[A Case of MSI-High Locally Advanced Colorectal Cancer with Pathological Complete Response to Pembrolizumab].

We report the case of an 80-year-old man who presented with acute abdominal pain and was emergently hospitalized. Contrast-enhanced computed tomography revealed a tumor in the ascending colon with free intraperitoneal air and ascites, without evidence of distant metastasis. A diagnosis of acute generalized peritonitis due to perforation of an ascending colon tumor was made, and the patient underwent emergency surgery consisting of primary closure of the perforation and creation of a diverting ileostomy. Postoperative colonoscopy identified a tumor in the ascending colon. Histopathological examination of biopsy specimens confirmed adenocarcinoma with HER2 positivity, RAS mutation, and microsatellite instability- high(MSI-high). The clinical stage was cT4bN1bM0(duodenum), Stage Ⅲc. Pembrolizumab therapy was initiated 1 month after surgery. Progressive tumor regression was observed during treatment, and after 14 courses, colonoscopy demonstrated marked tumor shrinkage with persistent severe luminal stenosis. After 17 courses in total, laparoscopic right hemicolectomy and ileostomy closure were performed. Histopathological examination of the resected specimen showed no residual carcinoma. The histological response was classified as Grade 3 corresponding to a pathological complete response(pCR). No adjuvant chemotherapy was administered. At 5 months after the operation, the patient is alive without any recurrence. This case suggests the potential efficacy of pembrolizumab for MSI-high locally advanced colorectal cancer.