What Did the FDA Approve for Stage III dMMR Colon Cancer?
On October 8, 2026, the FDA approved atezolizumab (Tecentriq) in combination with chemotherapy as an after-surgery (adjuvant) treatment for stage III mismatch repair deficient (dMMR) colon cancer. In the clinical trial behind this approval, called ATOMIC, adding atezolizumab to standard chemotherapy cut the risk of cancer returning or death by half. If you or someone you love has stage III colon cancer, here’s what this means, who it helps, and the questions worth bringing to your care team.
Who Is This Treatment For?
This approval is for a specific group of colon cancer patients:
- Stage III colon cancer — cancer that has reached nearby lymph nodes but not distant organs
- dMMR or MSI-H tumors — about 15% of colon cancers
- After surgery — the tumor appears to have been has been fully removed, and this treatment lowers the chance it comes back even if some cancer cells remain in your body after the operation
- Adults and children age 2 and older. A version given as a shot under the skin (Tecentriq Hybreza) is approved for people age 12 and older who weigh at least 88 pounds (40 kg).
If your tumor is not dMMR/MSI-H, or your cancer is a different stage, this specific treatment does not apply — which is exactly why testing matters (more on that below).
Why Is This Approval Important?
In the ATOMIC trial, adding atezolizumab to chemotherapy reduced the risk of cancer returning or death by 50% compared to chemotherapy alone. Three years after treatment, 86% of patients who received the combination were cancer-free, compared to 76% who received chemotherapy alone.
This matters because roughly 1 in 3 people with stage III colon cancer have their cancer come back within five years after surgery. For the dMMR group, this is the first after-surgery immunotherapy option proven to narrow that gap.
dMMR tumors carry many genetic changes, which makes them easier for the immune system to spot. Atezolizumab helps your immune system find and attack these tumors — which is why immunotherapy tends to work especially well for this group.
What We Know, and What We’re Still Learning
Here’s an important nuance to understand as you weigh this option. In the trial, this treatment clearly reduced the chance of cancer coming back — that’s the result it was designed to measure, and the evidence for it is strong. What the trial has not yet shown is whether it helps people live longer overall. So far, survival has been similar in both groups, but patients haven’t been followed long enough to answer that question fully.
What does this mean for you? It means this treatment has a proven benefit — staying cancer-free longer — which matters a great deal on its own. It also means the longer-term survival picture is still coming into focus, which is normal for a treatment this new. This is a worthwhile thing to talk through with your care team: how much weight to give a proven reduction in recurrence while the survival data continue to mature.
What Is dMMR / MSI-H — and How Do You Know If You Have It?
dMMR (mismatch repair deficient) and MSI-H (microsatellite instability–high) describe tumors with a high number of genetic changes. You cannot tell if a tumor is dMMR/MSI-H from a scan, a symptom, or the cancer’s stage. The only way to know is a biomarker test done on the tumor tissue.
This is the single most important takeaway. Without biomarker testing, there is no way to know whether this treatment — one that cut recurrence risk in half — is even an option for you.
Why Biomarker Testing Matters So Much
Biomarker testing is the gateway to this treatment and to others. One test can:
- Open the door to targeted treatments like this new immunotherapy approval
- Guide treatment in other settings, including metastatic colorectal cancer
- Reveal inherited risk (such as Lynch syndrome) that may matter for your family
Fight CRC encourages every colorectal cancer patient to ask their care team one question: “Has my tumor had biomarker testing, and what were the results?”
Thinking About This Treatment? Questions to Bring to Your Care Team
Deciding on treatment is a personal decision you and your care team make together. This immunotherapy combination offers a meaningful benefit — in the ATOMIC trial, it cut the risk of cancer returning or death by half — but, like all cancer treatments, it has side effects, and adding immunotherapy to chemotherapy can increase them. The right choice depends on your biomarker results, your overall health, and what matters most to you.
Here are questions and things to think about as you talk with your oncologist.
Start with your biomarker results
- Has my tumor been tested for dMMR or MSI-H status, and what were the results?
- Based on my results, am I a candidate for this treatment?
