In a major step forward for colorectal cancer treatment, researchers at the Mayo Clinic have found that combining immunotherapy with chemotherapy can dramatically improve outcomes for people with a specific type of stage 3 colon cancer. The study, which focused on patients whose tumours showed what’s known as “deficient mismatch repair” (or dMMR), showed that this combined approach reduced the risk of cancer coming back—or causing death—by around 50% compared to chemotherapy alone. For this group of patients, who haven’t always responded well to traditional treatment, this is a genuinely hopeful development.
The findings were recently shared in a press release from the Mayo Clinic and are expected to influence how this form of cancer is treated in the future. It’s a change that could shift the standard of care for thousands of people each year.
What is dMMR—and why does it matter?
To understand why this research is important, it’s useful to know what dMMR means. Normally, our cells have internal proofreading systems that correct errors when DNA is copied. In some people with colon cancer—around 15%, according to the American Cancer Society—that system is faulty. That’s what scientists call deficient mismatch repair. These dMMR tumours are often more resistant to standard chemotherapy, which makes treatment more challenging.
The condition is also strongly linked to Lynch syndrome, an inherited disorder that significantly raises the risk of colon and several other cancers. For people with dMMR colon cancer, particularly at stage 3 when the cancer has spread to nearby lymph nodes but not yet to other organs, treatment decisions can have a big impact on long-term survival.
What the study found
In the Mayo Clinic-led trial, 712 patients with stage 3 colon cancer and dMMR were enrolled after undergoing surgery to remove their tumours. The patients were split into two groups. One group received standard chemotherapy (which is currently the recommended treatment). The other group was given the same chemotherapy, plus the immunotherapy drug atezolizumab—more commonly known by its brand name, Tecentriq.
Atezolizumab is a type of immunotherapy known as a “checkpoint inhibitor.” It works by helping the immune system detect and attack cancer cells that might otherwise go unnoticed. In simple terms, it takes the brakes off the body’s natural defences.
After follow-up, the results were striking: the combination treatment cut the risk of recurrence and death by half compared to chemotherapy alone. That’s not just a slight improvement—it’s a substantial leap. And perhaps just as importantly, patients didn’t experience a notable increase in side effects. As reported by Reuters, this marks a strong case for changing how we treat this type of colon cancer.
What this could mean for patients
For people diagnosed with dMMR stage 3 colon cancer, this is an encouraging sign that better, more personalised treatment is on the horizon. If these findings are included in updated clinical guidelines—which the Mayo team intends to recommend to the National Comprehensive Cancer Network (NCCN)—it could become the new standard treatment protocol.
Lead investigator Dr Frank Sinicrope noted that this could have widespread impact: “Our results support the use of immune checkpoint blockade in earlier-stage colon cancer, and these findings could alter treatment practice.” The study highlights the importance of identifying the right treatment for the right patient at the right time—something known as precision oncology.
Why this is part of a bigger trend
This isn’t the first time immunotherapy has shown promise in treating dMMR tumours. What makes this trial different is that it targets people with stage 3 disease—when there’s still a good chance of beating the cancer, but the risk of recurrence remains high. Previous studies had mostly focused on later-stage, metastatic cancer.
The new research is part of a broader movement in cancer care that aims to use the body’s own immune system to fight disease. And while not every type of cancer responds to immunotherapy, patients with dMMR tumours are particularly good candidates. Their cells tend to mutate more rapidly, producing more abnormal proteins that the immune system can learn to recognise—if it’s given the right tools.
Checkpoint inhibitors like atezolizumab give the immune system those tools. They don’t kill cancer cells directly, but they do remove the defences cancer cells use to hide. It’s a clever and increasingly effective strategy.
The importance of genetic testing
One of the most important lessons from this study is the value of genetic testing in cancer diagnosis and treatment. Without testing for dMMR status, doctors wouldn’t know which patients might benefit from immunotherapy. Yet not all patients are routinely screened for this.
As Medical News Today explained, widespread testing for dMMR could lead to more personalised, and more successful, treatment plans. It could also prevent patients from undergoing chemotherapy that might not be effective on its own.
Moving forward, routine testing for tumour genetics could become a standard part of cancer care—not just for colon cancer but for many types of cancer where targeted treatments are available.
What’s next?
The Mayo Clinic team hopes these findings will lead to fast updates in treatment guidelines. They’re also planning additional studies to see if immunotherapy could benefit even more patients—such as those with earlier-stage cancer or people whose tumours have other genetic traits.
In the meantime, these results offer hope to thousands of people facing stage 3 colon cancer with dMMR. They prove that by understanding a tumour’s genetic profile and using that knowledge to tailor treatment, it’s possible to get better outcomes without adding unnecessary risk.
This isn’t a cure, and it won’t apply to every colon cancer case. But for a subset of patients who historically haven’t had great treatment options, it’s a major step forward. It’s also a sign that cancer care is moving in a direction where treatment is no longer one-size-fits-all. And that’s something everyone can be hopeful about.