A Case of Unresectable Gastric Cancer Successfully Treated with Conversion Surgery After Immunotherapy Including Treatment.

A 72-year-old woman diagnosed as unresectable advanced gastric cancer with both of direct invasion to liver and pancreas and peritoneal dissemination, received 11 courses of S-1 and oxaliplatin (SOX) + nivolumab combination therapy according to the Japanese Gastric Cancer Treatment Guideline because the tumor showed HER2-negative, microsatellite instability (MSI)-high, and PD-L1-positive. Since post-treatment endoscopic and other imaging findings showed a prominent reduction of the main tumor, release of surrounding organ infiltration, and disappearance of disseminated metastases, we planned the conversion surgery and could perform laparoscopic distal gastrectomy with D2 lymph node dissection. Postoperative histopathological examination showed no residual tumor cells both in the primary tumor and dissected lymph nodes. Postoperatively, the patient was treated with SOX therapy for 1 year and remains alive at 16 months postoperatively without recurrence.