Surgical resection of inoperable Lynch syndrome-related colorectal cancer following pembrolizumab treatment: Report of two cases

Recent therapeutic advances have dramatically improved outcomes in patients with colorectal cancer (CRC) with mismatch repair deficiency (dMMR) or high microsatellite instability (MSI). Approximately 20% of patients with MSI-high CRC have Lynch syndrome, but few studies have reported the conversion of unresectable CRC to surgery in patients with Lynch syndrome. Here, we present two patients with dMMR/MSI-high CRC who successfully converted to surgery for tumor resection. Case 1 was a 73-year-old male patient diagnosed with MSI-high rectal cancer, which was unresectable due to extensive posterior infiltration. Pembrolizumab treatment led to a reduction in tumor size, allowing laparoscopic residual rectal resection. Case 2 was a 33-year-old female patient diagnosed with Stage IIIC descending colon cancer with iliopsoas muscle infiltration. The family history suggested a hereditary predisposition to CRC, and MSI testing led to the diagnosis of MSI-high CRC. The iliopsoas muscle infiltration prevented curative resection, but pembrolizumab treatment led to a reduction in tumor size, and the patient underwent laparoscopic partial resection of the descending colon. In both patients, postoperative genetic testing led to the definitive diagnosis of Lynch syndrome. The two cases illustrate the remarkable response of Lynch syndrome-associated CRC to pembrolizumab treatment, which allowed for conversion resection.