[A Case of Conversion Surgery Following Successful SOX plus Nivolumab Therapy for Advanced Gastric Cancer with Peritoneal Dissemination].

A 66-year-old man was referred to our hospital for further evaluation and treatment of advanced gastric cancer, which was diagnosed after the patient presented with a history of a loss of appetite. The patient was diagnosed with advanced gastric cancer with pyloric stenosis and peritoneal dissemination(cT4aN+M1, P1, cStage ⅣB;Japanese Classification of Gastric Carcinoma, 15th edition). After undergoing gastrojejunostomy, the patient received 5 courses of SOX plus nivolumab. The tumor markedly regressed and the peritoneal dissemination disappeared. No peritoneal metastasis(P0)or positive cytology(CY0)was observed on staging laparoscopy, and conversion surgery was deemed feasible. The patient underwent distal gastrectomy with D2 dissection and partial resection of transverse colon to achieve R0 resection. Pathological findings revealed ypT3N2, P0, and CY0, corresponding to ypStage ⅢA. The histological therapeutic effect was graded as Grade 1b. Postoperative adjuvant chemotherapy with oral S-1 was administered for 6 months. The patient remained recurrence-free for 8 months after surgery. Recent research supports that R0 resection through conversion surgery can prolong survival, even in patients with advanced gastric cancer with peritoneal dissemination. Additionally, immune checkpoint inhibitor-based chemotherapy is recommended as first-line treatment for unresectable advanced gastric cancer, with increasing reports of favorable responses. This case is a valuable example in which SOX plus nivolumab therapy was effective, allowing curative resection via conversion surgery in a patient with peritoneal dissemination of advanced gastric cancer.