The overall goal of this imaging trial is to prospectively compare the usefulness of 18F-FDG PET and 18F-FET PET in evaluating treatment response and predicting prognosis in patients with central nervous system lymphoma.

Inclusion Criteria:

In order to be eligible for participation in this trial, the subject must:

• Be ≥ 19 years of age.
• Patients diagnosed with central nervous system lymphoma by surgery/biopsy or suspected of central nervous system lymphoma by anatomical imaging examination such as MRI.
• Patients who have results or are planning to examine 18F-FDG PET in the above patients.

Exclusion Criteria:

The subject must be excluded from participating in the trial if the subject fulfil any single criteria described below:

• Pediatric and adolescent patients under 19 years of age.
• The patient is pregnant, is attempting to become pregnant, or is nursing (breast-feeding) to children.
• Patients with diagnosed or suspected lymphoma lesions in organs other than the cetral nervous system.
• Based on the investigator's judgement, if the patient makes it impossible to collect complete data for a clinical trial due to personal or other reasons.
• Patients who cannot stably have PET/CT due to claustrophobia, dyskinesia, anxiety disorder, seizure, etc.
• Based on the investigator's judgement, if the patient has a serious and/or uncontrolled and/or unstable medical disease (e.g., congestive heart failure, acute myocardial infarction, severe lung disease, chronic renal disease or chronic liver disease, etc.)