By Xinyu Gao
As colon cancer continues to affect more than 1.9 million people worldwide each year, researchers are urgently seeking new ways to improve survival rates—especially for aggressive tumor types that respond poorly to standard chemotherapy.
One such subtype is deficient mismatch repair (dMMR) colon cancer, caused by genetic errors that occur during DNA replication. dMMR tumors account for 5 percent to 15 percent of colon cancer cases and are known to respond less effectively to traditional chemotherapy compared with other forms of the disease.
But new findings unveiled at the 2025 Annual Meeting of the American Society of Clinical Oncology (ASCO) suggest that adding immunotherapy to standard post-surgical chemotherapy may significantly improve patient outcomes.
A phase 3 trial shows a 50% reduction in recurrence and mortality
In the phase 3 ATOMIC trial, researchers enrolled 712 patients with stage III dMMR colon cancer. Participants averaged 64 years old and had all undergone surgery to remove the tumor. However, cancer cells were still present in nearby lymph nodes—a hallmark of stage III disease—though the cancer had not yet spread to distant organs.
Success
You are now signed up for our newsletter
Success
Check your email to complete sign up
Patients were randomly assigned to one of two groups:
- Chemotherapy alone, or
- Chemotherapy combined with atezolizumab, an immunotherapy drug that blocks PD-L1, helping the immune system recognize and attack tumor cells.
By the end of the study, the results were striking: Patients receiving the combined immunotherapy regimen had a 50 percent lower risk of cancer recurrence or death compared with those who received chemotherapy alone. These findings were measured through disease-free survival, a key indicator in cancer research.
Dr. Frank Sinicrope, an oncologist at the Mayo Clinic and lead investigator on the trial, emphasized the significance of the findings: “This represents a major step forward in adjuvant treatment for stage III dMMR colorectal cancer. We are providing patients with a new therapeutic option that reduces recurrence risk and improves survival. This changes how we approach colorectal cancer care by delivering meaningful benefits earlier in the disease course.”
Experts say the results could shift global treatment guidelines, positioning immunotherapy as a potential new standard regimen for this subtype of colon cancer.
What comes next when treating colon cancer?
Researchers note that longer-term follow-up is still needed, but the trial’s outcome marks one of the most promising advancements for stage III colorectal cancer in recent years. Further studies may also investigate combining immunotherapy with other agents or using it at earlier disease stages.
For now, the results offer renewed hope to thousands of patients worldwide diagnosed each year with the dMMR subtype of colorectal cancer.
