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Colorectal Cancer — SCE Medical Oncology MCQ

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ModerateGI CancersColorectal CancerSCE Medical Oncology

A 70‑year‑old man with metastatic colorectal cancer (MSS, RAS‑mutant) has progressed on FOLFOX‑bevacizumab and FOLFIRI. He is ECOG PS 0. A trial of a novel anti‑CTLA‑4 + anti‑PD‑1 combination for MSS mCRC is enrolling. What should be discussed about the expected response rate of immunotherapy in MSS colorectal cancer?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: A — MSS CRC has traditionally shown very low response rates (<5%) to standard checkpoint inhibitors, though novel combinations are being explored

Explanation lettering: E = shown as D · D = shown as E

Microsatellite‑stable (MSS) colorectal cancer is immunologically 'cold' and has shown minimal benefit from standard checkpoint inhibitor monotherapy, with overall response rates typically below 10%, and often under 5%. Novel combinations (e.g. anti‑CTLA‑4 plus anti‑PD‑1, or with TKIs) are under active investigation due to the limited activity of monotherapy. Option A correctly states this dual message and is the only defensible choice. Options B, D, C and E are incorrect: anti‑CTLA‑4 monotherapy has no proven efficacy (B), MSS disease does not reliably respond regardless of immunotherapy (C), pembrolizumab monotherapy is ineffective in MSS (D), and response rates in MSS are far lower than in MSI‑H (E).

Reference: The Efficacy of Immune Checkpoint Inhibitors in Microsatellite Stable Colorectal Cancer: A Systematic Review, 2024; CheckMate 142 phase II in MSS mCRC, 2026