For decades, the standard playbook for colon cancer has been straightforward: cut out the tumor first, then decide whether chemotherapy is needed afterward. A growing body of research is now flipping that sequence on its head — and the early results are striking enough to reshape how oncologists treat a significant subset of patients.
A series of clinical trials reported through 2024 and 2025, with follow-up data presented in early 2026, suggest that giving immune checkpoint inhibitors before surgery — an approach called neoadjuvant immunotherapy — can dramatically reduce the chance that colon cancer comes back. In some studies, tumors disappeared so completely that researchers questioned whether surgery was still necessary at all.
What Is Neoadjuvant Immunotherapy?
Immunotherapy drugs known as immune checkpoint inhibitors — including pembrolizumab, nivolumab and ipilimumab — work by releasing the brakes on the body’s own T cells, allowing them to recognize and attack tumor cells. These drugs have been used for years in advanced melanoma, lung cancer and other malignancies.
“Neoadjuvant” simply means treatment given before the main therapy, which in colon cancer is usually surgery. The logic of giving immunotherapy first is biological: when the tumor is still in place, it acts as a kind of training ground for the immune system, exposing T cells to a full repertoire of tumor antigens that can then patrol the body long after surgery.
The Research Driving the Shift
The most influential data come from the NICHE-2 trial, published in The New England Journal of Medicine, which enrolled patients with non-metastatic colon cancer that carried a specific genetic feature called mismatch repair deficiency (dMMR). Patients received two doses of immunotherapy in the weeks before scheduled surgery.
The results, according to the published trial, were unprecedented for early-stage colon cancer: roughly two-thirds of tumors showed a pathologic complete response, meaning no viable cancer cells were detectable in the surgical specimen. Three-year follow-up data presented at the American Society of Clinical Oncology (ASCO) annual meeting showed disease-free survival rates above 95 percent — far higher than historical benchmarks for this patient group.
A separate study from Memorial Sloan Kettering Cancer Center, also published in NEJM, reported that every one of the first dozen patients with dMMR rectal cancer treated with the immunotherapy dostarlimab experienced complete tumor regression without surgery or radiation. Follow-up extending into 2026 has continued to show durable responses.
Who Benefits Most
The dramatic results are largely confined to a specific tumor subtype. Mismatch repair-deficient or microsatellite instability-high (MSI-H) tumors account for roughly 15 percent of colon cancers, according to the National Cancer Institute. These tumors carry an unusually high number of mutations, which translates into more abnormal proteins on their surface — making them easier for the immune system to recognize once checkpoint inhibitors are administered.
Patients with these tumors often have Lynch syndrome, an inherited condition that raises lifetime cancer risk, or develop the genetic feature spontaneously. Standard tumor testing performed after diagnosis can identify candidates.
For the larger group of patients with mismatch repair-proficient (pMMR) colon cancer, immunotherapy alone has historically been less effective. However, ongoing trials are testing combinations with chemotherapy or radiation to broaden the approach.
Why the Timing Matters
Surgeons and oncologists have long debated whether intervention order influences outcomes. Research published in Nature Medicine has suggested that immunotherapy delivered before surgery generates a broader and longer-lasting immune response than the same drugs given after the tumor has been removed. The reasoning: once the tumor is gone, there are fewer antigens for the immune system to learn from.
A 2025 review in The Lancet Oncology noted that the neoadjuvant approach also offers a real-time window to assess whether a patient is responding, allowing clinicians to adjust treatment quickly rather than waiting months for recurrence data.
What This Means for Patients
Colon cancer remains one of the most common cancers in the United States, with the American Cancer Society estimating more than 150,000 new cases and approximately 53,000 deaths each year. Recurrence after surgery is a major driver of mortality, particularly for patients with locally advanced disease.
If neoadjuvant immunotherapy continues to deliver these results, it could:
- Reduce the extent of surgery required, preserving more healthy bowel tissue
- Lower long-term recurrence rates for patients with dMMR tumors
- Spare some patients from chemotherapy after surgery
- In select rectal cancer cases, potentially eliminate the need for surgery altogether
However, immune checkpoint inhibitors carry their own risks. Side effects can include immune-related inflammation of the lungs, liver, thyroid, colon and other organs. Most are manageable when caught early, but a small percentage can be serious.
Cautions and Open Questions
Researchers note that the longest follow-up data are still relatively short — typically three to five years. Whether responses will hold over decades, particularly for younger patients with Lynch syndrome, remains unknown. Trials are also exploring whether shorter or lower-dose regimens can achieve the same benefit with fewer side effects.
The approach is currently most established at major cancer centers with experience in immunotherapy. Access varies, and insurance coverage for off-label or trial-based use can be inconsistent.
The Practical Takeaway
For patients newly diagnosed with colon or rectal cancer, the most important early step is asking whether the tumor has been tested for mismatch repair status or microsatellite instability. National guidelines from the National Comprehensive Cancer Network now recommend universal testing for all colorectal cancers — but practice varies.
Patients whose tumors test positive may be candidates for clinical trials or established neoadjuvant immunotherapy protocols. A second opinion at an academic cancer center can help clarify options.
The broader story is one that increasingly defines modern oncology: cancers that look identical under a microscope can behave very differently based on their molecular fingerprint. Identifying those differences early — and matching treatment to biology rather than to historical defaults — is changing what is possible.
Disclosure: This content is for informational purposes only and is not medical advice. Always consult a qualified healthcare provider before making changes to your health regimen.