[18F]FET PET for Characterization of Progressive or Recurrent Glioma From Treatment Related Changes

Inclusion Criteria: 1. Signed patient informed consent with secondary use of data permitted. Pediatric patients (\<18 years of age) will provide assent along with parental/legal guardian /caregiver consent 2. Male or Female of any age at the time of \[18F\]FET PET imaging. While there is no age restriction, it is expected that enrollment of patients ≤ 3 years old will be rare due to the very low incidence of glioma in this age group. 3. Documented histologic diagnosis or clinical suspicion of glioma (Grades 1-4) based on local clinical assessment. 4. Underwent \[18F\]FET PET imaging as part of routine clinical care after completion of primary treatment (surgery, radiation therapy, and/or chemotherapy). 5. Evidence of radiographic suspicion of recurrence or progression on MRI at the time of \[18F\]FET PET imaging. 6. The adult patient, 18 years of age or older, has received nominal injected dose of 4 to 7 mCi (148 - 259 MBq ) of \[18F\]FET per imaging time point. 7. The pediatric patients, 0-17 years old, must have received a pediatric dose adjusted based upon the patient's body weight. Exclusion Criteria: Participants will be excluded if they did not meet any of the inclusion criteria or institutional requirements for participation. 1. Did not meet all inclusion criteria. 2. Did not undergo \[18F\]FET PET imaging as part of routine clinical care. 3. Imaging data or associated clinical information is incomplete, non-evaluable, or of insufficient quality for analysis. 4. Any institutional or regulatory requirement that precludes data sharing or inclusion in retrospective research studies.