RÉSUMÉ
The standard of care for patients with operable gastric adenocarcinoma is perioperative chemotherapy and surgical resection. The deficient mismatch repair (dMMR)/microsatellite instability (MSI-H) phenotype is a major predictive biomarker for immune checkpoint inhibitors (ICIs) efficacy in advanced disease. Several phase II and III trials suggest a promising future role of immunotherapy with or without chemotherapy in the neoadjuvant/adjuvant setting, especially in MSI-H localized gastric adenocarcinomas. We present a 38-year-old man diagnosed in March 2022 with poorly differentiated gastric adenocarcinoma clinical stage III (cT4 N0 M0) with deficiency of MLH1 and PMS2, combined positive score (CPS) of 100 and negative HER2 immunohistochemistry, had poor tumor response to preoperative 5-FU, leucovorin, oxaliplatin, and docetaxel (FLOT). It was considered unresectable because of the involvement of the colon, mesocolon, duodenum, pancreas, and retroperitoneum. Then, first-line systemic treatment with 5-FU, leucovorin, and oxaliplatin (FOLFOX)-nivolumab was initiated in August 2022, with a significant radiologic tumor reduction after six cycles, allowing a curative surgery in March 2023 with complete pathologic tumor response, followed by capecitabine and nivolumab for one year, maintaining radiological remission in the last follow-up in April 2024. With this case report, we conclude that it is likely that chemoimmunotherapy or immunotherapy alone may be alternative neoadjuvant treatment choices for MSI-H locally advanced gastric cancer patients.