CD274 and glioblastoma: Meta analysis: pooled result: high PD-L1 expression associated with worse OS in glioma patients (P = 0.032); subgroup analysis: high PD-L1 expression in glioblastoma (GBM) associated with worse OS (P = 0.030); index subgroup analysis: neither PD-L1 protein (P = 0.068) nor gene (P = 0.322) significantly associated with OS