A Pilot Study to Evaluate PBR PET in Brain Tumor Patients Treated With Chemoradiation or Immunotherapy

Inclusion Criteria: * Participants must have evidence of metastatic melanoma to the brain for Cohort A or histologically confirmed GBM for Cohorts Band C. * Those with newly diagnosed GBM but suspected to have pseudoprogression after completion of chemoradiation can enroll in Cohort C. * Participants must have measurable brain disease, defined as at least one lesion that is 10 mm in diameter. * Age \> 18 years. * ECOG performance status ≤2 (Karnofsky ≥60%, see Appendix A) * Life expectancy of greater than 3 months. * Participants must have normal organ and marrow function as defined below: * leukocytes ≥3,000/mcL * absolute neutrophil count ≥1,500/mcL * platelets ≥100,000/mcL * total bilirubin within normal institutional limits * AST(SGOT)/ALT(SGPT) ≤2.5 × institutional upper limit of normal * creatinine within normal institutional limits \--- OR * creatinine clearance ≥60 mL/min/1.73 m2 for participants with creatinine levels above institutional normal. * For Cohort A, only patients with metastatic melanoma to the brain for whom their treating physician has planned to give immunotherapy as monotherapy are eligible for this study. This can be in the setting of a clinical trial or not. * For Cohort B, only patients with GBM for whom their treating physician has planned to give immunotherapy are eligible for this study. This can be in the setting of a clinical trial or not. * For Cohort C, patients with newly diagnosed GBM who have completed standard temozolomide + radiation and have suspected pseudoprogression within the first 3 months of completing chemoradiation can enroll. * Patient must be able to undergo MRI and PET scans. * Patient must be maintained on a stable corticosteroid regimen for 5 days prior each MR-PET scan. * High or mixed affinity binders (Ala/Ala or Ala/Thr) based on genotyping result from PBR affinity test. This blood test will be performed as part of the screening process after consent has been obtained. * The effects of PBR on the developing human fetus are unknown. For this reason and because radiopharmaceuticals agents are known to be teratogenic, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately. Radiopharmaceutical agents are known to be teratogenic. * Ability to understand and the willingness to sign a written informed consent document. Exclusion Criteria: * History of allergic reactions attributed to compounds of similar chemical or biologic composition to PBR. * Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements. * Pregnant women are excluded from this study because PBR is a radiopharmaceutical agent with the potential for teratogenic or abortifacient effects. Because there is an unknown but potential risk for adverse events in nursing infants secondary to exposure of the mother to PBR, breastfeeding should be discontinued. These potential risks may also apply to other agents used in this study. * HIV-positive participants are excluded because their immune system is compromised and may affect the interpretation of the imaging data. * Patients who are not suitable to undergo MRI or PET or use gadolinium contrast due to: * Claustrophobia * Presence of metallic objects or implanted medical devices in body (i.e. cardiac pacemaker, aneurysm clips, surgical clips, prostheses, artificial hearts, valves with steel parts, metal fragments, shrapnel, tattoos near the eye, or steel implants) The craniotomy patients will all have titanium but this is MRI compatible * Sickle cell disease * Renal failure * Reduced renal function, as determined by creatinine clearance \< 30 mL/min based on a serum creatinine level obtained within 28 days prior to registration