Comparison between the first-line and second-line immunotherapy drugs in the progression-free survival and overall survival in advanced non-small cell lung cancer: a systematic review and meta-analysis of randomized controlled trials - Wu, Shengyu; Wang, Lei; Li, Wei; Chen, Bin; Liu, Yu; Wang, Hao; Zhao, Sha; Ye, Lingyun; Sun, Hui; He, Yayi (Annals of Palliative Medicine, 2020)