Microsatellite Instability-high Signet Ring Cell Carcinoma of the Colon Treated With Immunotherapy: Report of a Case

Background: Immune checkpoint inhibitors (ICIs) are attracting increasing attention as a novel and potentially curative therapy for microsatellite instability-high (MSI-H) colorectal cancer (CRC). Case Report: An 80-year-old female visited our hospital with complaints of lower abdominal pain due to bowel obstruction caused by descending colon cancer. After 1 month of metallic stent detention, she underwent radical surgery, although laparotomy showed broad peritoneal dissemination. Based on the genetic finding of MSI-H status, pembrolizumab therapy was administered in two cycles. Unfortunately, the therapy was ineffective, and the patient died after being discharged 5 months after surgery. Conclusion: The findings in this case of MSI-H CRC with a poor response to an ICI suggest the importance of confirming HLA status, including beta-2-microglobulin and HLA expression, before starting ICI therapy in cases of MSI-H CRC.of intracytoplasmic mucin and a displaced nucleus, with a reported incidence of 0.7%-1.0% among all cases of CRC (3). SRCC is known to be more aggressive than conventional adenocarcinoma because the rate of nodal lymph node metastasis is significantly higher in SRCC than in adenocarcinoma (4). Older age, male sex, and peritoneum metastasis have also been reported to be associated with a poor clinical course (5). Higher and lower frequencies of BRAF and KRAS mutations, respectively, have been observed more frequently in SRCC than in adenocarcinoma (5). MSI-H status is also substantially more prevalent in SRCC than in adenocarcinoma (6), which suggests that immunotherapy could be effective for treating MSI-H SRCC cases. We herein report a case of MSI-H SRCC with a poor response to an ICI.