Management of HER2-positive and microsatellite instability-high advanced gastric cancer: a case report.

Chemotherapy for advanced gastric cancer has progressed significantly in the past few decades. Biomarker-specific drugs, including anti-human epidermal growth factor receptor 2 (HER2) drugs for HER2-positive patients and immune checkpoint inhibitors for those with microsatellite instability-high (MSI-H), have become common. However, patients who are positive for HER2 and have MSI-H are extremely rare, and there are no established treatments for these patients. We present the case of a 75-year-old, male patient with gastric cancer with lymph node metastases and liver infiltration. Biomarker analysis revealed HER2 3 + , loss of MLH1, and MSI-H. After three cycles of S-1, oxaliplatin, and trastuzumab, the primary tumor and metastases shrank markedly. He subsequently underwent gastrectomy and hepatectomy as conversion surgery, achieving a pathologically complete response. He has been recurrence-free for seven months postoperatively. The present case demonstrated the efficacy of trastuzumab-containing chemotherapy followed by conversion surgery in a patient with HER2-positive, MSI-H, advanced gastric cancer.