Conversion surgery for microsatellite instability-high gastric cancer with a complete pathological response to pembrolizumab: a case report

BACKGROUND: Immune checkpoint inhibitors are reportedly effective in treating microsatellite instability (MSI)-high gastric cancer. There are a few case reports of conversion surgery (CS) with nivolumab but none with pembrolizumab. Herein, we describe a patient with MSI-high gastric cancer who was successfully treated with pembrolizumab and underwent CS with a pathological complete response. CASE PRESENTATION: A 69-year-old man was diagnosed with stage III gastric cancer (T3N2M0) based on contrast-enhanced computed tomography, which revealed a neoplastic lesion and enlarged perigastric lymph nodes in the gastric lesser curvature. The anterior superior lymph node of the common hepatic artery (CHA) was determined to be unresectable due to invasion of the pancreatic head and CHA. Histopathologically, the biopsied tissue showed moderately differentiated adenocarcinoma, then determined to be MSI-high. After three courses of mFOLFOX6 therapy, the patient was diagnosed with progressive disease. Since one course of paclitaxel plus ramucirumab therapy caused grade 3 fatigue, his second-line therapy was switched to pembrolizumab monotherapy. After three courses, the primary tumor and perigastric lymph nodes had shrunk, and it was determined as a partial response. The anterior superior lymph node of the CHA became resectable based on the improvement of infiltration of the pancreatic head and CHA due to shrinkage of the lymph node. Tumor markers remained low; hence, distal gastrectomy plus D2 lymphadenectomy was performed at the end of six courses. Anterior superior lymph node of the CHA was confirmed by intraoperative ultrasonography, and the resection was completed safely. The gross examination of the resected specimen revealed an ulcer scar at the primary tumor site. The histopathological examination showed no viable tumor cell remnants in the primary tumor, which had a grade 3 histological response, and resection margins were negative. The lymph nodes showed mucus retention only in the anterior superior lymph node of the CHA, indicating the presence of metastasis, but no viable tumor cells remained. The patient commenced 6 months of adjuvant pembrolizumab monotherapy 3 months after surgery. Twenty months after surgery, there was no evidence of recurrence. CONCLUSIONS: Conversion surgery following pembrolizumab monotherapy has a potential utility for the treatment of MSI-high gastric cancer.