Peroral cholangioscopy of programmed cell death-1 inhibitor-related sclerosing cholangitis: three case reports - Onoyama, Takumi; Takeda, Yohei; Kato, Masayuki; Edano, Mirai; Tarumoto, Ryohei; Matsumoto, Kazuya; Isomoto, Hajime (Endoscopy, 2019)