Implantation of iodine125-Seeds Combined With Chemotherapy in the Treatment of Metastatic Pancreatic Carcinoma

The goal of this clinical trial is to evaluate the clinical efficacy and safety of endoscopic ultrasonography (EUS)-guided radioactive iodine 125 seeds in combination with AG regimen chemotherapy for the treatment of metastatic pancreatic cancer. The main questions it aims to answer are: * whether the combination of minimally invasive endoscopy-guided local radiation therapy with chemotherapy may improve overall survival * the adverse events of the combination therapy Participants will receive the implantation of radioactive seeds under EUS guide. 48h after implantation, chemotherapy with Gem/nab-P given on days 1 and 8 of each 21-day cycle will be conducted. Researchers will compare the I125+AG group with the group that takes AG chemotherapy alone to see if the overall survival can be improved.