Rectal Cancer Biomarkers
MSI-H rectal cancer is producing some of the most remarkable results in oncology — recent trials show immunotherapy alone achieving complete responses in some patients, potentially avoiding surgery.
Reviewed by XpertPatient Medical Advisory · Updated June 2026 · Sources: NCI · ACS · ASCO · NIH/PMC
Start Here
What is a biomarker?
A biomarker in rectal cancer is a measurable signal in your tumor tissue that tells your doctor how your cancer is likely to behave — and which treatments it will respond to. The two most important ones are MSI status and RAS mutation status.
Rectal cancer recently produced one of the most remarkable results in all of oncology. Patients with a specific biomarker — MSI-H — treated with immunotherapy had their tumors disappear completely, without surgery or radiation. Knowing your biomarker results is essential.
What this means for you: Two tests matter most: MSI/dMMR status and RAS mutation status. Together they determine whether immunotherapy, targeted drugs, or standard chemotherapy is the right approach for your cancer.
The Tests Your Doctor Orders
🔍 Does your cancer respond dramatically well to immunotherapy?
MSI-H rectal cancer is rare — about 5–8% of cases — but responds dramatically to immunotherapy. A 2022 MSK trial found that every MSI-H rectal cancer patient treated with dostarlimab had a complete response. The tumor disappeared entirely in all patients. Some avoided surgery and radiation completely.
🔍 Do you have a mutation that blocks certain drugs from working?
RAS mutations are found in about 50% of rectal cancers. If mutated, drugs like cetuximab and panitumumab will NOT work. Knowing this before treatment protects you from ineffective therapy.
🔍 Is there a mutation that tells your doctor about cancer behavior?
TP53 mutations are common in rectal cancer and serve as a prognostic marker — they help your doctor understand how the cancer is likely to behave and plan treatment intensity.
Watch & Learn
Common Questions
Rectal Cancer Biomarkers — Your Questions Answered
What happened in the MSK dostarlimab trial?
In a 2022 NEJM paper, 12 patients with MSI-H locally advanced rectal cancer were treated with dostarlimab for 6 months. All 12 had a complete clinical response — no tumor detectable on any scan or exam. None needed surgery or radiation. Subsequent follow-up confirmed similar results in more patients.
If I have MSI-H rectal cancer, do I still need surgery?
Possibly not. MSI-H rectal cancer patients who achieve a complete response to immunotherapy are being managed with close monitoring instead of surgery — called ‘watch and wait.’ Discuss this with your oncologist at a major cancer center.
What if my rectal cancer is MSS?
Most rectal cancers are MSS (microsatellite stable). Standard treatment for locally advanced MSS rectal cancer is chemoradiation followed by surgery. For metastatic MSS rectal cancer, RAS and BRAF testing guides which drugs can be used.
Should I see a specialist?
Yes. Rectal cancer — especially if MSI-H — requires a multidisciplinary team including a colorectal surgeon, radiation oncologist, and medical oncologist. Seek care at a center with experience in rectal cancer.
Know Your Biomarkers. Own Your Journey.
Free, survivor-led cancer education for newly diagnosed patients.
Visit the Rectal Cancer Center → Full Biomarkers Guide →
