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 <i>MSH2</i> expression, confirming microsatellite instability-high (MSI-H) status. Molecular tests were negative for <i>KRAS</i>, <i>NRAS</i>, and <i>BRAF</i> 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.