Complete and rapid response with the combination of immunotherapy and chemotherapy in a young adult patient with microsatellite instability-high metastatic gastric cancer

Gastric cancer (GC) is an aggressive malignancy; 5.0% of GC patients are diagnosed before the age of 40. These patients are more aggressive and advanced stage at the diagnosis. Microsatellite instability-high (MSI-H) status is usually seen in relatively older patients. We report a 31-year-old male patient presenting with an intra-abdominal mass and spleen lesions that radiologically mimic a gastrointestinal stromal tumor (GIST). He underwent surgery. Histological examination revealed poorly differentiated adenocarcinoma starting from the deep gastric mucosa. After surgery, rapidly progressive disease was observed. The patient with MSI-H and combined positive score (CPS) of 65% was treated with a combination of immunotherapy and chemotherapy; complete response was observed in approximately 3 months. This is a very rare GC case in young adult age with MSI-H status and responds to treatment in a short time. Predictive markers for immunotherapy efficacy are still being discussed; this case supports the predictive role of high CPS score and MSI-H phenotype in demonstrating treatment efficacy.