Long-term survival with nivolumab followed by irinotecan after total gastrectomy in alpha-fetoprotein-producing gastric cancer: a case report and review of the literature oleh Takayo Ota, Katsuya Sakashita, Ryugo Sawada, Kurumi Seki, Hiroyuki Maeda, Noriko Tanaka, Toshimasa Tsujinaka