Spontaneous colonic transection following pathologic complete response to pembrolizumab in high microsatellite instability colorectal cancer: A case report and review of literature

BACKGROUND High microsatellite instability (MSI-H) colorectal cancer (CRC), caused by deficient mismatch repair, accounts for about 15% of all CRC cases and is more common in right-sided tumors. While early-stage MSI-H CRC has a relatively good prognosis, advanced cases often respond poorly to standard chemotherapy. Immune checkpoint inhibitors, such as pembrolizumab, have shown strong and lasting effects in MSI-H CRC. Pembrolizumab is now approved as a first-line treatment for metastatic MSI-H CRC due to its superior outcomes compared to traditional chemotherapy. CASE SUMMARY A 44-year-old male with MSI-H transverse colon cancer presented with hematochezia, abdominal pain, and significant weight loss. Imaging revealed a bulky tumor with invasion of adjacent structures and multiple liver lesions. A diverting ileostomy was performed followed by 36 cycles of pembrolizumab. The patient achieved a clinical and radiologic complete response. One month after completing the treatment, the patient underwent laparoscopic right hemicolectomy. A spontaneous transection of the colon at the original tumor site was unexpectedly identified. Final pathology confirmed pathological complete response (ypT0N0) with fibrosis. The patient recovered well after surgery, and follow-up showed no evidence of recurrence. CONCLUSION Immune checkpoint inhibitors may cause delayed structural damage to bowel tissue even after apparent complete tumor regression.