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.)