CION Ameerpet
Immunotherapy · Colorectal & Lower GI Cancer
Immunotherapy for Colorectal Cancer — Only If You Are MSI-High
Immunotherapy helps one group of colorectal cancer patients: those whose tumour is MSI-High, also written dMMR. That is roughly 4 to 5 in every 100 advanced colorectal cancers. If your report says MSS or pMMR, checkpoint inhibitor immunotherapy alone is not the right treatment, and knowing that early matters.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed August 2026
- One line in the report decides it — MMR/MSI status, not the stage and not the symptoms, determines whether immunotherapy is even on the table.
- Roughly 5% of advanced cases qualify — about 4 to 5 in 100 metastatic colorectal cancers are MSI-High. Hearing that plainly prevents months of false hope.
- MSS is not a dead end — around 95% are MSS or pMMR. Chemotherapy, targeted therapy chosen on other molecular results, and trials remain the evidence-based path.
- Testing is standard, not extra — NCCN guidance advises MMR/MSI testing for every colorectal cancer patient. If you have not seen the result, ask for it.
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The short answer
Is Immunotherapy an Option for Colorectal Cancer?
Only for a small minority. Immunotherapy is an option in colorectal cancer only when the tumour is MSI-High, also reported as dMMR. That is roughly 4 to 5 in every 100 advanced colorectal cancers. For the other 95 or so, reported as MSS or pMMR, checkpoint inhibitor immunotherapy alone is not an evidence-based treatment.
Across all stages taken together, about 15 in 100 colorectal cancers are MSI-High. The proportion falls sharply in advanced, metastatic disease, which is exactly where the question usually comes up. These ranges follow NCCN and ESMO patient guidance current as of August 2026.
Saying this plainly is the point of this page. Families often spend months, and considerable money, chasing immunotherapy for a tumour that was never going to respond to it. One line in the pathology report settles the question. Reading that line early protects both hope and finances.
Eligibility is confirmed by an oncology team reading your actual tissue report, not by this page. If you have not been told your MMR or MSI result, that is the first thing to ask for.
Why one biomarker decides
Why Does MSI Status Decide Everything?
Because MSI-High tumours look different to the immune system. A faulty mismatch repair system lets DNA errors build up unchecked. The tumour then carries a very large number of abnormal proteins. Those abnormal proteins are what make the cancer visible to immune cells, and that visibility is exactly what checkpoint inhibitor immunotherapy depends on.
Mismatch repair is the cell's spell-check. Four proteins do most of that work: MLH1, MSH2, MSH6 and PMS2. When one is missing, small repeated stretches of DNA called microsatellites accumulate errors, and the tumour becomes unstable. That instability is what the letters MSI describe.
An MSS tumour has a working spell-check. It carries far fewer abnormal proteins, so immune cells largely do not recognise it as foreign. Releasing the immune system's brakes achieves very little when there is nothing for the immune system to see in the first place.
This is biology, not effort, access or money. An MSS tumour does not become immunotherapy-responsive by trying a different centre or paying more.
Did you know?
NCCN guidance recommends universal MMR or MSI testing for every patient diagnosed with colorectal cancer, not only those being considered for immunotherapy. The result also flags Lynch syndrome, an inherited condition that raises cancer risk for blood relatives. So the test does two jobs at once, and it is run on tissue that has usually already been collected.
The numbers, precisely
What Proportion of Patients Actually Qualify?
The proportions below follow ranges published in NCCN and ESMO patient guidance, current as of August 2026. They describe how common the biomarker is. They are not response figures and they are not survival figures.
| Setting | Approximate proportion MSI-High / dMMR | Why it matters |
|---|---|---|
| All colorectal cancers, all stages | About 15 in 100 | The figure most often quoted online, and the main reason expectations get set too high |
| Stage 2 colon cancer | Around 1 in 5 | Enriched compared with average; the result also guides how adjuvant chemotherapy is planned |
| Stage 4 (metastatic) colorectal cancer | About 4 to 5 in 100 | This is the setting where immunotherapy eligibility is usually being asked about |
| Lynch syndrome related colorectal cancer | Nearly all | An MSI-High result can be the first sign of an inherited syndrome; genetic counselling is offered to the family |
So when an article says 15% of colorectal cancers are MSI-High, that is accurate, but it is not the number that applies to an advanced case. For metastatic disease, roughly 5% is the honest figure.
For the small group who are MSI-High, the benefit is real. Published trial data summarised in NCCN and ESMO guidance describe objective response rates broadly in the 40 to 45 percent range for MSI-High advanced colorectal cancer treated with checkpoint inhibitor immunotherapy, current as of August 2026. A response rate describes tumour shrinkage in a proportion of patients. It is not a survival figure, and it is not a promise for any one person.
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Get Your MSI Status Read Before Anything Else Is Decided
A medical oncologist will go through your MMR/MSI report, explain what it means for treatment, and set out the realistic options either way. Free, confidential, with no commitment to start treatment.
Reading the report
What Does Your Pathology Report Actually Say?
