# Successful Conversion Surgery Following S-1 Plus Oxaliplatin Combined with Nivolumab Therapy for a Preoperatively Diagnosed Gastric Adenocarcinoma with Enteroblastic Differentiation: A Case Report

## TL;DR

A rare aggressive gastric cancer case responded well to a combination therapy, enabling surgery and improving patient outcomes.

## Contribution

A successful treatment strategy for Stage IV GAED with peritoneal metastases using immunotherapy and chemotherapy is reported.

## Key findings

- Combination therapy with S-1, oxaliplatin, and nivolumab led to tumor shrinkage and enabled conversion surgery.
- Post-surgery recurrence was managed effectively with nivolumab monotherapy, reducing ascites and maintaining partial response.

## Abstract

Gastric adenocarcinoma with enteroblastic differentiation (GAED) is a rare subtype of gastric cancer known for its aggressive nature compared to conventional gastric adenocarcinoma. Due to its rarity and high malignancy, reports of successful medication therapy for Stage IV GAED are scarce. In this case, we report a GAED patient with peritoneal dissemination who responded well to combination chemotherapy with nivolumab, leading to the possibility of conversion surgery.

A 74-year-old man presented with anemia and was diagnosed with GAED involving pancreatic infiltration and peritoneal dissemination. As first-line treatment, he underwent 9 cycles of S-1 and oxaliplatin chemotherapy combined with nivolumab. The tumor showed remarkable shrinkage. Staging laparoscopy revealed the disappearance of peritoneal nodules, and negative peritoneal cytology was confirmed intraoperatively. Consequently, conversion surgery was performed, involving laparoscopic distal gastrectomy with D2 lymph node dissection and Roux-en-Y reconstruction. Pathological examination showed ypT2N0M0, ypStage IB, with a chemotherapy response graded at 2a. Although peritoneal dissemination recurred 4 months after surgery, restarting nivolumab monotherapy significantly reduced ascites, and the patient maintained a partial response.

Stage IV GAED is associated with a poor prognosis; however, the advent of immune checkpoint inhibitors has expanded treatment options for these patients. In this case, we propose a personalized treatment strategy for GAED with peritoneal metastases that may improve clinical outcomes.