PDCD1 and head and neck squamous cell carcinoma: While immune checkpoint inhibitors targeting PD-1/PD-L1 have recently offered hope for advanced HNSCC, their objective response rate (ORR) remains limited to 15–20% [6], and existing biomarkers fail to reliably stratify beneficiaries due to spatiotemporal heterogeneity and suboptimal predictive efficacy [7].