EvidenceMismatch-repair deficient (MSI-high) colorectal cancer

Nivolumab-ipilimumab is a first-line standard for microsatellite-unstable metastatic colorectal cancer alongside pembrolizumab, with the trade-off of more immune toxicity for deeper and more durable control.

For mismatch repair-deficient rectal cancer, six months of a single antibody now replaces chemotherapy, radiotherapy and an operation, and the same appears to be true for early-stage mismatch repair-deficient cancers of other organs.

Combination checkpoint blockade became a first-line standard for mismatch repair-deficient metastatic colorectal cancer in 2025; the later all-lines comparison against nivolumab alone showed the CTLA-4 antibody adds to the PD-1 antibody, the first phase 3 to prove that in this disease.

For colon cancer that is mismatch-repair deficient (about 10-15% of colon cancers, more in older patients), a single short course of immunotherapy before surgery is now a reasonable standard and is far more effective than chemotherapy, which barely works in this subtype. It requires testing every colon cancer for mismatch repair at diagnosis, before surgery. Whether some patients can safely skip surgery, as in dMMR rectal cancer, is the next question.

Cercek's dostarlimab study is the clearest demonstration that immunotherapy can replace surgery in a solid tumour: patients with dMMR rectal cancer can keep their rectum and avoid the permanent effects of pelvic radiotherapy and surgery. Non-operative management after PD-1 blockade is now in guidelines for this group, and MMR testing before treatment of rectal cancer is essential. The approach applies only to the 5-10% of rectal cancers that are dMMR.

Daily aspirin is recommended for people with Lynch syndrome; the optimal dose is being tested in CaPP3.

First-line chemotherapy-free treatment for mismatch repair-deficient metastatic colorectal cancer, approved in June 2020; the five-year follow-up published in 2024 reported median overall survival of 77.5 months despite 62 percent crossover.

Every colorectal cancer should be tested for mismatch repair deficiency, because patients whose metastatic tumour is dMMR should receive pembrolizumab rather than chemotherapy as first treatment, gaining a much better chance of durable remission with fewer side effects. About a third of dMMR tumours do not respond initially, so early scans are essential and chemotherapy remains available. The trial does not apply to the 95% of metastatic colorectal cancers that are mismatch-repair proficient (the deficient share is higher in localised disease, about 15%).