[A Case of an Older Patient with MSI-High Colorectal Cancer with Distant Lymph Node Metastasis Successfully Treated with Pembrolizumab].

The patient was a 90-year-old man with a PS score of 0 and no underlying disease. He visited our hospital with a fever and was diagnosed with cancer of the ascending colon with abscess formation due to retroperitoneal perforation. First, he underwent ileostomy with double orifices and abscess puncture drainage, followed by right semicolon resection plus ileostomy closure to resect the primary tumor. Three months after the surgery, metastases were found in the left cervical supraclavicular and periaortic lymph nodes. The primary tumor was a poorly differentiated adenocarcinoma, RAS wild, BRAF mutation(+), and MSI-high(+). Therefore, pembrolizumab therapy was initiated, which resulted in a complete response(CR) at the end of 6 courses. After 2 years of pembrolizumab treatment, the patient's CR status was maintained, with only Grade 1 hypothyroidism. We believe that pembrolizumab therapy for MSI-high, unresectable, advanced, recurrent colorectal cancer is useful in older patients with good PS, no major underlying disease, and a medical system that can respond to irAE.