Striking Discordance Between Morphological Residual Disease and Histological Pathological Complete Response: A Case Report of Pembrolizumab Treatment in Mismatch Repair-Deficient Colorectal Cancer With Liver Metastases.

Immune checkpoint inhibitors (ICIs) have demonstrated high response rates in mismatch repair-deficient (dMMR) colorectal cancer (CRC). However, radiological or macroscopic findings may suggest residual tumor even when a pathological complete response (pCR) has been achieved. We report a case of dMMR CRC with liver metastases in which pCR was confirmed in both the primary tumor and metastatic lesions after preoperative pembrolizumab therapy. A 70-year-old man with diabetes mellitus and hypertension presented with anemia and was diagnosed with sigmoid colon cancer. Further evaluation revealed cStage IVa dMMR CRC with multiple liver metastases. Laparoscopic transverse colostomy was performed for tumor-related obstruction. Although resection of both the primary tumor and liver metastases was initially considered, treatment of ischemic heart disease with percutaneous coronary intervention was prioritized, and pembrolizumab was administered for six courses during this period. After marked reduction of the primary tumor and liver metastases, robot-assisted sigmoid colectomy followed by laparoscopic right hepatectomy was performed. Although preoperative imaging and intraoperative findings suggested residual tumor, pathological examination revealed no viable cancer cells at either site. This case highlights the discrepancy between morphological residual disease and pathological response after ICI therapy and underscores the importance of comprehensive response assessment.