Conversion Surgery for Advanced Gastric Cancer with Para-Aortic Lymph Node Metastases Following Treatment with Capecitabine, Oxaliplatin, and Pembrolizumab: A Case Report

INTRODUCTION: Immune checkpoint inhibitors (ICIs) have recently emerged as an important treatment option for various cancers. In 2024, pembrolizumab was approved as a first-line treatment for unresectable or recurrent gastric cancer. Conversion surgery following ICI-based chemotherapy has been reported; however, cases involving pembrolizumab-based first-line therapy remain rare. Here, we report a case of conversion surgery after treatment with capecitabine plus oxaliplatin (CAPOX), combined with pembrolizumab for unresectable advanced gastric cancer. CASE PRESENTATION: An 82-year-old man presented with anorexia and was referred to our department for surgical evaluation. Upper gastrointestinal endoscopy revealed circumferential type 3 gastric cancer extending from the lower gastric body to the antrum with pyloric stenosis. Contrast-enhanced CT showed para-aortic lymph node metastases, resulting in a diagnosis of stage IVB gastric cancer (cT4aN2M1, 15th edition of the Japanese Classification of Gastric Carcinoma [JGCA]). First, we performed a laparoscopic gastrojejunal bypass to treat anorexia and oral intake difficulties due to pyloric stenosis. The patient then received 5 cycles of CAPOX plus pembrolizumab. Subsequent upper gastrointestinal endoscopy revealed significant scarring with residual cancer cells, and contrast-enhanced CT showed significant shrinkage of the primary tumor lesion and para-aortic lymph nodes. Because R0 resection was achievable, we performed conversion surgery involving open distal gastrectomy with D2 and para-aortic lymphadenectomy. Postoperative pathological findings revealed a small number of residual cancer cells in the submucosa, with no viable cancer cells detected in the para-aortic lymph nodes (ypT1bN0M0, ypStage IA). The pathological response grade was 2b according to the 15th edition of the JGCA. At 6 months postoperatively, the patient remains alive and recurrence-free. CONCLUSIONS: Conversion surgery after CAPOX plus pembrolizumab chemotherapy is a potential therapeutic strategy for unresectable advanced gastric cancer.