Decitabine and Anti-PD-1 in R/R DLBCL

To evaluate the effectiveness and safety of patients with relapsed/refractory diffuse large B cell lymphoma with extranodal (esp. central nervous system) involvement treated with Low-Dose Decitabine plus anti-PD-1 regimen. 3 weeks for a cycle, with a total of 2 years or until the disease progress esor unacceptable toxicity occurs, or the patient decides to withdraw from the trial. Enrollment was planned to be completed within 2 years, and all trials were followed up for 12 months after the last enrolled patient completed treatment. All cases will be followed up and the long-term curative effect will be observed and recorded.