Biomarker Resource Center
Rectal Cancer Biomarkers
Reviewed by XpertPatient Medical Advisory · Updated August 2026 · Primary Source: cancer.gov
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What Is a Biomarker — and Why Does It Matter for Your Treatment?
A biomarker in rectal cancer is a signal found in your tumor tissue. Two biomarkers matter most: MSI status and RAS mutation status. Together they tell your doctor which treatment approaches will work best for your cancer.
Think of it like a clue. Your doctor collects clues from your biomarker test results. Those clues help build a picture of your cancer. That picture helps you have a more informed conversation with your care team about next steps.
How a Biomarker Test Works
A biomarker is a signal your cancer leaves behind. Your doctor finds it through a biopsy or blood test, reads your results, and uses them to choose the treatment most likely to work for your specific tumor.
Why Do Biomarkers Matter in Rectal Cancer?
Rectal cancer recently produced one of the most remarkable results in all of cancer medicine. Patients with a specific biomarker — MSI-H — treated with immunotherapy had their tumors disappear completely without surgery. Knowing your results can change everything.
The most important rectal cancer biomarkers are MSI status and RAS mutation status. Together they shape your entire treatment approach.
A good question to ask your doctor: Ask your doctor for both your MSI result and your RAS result before treatment begins.
Quick Poll
Have you been biomarker tested?
MSI and RAS results are the two most important biomarkers in determining your rectal cancer treatment path.
Source: Adekunle et al., JAMA Network Open, 2024 · MSI testing rate in metastatic colorectal cancer (n=41,061) · jamanetwork.com
The Tests Your Doctor May Order
Does your cancer respond dramatically well to immunotherapy?
MSI-H rectal cancer is found in about 5 to 8% of cases. A 2022 clinical trial found that patients with MSI-H rectal cancer who received immunotherapy had their tumors disappear completely — without surgery or radiation. Ask your doctor whether MSI testing has been done and what your result is.
Do you have a mutation that affects which approaches may help?
RAS mutations are found in about half of rectal cancers. This result tells your doctor which approaches are appropriate and which are not. Ask your doctor what your RAS result is.
Is there a specific mutation linked to more aggressive disease?
BRAF V600E is found in some rectal cancers and is linked to more aggressive behavior. There are approaches specifically for BRAF-mutated colorectal cancer. Ask your doctor whether BRAF testing was done and what your result means.
Could you avoid surgery if immunotherapy works?
For MSI-H rectal cancer patients who have a complete response to immunotherapy, a watch-and-wait approach is being used at major cancer centers. Ask your doctor whether this approach may be relevant for your situation.
Sources: National Cancer Institute · Fight Colorectal Cancer · American Cancer Society
Watch & Learn
Common Questions
Rectal Cancer Biomarkers — Your Questions Answered
What happened in the 2022 rectal cancer immunotherapy trial?
In a 2022 NEJM paper, patients with MSI-H locally advanced rectal cancer received immunotherapy for 6 months. All had a complete response — no tumor was found on any scan or exam. None needed surgery or radiation.
If I have MSI-H rectal cancer, do I still need surgery?
Possibly not. Patients with a complete response to immunotherapy are being monitored closely instead of having surgery at some major cancer centers. Discuss this with your oncologist.
What if my rectal cancer is not MSI-H?
Standard treatment for locally advanced MSS rectal cancer is chemoradiation followed by surgery. For metastatic MSS rectal cancer, RAS and BRAF testing guides which approaches can be used.
Should I see a specialist?
Yes. Rectal cancer requires a team including a colorectal surgeon, radiation oncologist, and medical oncologist. Seek care at a center with experience in rectal cancer.
Knowledge Check
Test what you’ve learned
Three quick questions to help you feel more confident talking with your care team.
What did the 2022 rectal cancer immunotherapy trial find?
Based on what you just read, what happened to MSI-H rectal cancer patients who received immunotherapy?
What does a RAS mutation mean for rectal cancer treatment?
Based on what you just read, how does a RAS mutation affect your care options?
What is watch-and-wait in rectal cancer?
Based on what you just read, what does watch-and-wait mean?
Keep these in mind for your next appointment.
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