PDCD1 and primary central nervous system lymphoma: Checkpoint inhibitor antibodies blocking PD-1 (nivolumab, pembrolizumab and Sintilimab) or PD-L1 (atezolizumab, avelumab) have been classified as Level 3 evidence for use in salvage therapy for extranodal lymphoma, including MALT [52], NKTCL [51–55], PCNSL [56], PTCL [52, 54].