CD274 and esophageal squamous cell carcinoma: However, PFS was revealed as an invalid surrogate for OS in advanced ESCC treated immunochemotherapy as first-line treatment (patient level τ = 0.436, trial level squared correlation R2 = 0.436 in ITT and R2 = 0.072 in PD-L1 + , arm level R2 = 0.264 in ITT and R2 = 0.294 in PD-L1 +).