FDA Approves Atezolizumab Plus Chemotherapy After Surgery for Stage III dMMR Colon Cancer

First Postoperative Immunotherapy Approval in Colon Cancer: FDA Approves Atezolizumab + FOLFOX for Stage III dMMR Disease

On October 8, 2026, the U.S. Food and Drug Administration (FDA) approved atezolizumab (Tecentriq) in combination with fluoropyrimidine- and oxaliplatin-based chemotherapy for postoperative (adjuvant) treatment of stage III mismatch repair-deficient (dMMR) colon cancer. The approval also covers children aged 2 years and older and the subcutaneous formulation, Tecentriq Hybreza. This marks the first time an immunotherapy drug has been approved as preventive postoperative treatment in colon cancer. We have previously covered the results of the ATOMIC trial on which the approval is based, the uses of Tecentriq and the approval of dostarlimab in dMMR rectal cancer.

Why does this matter?

Colon cancer is one of the most common cancers worldwide. In stage III disease, which has spread to the lymph nodes but not to distant organs, the standard treatment is surgery followed by 3–6 months of oxaliplatin-based chemotherapy (FOLFOX or CAPOX). Despite this treatment, the disease recurs in some patients.

In about 10–15% of stage III colon cancers, the system that repairs copying errors in DNA (mismatch repair) does not work. These tumors are called dMMR or microsatellite instability-high (MSI-H). Some of them are associated with hereditary Lynch syndrome. Because these tumors carry a large number of mutations, they are easily recognized by the immune system and respond very well to immunotherapy. On the other hand, the benefit of chemotherapy in stage III dMMR tumors is more limited than in other colon cancers, and the risk of recurrence remains high, particularly in those with extensive lymph node involvement.

How is atezolizumab used in stage III dMMR colon cancer?

In stage III dMMR colon cancer, atezolizumab is given after surgery together with mFOLFOX6 chemotherapy (oxaliplatin, leucovorin and fluorouracil) for 6 months, followed by 6 months on its own; total treatment lasts about 1 year. In adults, the intravenous dose is 840 mg every 2 weeks, 1200 mg every 3 weeks or 1680 mg every 4 weeks; the subcutaneous formulation is given as a single injection every 3 weeks.

What is the approval based on? The ATOMIC trial

Results

At a median follow-up of 40.9 months, adding atezolizumab halved the risk of recurrence or loss of life (HR 0.50; 95% CI 0.35–0.73; p=0.0001). 3-year disease-free survival was 86.3% in the atezolizumab arm and 76.2% in the chemotherapy-only arm; in other words, about 10 more of every 100 patients remained free of recurrence. Median disease-free survival was not reached in either arm.

In overall survival, however, there was no significant difference: at a median follow-up of 45.8 months, 5-year overall survival was 89.7% vs 87.9% (HR 0.90; 95% CI 0.55–1.47). One possible explanation is that patients who relapsed in the control arm could be treated effectively with immunotherapy in the advanced setting. So, for now, the benefit of atezolizumab appears to lie less in prolonging survival than in reducing recurrence and the burden of further treatment that comes with it.

Safety

Adding atezolizumab increased adverse events. Grade 3 or higher adverse events occurred in 84.1% vs 71.9%. Prominent immune-related adverse events included hypothyroidism (19.9% vs 3.9%), inflammation of the bowel (colitis; 6.6% vs 0.6%) and adrenal insufficiency (1.2% vs 0%). Treatment-related loss of life (sudden loss of life and sepsis) was reported in 2 patients in the atezolizumab arm. Adverse events affecting the hormone-producing glands can be permanent and may require lifelong hormone therapy (side effects of immunotherapy).

Approved doses

The atezolizumab-alone phase is stopped earlier if the disease recurs or unacceptable toxicity develops. A subcutaneous dose has not been established for children under 12 years or patients weighing less than 40 kg.

How should the results be interpreted?

Strengths

• A large, biomarker-selected phase 3 trial, the first designed specifically for dMMR stage III colon cancer.
• The halving of recurrence risk and an absolute difference of about 10 percentage points at 3 years are clinically meaningful.
• It was conducted by an academic research group (Alliance) and published in a peer-reviewed journal.
• dMMR testing can be done as part of routine pathology workup, which makes patient selection straightforward.

