Complete pathological response with pembrolizumab in recurrent sigmoid adenocarcinoma

Genetic evaluation for mismatch repair deficiency (dMMR) or microsatellite instability (MSI) is now routinely performed as part of the workup of colorectal cancer. The traditional approach to advanced disease was chemotherapy-based agents; however, dMMR/MSI patients were found to respond more poorly. Immunotherapy, such as pembrolizumab, a programmed cell death protein-1 inhibitor, has evolved as a superior option. This report describes a 48-year-old with a history of stage IIC sigmoid adenocarcinoma who developed recurrent uterine and peritoneal implants following surgical resection and adjuvant chemotherapy. Given the concern for recurrent disease, the appropriateness of colostomy reversal remained uncertain. The patient subsequently underwent 20 cycles of pembrolizumab followed by colostomy reversal with concurrent resection of suspected implants. Final pathology revealed a complete pathological response (cPR) with no evidence of residual malignancy. This case highlights the potential for pembrolizumab to induce a cPR in dMMR colorectal cancer despite the absence of radiographic regression.