Pathological complete response following immunotherapy in dMMR/MSI-H ascending colon primary squamous cell carcinoma: a case report.

Primary colon squamous cell carcinoma (SCC) is an extremely rare malignancy and associated with a poor prognosis. This case report describes a patient with deficient mismatch repair/microsatellite instability-high (dMMR/MSI-H) ascending colon SCC, who was treated at our institution. After receiving four cycles of programmed cell death -1 (PD-1) blockade monotherapy, the tumor exhibited significant regression, and pathological complete response (pCR) was achieved following surgical resection. This case demonstrates that Anti-PD-1 therapy can induce clinically meaningful tumor regression even in this rare colon SCC subtype, suggesting a potential treatment strategy for dMMR/MSI-H colon SCC. These findings may provide valuable insights for clinicians managing similar cases. However, the feasibility and safety of immunotherapy in dMMR/MSI-H primary colon SCC require further validation through additional clinical studies.