This study is designed to prospectively analyze [18F]FET PET image data obtained retrospectively in the routine clinical care of glioma patients. The study will analyse the data from participants with grade (1-4) glioma after primary treatment according to local clinical practice and with suspicion of progression/recurrence on magnetic resonance imaging (MRI).

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.

All patients who are enrolled in the study will be included in the FAS.