The FDA has approved Roche’s immunotherapy Tecentriq (atezolizumab), and its under-the-skin version Tecentriq Hybreza, in combination with chemotherapy for people with stage III colon cancer whose tumors have a DNA-repair defect known as deficient mismatch repair (dMMR). It is given after surgery to reduce the chance of the cancer coming back, and is the first immunotherapy-based combination approved for this purpose.

What the trial showed

The approval is based on the Phase 3 ATOMIC trial, which enrolled about 700 patients whose stage III dMMR colon cancer had been surgically removed. Half received standard chemotherapy (a regimen called mFOLFOX6) for six months; half received the same chemotherapy plus Tecentriq, continued for a total of a year. Adding Tecentriq reduced the risk of recurrence or death by 50%. In results presented in 2025, about 86% of patients on the combination were alive and free of disease at three years, compared with about 77% on chemotherapy alone.

What stage III means

In stage III colon cancer, the tumor has spread to nearby lymph nodes but not to distant organs. Just under a quarter of colon cancers are found at this stage. Surgery removes the visible cancer, and chemotherapy afterwards, called adjuvant therapy, aims to kill any stray cells left behind. Even so, about one in three patients sees the cancer return within five years, usually in the liver or lungs, when it is much harder to cure.

What dMMR is, and why it matters

Every time a cell divides, it copies its DNA, and a set of proteins called the mismatch repair system fixes the copying errors. In roughly 15% of stage III colon cancers, that system is broken. These dMMR tumors, also described as MSI-high, accumulate thousands of mutations. That turns out to be a weakness: the mutations produce abnormal proteins that make the tumor look foreign to the immune system. Such tumors respond unusually well to checkpoint inhibitors like Tecentriq, which release a brake on the immune system’s T cells — and, notably, they tend to respond less well to chemotherapy alone.

How immunotherapy reached early-stage disease

Checkpoint inhibitors first proved themselves in dMMR colorectal cancer that had already spread, where they outperformed chemotherapy. In locally advanced rectal cancer with the same defect, immunotherapy alone has made tumors disappear entirely in small studies, allowing some patients to avoid surgery. ATOMIC answers the next question: whether adding immunotherapy after surgery, when no cancer is visible, prevents relapse. Roche’s chief medical officer, Levi Garraway, called the combination a “new standard of care,” and lead US investigator Frank Sinicrope of Mayo Clinic described the results as practice-changing for this group.

Who should be tested

The approval makes mismatch repair testing more important than ever. Guidelines already recommend that every colorectal cancer be tested for dMMR or MSI, usually on tissue removed at surgery or biopsy, partly because the result affects treatment and partly because it can reveal Lynch syndrome, an inherited condition that raises the risk of several cancers and has implications for relatives. Patients with colon cancer can ask their care team whether their tumor has been tested and what the result was.

Side effects and trade-offs

Immunotherapy has a different side-effect profile from chemotherapy. By loosening the immune system’s brakes, it can cause the immune system to attack healthy organs, leading to inflammation of the thyroid, skin, bowel, liver or lungs. Most such effects are manageable when caught early, but some are serious, and a few, such as thyroid or adrenal problems, can be permanent and require lifelong hormone replacement. Treatment also lasts a year rather than six months. For a patient who would have been cured by surgery and chemotherapy alone, those are real costs — but there is currently no reliable way to know in advance who those patients are.

What it doesn’t cover

The approval applies only to the minority of colon cancers with dMMR. The roughly 85% of stage III tumors with intact mismatch repair generally do not respond to checkpoint inhibitors on their own, and standard chemotherapy remains the treatment after surgery. Researchers are testing combinations intended to make those tumors visible to the immune system, so far with limited success.

Open questions

Several questions remain. It is not yet known whether the benefit will translate into longer overall survival, which takes more follow-up. Some specialists ask whether chemotherapy is needed at all for dMMR tumors, or whether immunotherapy given before surgery, which has produced striking tumor shrinkage in early studies, might work even better. Blood tests that detect traces of tumor DNA after surgery may eventually help decide who needs additional treatment.

What it means

For people with stage III dMMR colon cancer, the approval offers a meaningfully better chance of staying cancer-free. Roche is also seeking approval in Europe. Commercially, the new use arrives late in Tecentriq’s life: key patents begin to expire in 2029 and analysts expect sales to decline after a 2028 peak. For patients, the practical step is simple — make sure the tumor has been tested, and discuss the result with an oncologist. This is regulatory news, not medical advice.