Ultrasound-Guided High-Intensity Focused Ultrasound Combined With PD-1 Blockade in Patients With Liver Metastases From Lung Cancer: Protocol for a Single-Arm Phase 2 Trial - Chao Hu; Qiang Fu; Fei Fei Gao; Jian Zeng; Wei Xiao; Hui Li; Li Peng; Xi Huang; Li Yang; Wen Zhi Chen; Ming Yan Jiang (JMIR Research Protocols)