A case of dramatic response to pembrolizumab monotherapy in dMMR unresectable colorectal cancer diagnosed following abdominal hemorrhage due to segmental arterial mediolysis.

We present a case of mismatch repair deficient (dMMR) descending colon cancer complicated by peritoneal dissemination and adjacent organ invasion, initially considered unresectable. The patient, a 68-year-old male, experienced acute intra-abdominal hemorrhage due to spontaneous rupture of a pseudoaneurysm from segmental arterial mediolysis (SAM). Emergency surgery confirmed unresectability, prompting immunotherapy with pembrolizumab. Remarkably, after four cycles, significant tumor regression allowed for curative resection, with complete pathological response. Immunohistochemistry revealed a high density of tumor-infiltrating CD8 + T cells and PD-L1-positive stromal cells, indicating robust immune activation. This case underscores immunotherapy's emerging role in converting unresectable dMMR colorectal cancer into resectable disease. This highlights the significance of multidisciplinary management, incorporating immunotherapy, surgical interventions, and vascular assessment to optimize clinical outcomes in dMMR colorectal cancer complicated by vascular pathologies such as SAM. Further investigation into the relationship between vascular pathologies, such as SAM, and malignancy is warranted.