Have your biomarker report? ChatCRC can help you make sense of it.
You can upload your biomarker results to ChatCRC’s free biomarker report reader to better understand what they mean — so you can walk into your next appointment with clearer questions. ChatCRC helps you understand your report; your care team decides what it means for your treatment. Start at chatbot.fightcolorectalcancer.org or text 318-242-8272 (318-CHATCRC).
Understanding the benefit for me
- Given my specific situation, how much might this treatment lower my chance of the cancer coming back?
- How does this option compare to chemotherapy alone for someone like me?
- This treatment has been shown to lower the chance of cancer coming back. It has not yet been shown to help people live longer overall — that data is still maturing. How should we weigh that as we decide?
Understanding the side effects
- What side effects are most common with this combination, and which are most serious?
- Because this treatment uses my immune system, what immune-related symptoms should I watch for and report right away? (Immune-related side effects — like new diarrhea, rash, shortness of breath, or unusual fatigue — can come on quickly and are most manageable when caught early.)
- How will you monitor me during treatment (for example, thyroid, blood sugar, and blood counts)?
- What happens if I have a reaction — would we pause, treat it, or stop?
Thinking about my life and priorities
- How is this treatment given, and how long does it last?
- How might it affect my daily life, work, and the people who care for me?
- What support is available if side effects become hard to manage?
Getting a full picture
- Would a second opinion be helpful before I decide?
- Are there clinical trials I should consider?
Remember: there is no single right answer that fits everyone. These questions are here to help you and your care team find the choice that’s right for you.
Frequently Asked Questions
What is atezolizumab (Tecentriq)?
Atezolizumab is an immunotherapy drug that helps the immune system recognize and attack cancer cells. The FDA approved it with chemotherapy for stage III dMMR colon cancer on October 8, 2026.
Is atezolizumab chemotherapy?
No. Atezolizumab is an immunotherapy. In this approval, it is given in combination with chemotherapy (a fluoropyrimidine and oxaliplatin), not as a replacement for it.
How is atezolizumab given for colon cancer?
In the ATOMIC trial, patients received atezolizumab by IV alongside chemotherapy for about six months, followed by six months of atezolizumab alone — twelve months of treatment in total. A version given as a shot under the skin (Tecentriq Hybreza) is also approved for eligible patients. Your care team will confirm the exact schedule and dose that’s right for you.
Who qualifies for this treatment?
Adults and children age 2 and older with stage III dMMR colon cancer who have had surgery to remove the tumor. The under-the-skin version is for people age 12 and older who weigh at least 40 kg (88 lbs).
How do I find out if my tumor is dMMR or MSI-H?
Ask your oncologist whether your tumor tissue has been tested for mismatch repair (MMR) or microsatellite instability (MSI) status. This testing is done on the tumor sample, often from your surgery or biopsy.
ChatCRC can help you understand your biomarker results.
Our free AI chatbot includes a biomarker report reader, a clinical trial finder, and trusted colorectal cancer information — available 24/7. Start at chatbot.fightcolorectalcancer.org or text 318-242-8272 (318-CHATCRC).
Explore more in our Resource Library.
Medical Review & Disclaimer
Medically reviewed by Dr. Richard Goldberg [October 12, 2026]. This article is for educational purposes only and is not medical advice. Treatment decisions should always be made with your own oncology care team.
Sources
1. U.S. Food and Drug Administration. “FDA approves atezolizumab in combination with chemotherapy for Stage III mismatch repair deficient colon cancer.” October 8, 2026. fda.gov
2. Sinicrope FA, et al. “Atezolizumab plus FOLFOX for Stage III Mismatch Repair–Deficient Colon Cancer.” New England Journal of Medicine. 2026;394(12):1155-1166. doi.org/10.1056/NEJMoa2507874 (ATOMIC trial, NCT02912559)
3. Roche/Genentech, stage III relapse rates, June 11, 2026.
4. Fight Colorectal Cancer. “Fight CRC Expands ChatCRC with Biomarker Report Reader and Clinical Trial Finder.” fightcolorectalcancer.org