Study of Lenalidomide/Rituximab Maintenance for Transplantation Ineligible Patients With PCNSL.

Inclusion Criteria: 1. Those who have been diagnosed with histopathological primary central nervous system lymphoma and who have completed standard chemotherapy for induction of remission of primary central nervous system lymphoma have reached a complete or partial response. 2. Those who are unable to transplant autologous hematopoietic stem cells for the following reasons * If you are 65 years of age or older or if you are judged to have a weak systemic condition before receiving high-dose chemotherapy * Refusal of autologous hematopoietic stem cell transplantation after high-dose chemotherapy 3. Adequate laboratory functional values * Absolute neutrophil count ≥ 1000/ul * Platelet count ≥ 50,000/ul * Hemoglobin ≥ 9.0 g/dL * Serum calcium ≤ 12.0mg/dL * Serum creatinine ≤ 1.5 X UNL * AST/ALT ≤ 2.5 X UNL * Total bilirubin ≤ 1.5 X UNL 4. Hepatitis B patients with combination of prophylactic antiviral therapy 5. ECOG PS 0-2 6. Those who can take oral medication 7. Written informed consent under institutional guidelines. 8. Female patients of child-bearing potential (FCBP) must have two negative pregnancy tests (sensitivity of at least 25 mIU/mL) prior to starting lenalidomide. The first pregnancy test must be performed within 10 to 14 days prior to the start of lenalidomide, and the second pregnancy test must be performed within 24 hours prior to the start of lenalidomide. 9. Effective method of contraception should be used during and for 28 days following the last dose of the drug \- FCBP is defined as a sexually mature woman who: 1) has not undergone a hysterectomy or bilateral oophorectomy or 2) has not been naturally postmenopausal (amenorrhea following cancer therapy does not rule out childbearing potential) for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months). 10. Male patients must use an effective barrier method of contraception during study and 28 days following the last dose if sexually active with a FCBP. Exclusion Criteria: 1. If autotransplantation is planned after chemotherapy 2. Active congestive heart failure (New York Heart Association \[NYHA\] Class III to IV), symptomatic ischemia, or conduction abnormalities uncontrolled by conventional intervention. Myocardial infarction within six months prior to 1st day of 1st cycle. 3. Acute active infection requiring systemic antibiotics, antiviral (except antiviral therapy directed at hepatitis B) or antifungal agents. 4. Uncontrolled hepatitis C infection and/or hepatitis B (except for patients with hepatitis B surface antigen \[SAg\] or core antibody receiving and responding to antiviral therapy directed at hepatitis B: these patients are allowed). 5. . Known human immunodeficiency (HIV) seropositive 6. Those who are unable to take oral medication 7. Patients with a history of malignant tumors other than the target diseases except for the following cases * If the tumor has not been treated for at least 5 years or is disease-free * Patients at least 1 year after complete resection of basal cell carcinoma / squamous cell carcinoma or successful treatment of cervical epithelial cancer 8. Adverse reactions within 30 days prior to screening Severe gastrointestinal bleeding exceeding Grade 2 according to the Common Terms Criteria 4.03 version criteria 9. Occurrence of blood clots or embolism within 6 months before starting screening 10. Patients with hypersensitivity to THIS DRUG and other ingredients of THIS DRUG (e.g., angioedema, Stevens-Jones syndrome, toxic epidermal necrosis, etc.) 11. Patients with seizure disorder requiring medication 12. Female patients who are pregnant or lactating. 13. Patients with genetic problems such as galactose intolerance, lapp lactase deficiency, or glucose-galactose malabsorption. 14. Patients with hyperreactivity to rituximab.