Lenalidomide and Sintilimab for Relapsed/Refractory NK/T-cell Lymphoma

Inclusion Criteria: * Histopathology and immunohistochemistry confirmed diagnosis of NK/T-cell lymphoma according to WHO 2016 criteria. * Previously treated with pegaspargase-based regimens (refractory or relapsed after initial remission). * PET/CT or CT/MRI with at least one objectively evaluable lesion. * General status ECOG score 0-3 points. * The laboratory test within 1 week before enrollment meets the following conditions: Blood routine: Hb\>80g/L, PLT\>50×10（9）/L. Liver function: ALT, AST, TBIL ≤ 2 times the upper limit of normal. Renal function: Cr is normal. Coagulation : plasma fibrinogen ≥ 1.0g / L. Cardiac function: LVEF≥50%, ECG does not suggest any acute myocardial infarction, arrhythmia or atrioventricular conduction above I Blocking. * Sign the informed consent form. * Voluntary compliance with research protocols, follow-up plans, laboratory and auxiliary examinations. Exclusion Criteria: * Patients with a history of pancreatitis. * Active infection requires ICU treatment. * Concomitant HIV infection or active infection with HBV, HCV. Patients who are infected with HBV but not active hepatitis at the same time are not excluded. * Serious complications such as fulminant DIC. * Significant organ dysfunction: such as respiratory failure, NYHA classification ≥ 2 chronic congestive heart failure, decompensation Hepatic or renal insufficiency, high blood pressure and diabetes that cannot be controlled despite active treatment, and cerebral vascular thrombosis or hemorrhagic time within the past 6 months. * Pregnant and lactating women. * Had a history of autoimmune diseases, and disease was active in the last 6 months, and was still taking oral immunosuppression in the past three months. * Those who were known to be allergic to drugs in the study regimen. * Patients with other tumors who require surgery or chemotherapy within 6 months. * Other experimental drugs are being used.