Fluoroethyltyrosine for the Evaluation of Intracranial Neoplasm

PRIMARY OBJECTIVES: 1. To determine if FET Positron Emission Tomography (PET) scan can differentiate between benign treatment-related changes and recurrence in comparison to pathology in population 1 with recurrent metastatic disease and high-grade gliomas. 2. To determine if FET PET scan can differentiate between low-grade and high-grade gliomas in population 2. SECONDARY OBJECTIVES: 1. To determine if FET PET scan can differentiate between benign treatment- related changes and recurrence in comparison to pathology or imaging follow-up in population 1. 2. To determine if FET PET scan can differentiate between benign treatment- related changes and recurrence in comparison to pathology in population 1 with recurrent low-grade gliomas. EXPLORATORY OBJECTIVES: 1\. To assess relationships between serial FET PET scan and clinical outcome in population 1 with recurrent metastatic disease and gliomas. OUTLINE: Participants will receive a single PET scan lasting for about 40 minutes in an outpatient setting after an injection with FET. Adult participants may undergo up to two repeat FET PET scans. Adverse events will be recorded.