Conversion therapy for unresectable intrahepatic cholangiocarcinoma using gemcitabine plus S-1 combined with PD-1 inhibitors: a case report - Zhao, Shuangying; Zhang, Xiaodong; Luo, Jialiang; Yan, Huanjun; Zhang, Jianlei; Lin, Rongfeng; Zhu, Kelei (Frontiers in Oncology, 2025)