Pathological Complete Response in Locally-advanced MSI-H/dMMR Sigmoid Colon Cancer Treated With Pembrolizumab and...

Microsatellite instability-high (MSI-H) and mismatch repair-deficient (dMMR) colorectal cancers have shown significant responses to immune checkpoint inhibitors such as pembrolizumab, presenting novel opportunities for potentially curative strategies in cases previously deemed unresectable.

A 61-year-old Japanese man presented with abdominal pain and was diagnosed with locally advanced sigmoid colon cancer with abdominal wall invasion. The tumor also involved the small intestine and had formed a fistula extending to the skin. Imaging studies revealed no distant metastases or peritoneal dissemination. Due to difficulty in obtaining a biopsy endoscopically, an ultrasound-guided biopsy was performed during laparoscopic colostomy, and the tumor was diagnosed as adenocarcinoma. Polymerase chain reaction testing revealed MSI-H status, and immunohistochemistry confirmed dMMR. The patient received 12 courses of pembrolizumab, resulting in marked tumor regression. He subsequently underwent sigmoidectomy with en bloc abdominal wall resection and reconstruction using the right fascia lata. Histopathological examination showed no residual tumor cells in either the sigmoid colon or the abdominal wall, indicating a complete pathological response.

This case highlights the potential of pembrolizumab, in combination with aggressive surgical intervention, to achieve curative outcomes in patients with extensively invasive MSI-H/dMMR colorectal cancer.