GemCis Plus PD-1/PD-L1 Inhibitor in Advanced Biliary Tract Cancer: Efficacy and Immune Microenvironment （BTC-IM-1）

Inclusion Criteria: * Age ≥18 years. * Histologically confirmed unresectable/metastatic cholangiocarcinoma (intrahepatic, extrahepatic, or gallbladder). * No prior systemic anticancer therapy (chemotherapy, targeted therapy, or immunotherapy). * Planned to receive GemCis+PD-1/PD-L1 inhibitor as standard first-line treatment. * ≥1 measurable lesion per RECIST 1.1. * ECOG performance status 0-1. * Adequate organ function: * ANC ≥1.5 × 10⁹/L, platelets ≥100 × 10⁹/L, hemoglobin ≥9 g/dL. * Total bilirubin ≤1.5 × ULN, AST/ALT ≤3 × ULN (≤5 × ULN if liver metastases). * Creatinine ≤1.5 × ULN or CrCl ≥60 mL/min. * Willing to provide archival/fresh tumor tissue and peripheral blood samples. * Signed informed consent. Exclusion Criteria: * Prior systemic therapy. * Active autoimmune disease requiring immunosuppression. * Active infection requiring IV antibiotics. * HIV-positive or active HBV/HCV infection (HBsAg+ with HBV DNA ≥2000 IU/mL; HCV RNA+). * Symptomatic CNS metastases. * Pregnancy/lactation. * Any condition compromising protocol compliance or data interpretation per investigator.