CD274 and primary central nervous system lymphoma: Although we did not find a significant correlation between the prognosis of patients with brain metastatic NSCLC and the expression of PD-1, PD-L1 and B7-H3 in the current study, previous studies have confirmed that PD-1, PD-L1 and B7-H3 play an important role in primary brain tumors, including glioblastoma and primary central nervous system lymphoma, and are associated with patient prognosis [61–63].