Complete remission after immunotherapy in a rare colon cancer subtype—dMMR/MSI-H colon SCC.

Complete remission after immunotherapy in a rare colon cancer subtype—dMMR/MSI-H colon SCC.

A case report of dMMR/MSI-H ascending colon squamous cell carcinoma (primary SCC of colon is extremely rare) treated with 4 cycles of PD-1 blockade monotherapy. The tumor showed significant regression and pathological complete response (pCR) was achieved after surgical resection. Demonstrates that anti-PD-1 can induce clinically meaningful tumor regression in this rare subtype based on dMMR/MSI-H status.

Key Findings

• dMMR/MSI-H colon SCC is extremely rare but responds to PD-1 blockade
• 4 cycles of PD-1 blockade monotherapy led to significant tumor regression
• Pathological complete response achieved after surgical resection
• Anti-PD-1 activity in this rare subtype guided by dMMR/MSI-H status
• Feasibility and safety require further validation through clinical studies

Implications

dMMR/MSI-H testing should be performed for rare colon cancer subtypes, as immunotherapy may have activity regardless of histological subtype. This provides a treatment strategy for otherwise poorly managed primary colon SCC.

Caveats

Single case report; abstract-only. Extremely rare cancer—no randomized evidence. Limited generalizability.

Source: Frontiers in immunology — 2026-01-01