The Value of Molecular Residual Disease Monitoring Based on ctDNA in Advanced or Metastatic Pancreatic Cancer

The goal of this clinical trial is to explore the value of molecular residual disease (MRD) monitoring based on ctDNA in advanced or metastatic pancreatic cancer. The main questions it aims to answer are: * prognostic value of baseline MRD; * the role of MRD dynamic changes after treatment in guiding treatment. Peripheral blood derived from participants will be obtained for MRD test before first-line chemotherapy initiation and at the first imaging assessment after chemotherapy.