Emergency Colectomy for Obstruction in Microsatellite Instability Colonic Cancer: A Complete Response Following Neoadjuvant Immunotherapy.

A 40-year-old man with a recent deep vein thrombosis presented with chronic abdominal pain, rectal bleeding, night sweats, and significant weight loss. Imaging and colonoscopy revealed a stenosing transverse colon adenocarcinoma with loss of MSH2 expression, confirming microsatellite instability-high (MSI-H) status. Molecular tests were negative for KRAS, NRAS, and BRAF mutations. The patient received four cycles of pembrolizumab as neoadjuvant therapy but required emergency subtotal colectomy for recurrent obstruction. Histopathology showed no residual tumor, negative lymph nodes, and pT0N0M0 staging, indicating a complete pathological response. This case illustrates the potential of short-course neoadjuvant immunotherapy to induce complete regression in MSI-H colorectal cancer and supports its safe integration into the surgical management of such tumors, even in emergency settings.