Points to keep in mind

• No overall survival difference: No significant survival gain has been shown so far; longer follow-up is needed.
• Open-label design and investigator assessment: The primary endpoint was assessed by the investigators, not by independent central review.
• More adverse events, which may be permanent: Hormonal problems such as thyroid and adrenal insufficiency may require lifelong treatment.
• Duration of chemotherapy: The control arm received 6 months of FOLFOX; there was no comparison with 3 months of chemotherapy, which is now commonly used in lower-risk patients.
• New questions: The very high response rates with preoperative immunotherapy using nivolumab and ipilimumab (NICHE-2) suggest that some patients may one day not need postoperative chemotherapy at all; these approaches have not been compared head to head.

What does this mean in practice?

• dMMR testing in every colon cancer: Immunohistochemical staining for mismatch repair proteins (MLH1, MSH2, MSH6, PMS2) or MSI testing on the resected tumor is now essential for treatment decisions. Chemotherapy can be started while awaiting the result; in ATOMIC, 1 cycle of chemotherapy was allowed while the test result was pending.
• Evaluation for Lynch syndrome: Patients found to have dMMR should be evaluated for hereditary Lynch syndrome; this also matters for screening relatives (hereditary variant analysis).
• Shared decision-making: The marked reduction in recurrence risk should be discussed with the patient together with the additional year of treatment, hormonal adverse events that may be permanent, cost, and a survival benefit that has not yet been shown.
• Monitoring for adverse events: Thyroid hormones, blood glucose, and liver and kidney values should be monitored regularly throughout treatment; symptoms such as diarrhea, shortness of breath or extreme fatigue should be evaluated promptly (kidney side effects).
• Subcutaneous formulation: The subcutaneous injection can be given in a few minutes and shortens time spent in the hospital; efficacy similar to the intravenous formulation had previously been shown in lung cancer.

Practical tips for patients and their families

• If you have had surgery for stage III colon cancer, ask whether your tumor has been tested for dMMR/MSI and what the result was.
• If your tumor is dMMR, tell your care team whether there is any colon, uterine or ovarian cancer in your family; genetic counseling may be needed.
• Ask for the benefits and risks of adding immunotherapy to chemotherapy in numbers, especially permanent adverse events that may affect hormone-producing glands such as the thyroid.
• During treatment, contact your care team right away if you have symptoms such as diarrhea, abdominal pain, bloody stools, shortness of breath, cough, extreme fatigue, dizziness, skin rash or jaundice.
• Do not skip regular follow-up and your colonoscopy schedule after treatment; healthy eating and exercise can help lower the risk of recurrence.
• Do not hesitate to get a second opinion while making your decision.

Frequently asked questions

Which immunotherapy did the FDA approve for colon cancer?

On October 8, 2026, the FDA approved atezolizumab (Tecentriq) in combination with fluoropyrimidine- and oxaliplatin-based chemotherapy for postoperative treatment of completely resected stage III dMMR colon cancer. The approval covers adults, children aged 2 years and older, and the subcutaneous formulation in patients aged 12 years and older weighing 40 kg or more.

How much does adding atezolizumab reduce the risk of recurrence?

In the ATOMIC trial, the risk of recurrence or loss of life was halved (HR 0.50). 3-year disease-free survival was 86.3% vs 76.2%; in other words, about 10 more of every 100 patients remained free of recurrence. For now, however, there is no significant difference in overall survival.

How long does the treatment last?

After surgery, atezolizumab is given together with chemotherapy for 6 months, followed by 6 months of atezolizumab alone; about 1 year in total. The intravenous formulation is given every 2, 3 or 4 weeks; the subcutaneous formulation every 3 weeks.

Is this treatment available in Turkey?

Atezolizumab is licensed in Turkey, but there is not yet marketing authorization or SGK reimbursement for postoperative use in stage III dMMR colon cancer; this use requires off-label approval.

Glossary

dMMR / MSI-H

Tumors in which the mismatch repair system, which corrects DNA copying errors, does not work (dMMR) and, as a result, repetitive sequences in the tumor DNA become unstable (MSI-H).

Lynch syndrome

A syndrome in which an inherited change in mismatch repair genes increases the risk of colon, uterine, ovarian and other cancers.

Adjuvant therapy

Preventive treatment given after visible tumor has been removed by surgery to reduce the risk of recurrence.

Atezolizumab

An immunotherapy drug that blocks the PD-L1 protein on tumor cells, enabling the immune system to attack cancer cells.

mFOLFOX6

A chemotherapy regimen made up of oxaliplatin, leucovorin (folinic acid) and fluorouracil, given every 2 weeks.

Disease-free survival

The time after surgery during which the disease does not recur and no loss of life occurs.

Project Orbis

An international collaboration program through which the FDA reviews cancer drugs concurrently with the drug regulatory authorities of other countries.