recently, knowledge in molecular biology has permitted to identify different tumors subgroups of very different prognosis. Today, surgery removal of the tumor is the mandatory. Neuro-oncological treatment differs depending on the histological and molecular subgroup. With classical Magnetic Resonance Imaging (cMRI), distinction between different types of posterior fossa tumors remains difficult.

Positron Emission Tomography combined with MRI (PET/MRI) has proven its benefits for the management of brain tumors. The direct spatiotemporal correlation makes possible the assessment of metabolic, anatomical and functional information. PET/MRI would provide precisions on the pre-therapeutic characterization of tumors, which could permit to modify the patients' care.

L- [methyl-11C] methionine (11C-MET) is currently the gold-standard tracer used in neuro-oncology, but few data exists in children. 11C-MET PET/MRI assessment seems promising, but has not been studied enough. To the best of our knowledge, there is no specific study on primary brain tumors of the posterior fossa in children using 11C-MET PET/MRI.

The primary objectives of this first pilot study is to evaluate the practical feasibility of 11C-MET PET/MRI imaging (machine accessibility and imaging interpretability) before surgery in a population of children older than 5 years old with posterior fossa tumors.

The secondary objectives aim to describe :

• the PET/MRI parameters of the different tumors studied,
• and the patient's participation acceptation.

This feasibility study will allow us to standardize the PET/MRI measurements; this could allow us to discriminate, in a larger study, the different tumor subgroups before surgery.

Inclusion Criteria:

• Patients ≥ 5 years old and < 18 years old at enrollment,
• with primary posterior brain fossa tumor, metastatic or not,
• hospitalized in the Department of Pediatric Neurosurgery of Femme-Mère-Enfant Hospital, and for whom a surgery is scheduled,
• covered by national health insurance,
• patient's parents or legal guardians who have provided written informed consent prior to participation in the study.

Exclusion Criteria:

• Patients with contraindication for MRI (claustrophobia, carrying metallic object)
• Patients with contraindication for Gadolinium injection (including pregnancy and breastfeeding)
• patient with recurring posterior brain fossa tumor
• patient with brain stem infiltrative tumor