1. Establish a predictive model for the efficacy of immune checkpoint inhibitors (ICI) in Chinese patients with biliary tract cancers. By analyzing the dynamic changes of circulating tumor DNA (ctDNA) and other clinical and pathological features before and after ICI treatment in a cohort of patients with biliary tract tumors, a predictive model can be established to evaluate the efficacy of ICI treatment in the early stages or even before treatment, serving as a reliable tool for selecting patients who are likely to benefit from ICI treatment.
2. Investigate the clinical features of populations that benefit from different immune combination therapies. By comparing the differences and enrichment of mutations between patients receiving different treatment regimens, and if patients have sufficient pre-treatment tissue, further comparisons of differentially expressed genes and pathways may be made.

Inclusion Criteria:

• The subjects voluntarily participate in the study and agree to sign the informed consent form, are compliant, and cooperate with follow-up.
• Patients with advanced biliary tract cancers (including gallbladder cancer, intrahepatic cholangiocarcinoma (ICC), hilar cholangiocarcinoma, or distal cholangiocarcinoma) with histologically, cytologically, or clinically confirmed.
• Patients who have undergone curative surgery and experienced disease recurrence after more than 6 months; or patients who have completed adjuvant therapy (chemotherapy and/or radiotherapy) and have been disease-free for more than 6 months after completing adjuvant therapy are eligible for inclusion.
• Regardless of gender, age 18-80 years old.
• ECOG PS of 0-2 with expected survival time ≥12 weeks.
• According to RECIST V1.1, the patient had at least one measurable lesion.
• The patient is suitable for treatment with immune checkpoint inhibitors (PD-1/PD-L1 inhibitor).

Exclusion Criteria:

• Suffering from other malignant tumors.
• Previous systemic treatment received.
• Patients with serious organic diseases cannot receive the treatment designed in this study;
• Having a psychiatric disorder does not guarantee compliance with this study.
• Patients with symptomatic brain metastasis or complications related to brain metastasis or cognitive impairment.
• Adverse events (except hair loss of any grade) from previous antitumor therapy did not return to ≤ grade 1 or better (According to CTCAE version 5.0).
• Previously received any cell or organ transplant treatment.
• Allergy to the therapeutic drugs of this study or CT or MRI-enhanced contrast agents.
• Received local hepatobiliary treatment (including various types of ablations, percutaneous ethanol or acetic acid injection, high-intensity focused ultrasound, transarterial embolization, chemotherapy, or chemoembolization) within 14 days before the start of the study treatment.
• The physician determines that there are any contraindications to ICI treatment.
• During the second blood monitoring, the disease changes could not be judged (such as unexplained ascites, worsening symptoms); Or non-disease progression, but the next immunotherapy is not planned.