The purpose of this study is to evaluate the efficacy and safety of PD-1 blockade pembrolizumab for patients with relapsed or refractory Natural Killer(NK)/T Cell Lymphoma.

Patients with relapsed or refractory NK/T cell lymphoma usually have a bad prognosis. These patients cannot be treated successfully with the conventional chemotherapy. The investigators have been proceeding this trial to evaluate the efficacy and safety of the immune checkpoint inhibitor PD-1 blockade, pembrolizumab in the patients with relapsed or refractory NK/T cell lymphoma.

Inclusion Criteria:

• Age range 14-70 years old; ECOG performance status 0-2; Estimated survival time > 3 months
• Histological confirmed evidence of relapsed or refractory NK/T cell lymphoma
• Before enrollment, representative formalin-fixed paraffin-embedded tumor samples (or 15 tissue sections at least) and related pathological reports are needed
• Previous treatment with at least one chemotherapy regimen
• At least one measurable lesion
• None of other serious diseases, cardiopulmonary function is normal
• Pregnancy test of women at reproductive age must be negative
• Patients could be followed up
• None of other relative treatments including the traditional Chinese medicine, immunotherapy, biotherapy except anti-bone metastasis therapy and other symptomatic treatments.
• volunteers who signed informed consent.
• No anti-PD1 antibody contraindication (All of the following tests are required to be finished within 14 days prior to the first research): 2.5×109/L\