Inclusion Criteria  Participant is at least 8 years of age Participants with histologically confirmed medulloblastoma and are scheduled to receive 36 Gy craniospinal irradiation Participant and/or guardian can understand and is willing to sign a written informed consent document according to institutional guidelines Subjects have either have high-affinity or mixed-affinity TSPO binding sites as determined by the single-nucleotide polymorphism (SNP) rs6971 in the TSPO gene.  Exclusion Criteria:  Participants with a large pseudomeningocele (>4 cm in size) at the surgical site, complications from ventricular access (evidenced by hemorrhage and/or enhancement along the catheter), large subdural effusion in the supra-tentorial compartment (>1 cm in the maximum transverse dimension). All of these are expected to induce inflammation. Female participants of childbearing age must not be lactating due to theoretical potential harm to the infant from exposure to radiation. Subjects with low-affinity TSPO binding sites as determined by the single-nucleotide polymorphism (SNP) rs6971 in the TSPO gene. Pregnant subjects. Patients needing sedation for the PET scans.