Look for a line naming the mismatch repair proteins, or the words microsatellite instability. Two different laboratory methods are used, and they report the same biology in different words.
| What the report says | What it means | Is checkpoint immunotherapy relevant? |
|---|---|---|
| MSI-High (MSI-H) | Microsatellite instability high. The mismatch repair system is not working, so DNA errors have accumulated | Yes. This is the group for whom checkpoint inhibitor immunotherapy is considered |
| dMMR | Mismatch repair deficient. One or more of MLH1, MSH2, MSH6 or PMS2 is absent on immunohistochemistry | Yes. Treated as equivalent to MSI-High for treatment decisions |
| MSS | Microsatellite stable. The repair system is working normally. Roughly 95 in 100 advanced colorectal cancers | No. Checkpoint inhibitor immunotherapy alone is not an evidence-based option |
| pMMR | Mismatch repair proficient. All four proteins present on immunohistochemistry. The same meaning as MSS | No. Chemotherapy, targeted therapy or a clinical trial is the path |
| MSI-Low (MSI-L) | An older PCR category showing limited instability | No. Grouped with MSS for treatment decisions |
| Not mentioned | The report carries no MMR or MSI line at all | Unknown. Ask for the test before immunotherapy is discussed either way |
The testing is done on tumour tissue already taken at biopsy or surgery, so a fresh procedure is usually not needed. Turnaround is typically several working days once the laboratory has the tissue block, though it varies by laboratory and by method.
If the answer is MSS
What If You Are MSS or pMMR?
You are in the majority, and you still have established, evidence-based treatment. Being ruled out for immunotherapy is not the same as being out of options.
- Chemotherapy remains the backbone — combination chemotherapy is still the most established treatment for advanced MSS colorectal cancer, chosen on fitness, prior treatment and the pattern of disease.
- Other molecular results still matter — RAS, BRAF and HER2 testing guide targeted treatment choices in MSS disease, so a molecular report is never wasted even when MSI is stable.
- Surgery and local treatment can still apply — where spread is limited, surgery or local ablative treatment is weighed alongside systemic treatment at tumour board.
- Trials are where MSS immunotherapy is being studied — combination approaches are under investigation. This is said so you can ask about it. It is information, not an offer of enrolment, and no benefit is claimed for an investigational treatment.
- Ask for the reasoning in writing — you are entitled to understand why immunotherapy was ruled out, and MSS status is a specific, checkable reason rather than a judgement call.
If you are MSI-High
What Happens Before the First Immunotherapy Infusion?
Eligibility is confirmed, baseline safety testing is completed, then treatment is given as day care. None of these steps is unusual or optional. They are routine protocol.
1
Confirm MSI-High on tissue, not on assumption
The MMR or MSI result is confirmed on the tumour tissue report before anything is planned. Where the report is old, unclear, or from another centre, the slides are re-read rather than taken on trust.
2
Baseline hepatitis B and C screening
Screening for hepatitis B and C before starting immunotherapy is routine protocol, not a comment on your history. Immune-related side effects are sometimes treated with steroids or other immune-suppressing medicines, and a previous hepatitis infection can reactivate when the immune system is suppressed. Knowing the status in advance lets the team plan monitoring, or preventive antiviral treatment, rather than react to a problem later.
3
Baseline organ-function bloods
Thyroid, liver, kidney and blood-count baselines are recorded before the first cycle. Later changes can then be recognised as immune-related side effects rather than guessed at.
4
Treatment as day care, scans coordinated separately
Immunotherapy is administered as day care at CION centres, so an overnight stay is not usually needed. Response-assessment PET-CT is coordinated at partner imaging centres rather than performed in-house. How this runs in practice, including cost and scheme cover, is set out on the immunotherapy hub.
Answers before decisions
Know Where You Stand on the One Test That Decides This
MSI-High or MSS, you deserve to hear it plainly and to know what each answer changes. A medical oncologist will go through your report and the realistic options with you.
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Common questions
Immunotherapy for Colorectal Cancer and MSI — Your Questions Answered
Does immunotherapy work for colorectal cancer?
Only in a small, specific group. Checkpoint inhibitor immunotherapy is an evidence-based option in colorectal cancer when the tumour is MSI-High, also reported as dMMR. In advanced colorectal cancer that is roughly 4 to 5 in every 100 patients. Across all stages taken together, about 15 in 100 tumours are MSI-High. For the remaining tumours, reported as MSS or pMMR, checkpoint inhibitor immunotherapy alone is not an evidence-based treatment, and chemotherapy, targeted therapy and clinical trials remain the established path. The pathology report, not the stage or the symptoms, decides which group you are in.
What does MSI-High mean on a colorectal cancer report?
MSI-High means microsatellite instability high. The cell's mismatch repair system, its DNA spell-check, is not working, so short repeated stretches of DNA called microsatellites accumulate errors. The tumour ends up carrying a very large number of abnormal proteins. Those abnormal proteins are what make an MSI-High tumour visible to immune cells, which is why checkpoint inhibitor immunotherapy can help this group. If your report was done by immunohistochemistry it may instead use the term dMMR, meaning one or more of the four mismatch repair proteins, MLH1, MSH2, MSH6 or PMS2, is missing. dMMR and MSI-High mean the same thing for treatment decisions.
What proportion of colorectal cancer patients are MSI-High?
About 15 in 100 across all stages, but only about 4 to 5 in 100 in advanced, metastatic disease, following the ranges published in NCCN and ESMO patient guidance current as of August 2026. Stage 2 colon cancers are enriched, at around 1 in 5. The higher all-stage figure is the one usually quoted online, and it is the reason many families expect an eligibility that does not apply to an advanced case. If immunotherapy is being considered for metastatic disease, the roughly 5 percent figure is the relevant one.
What are my options if my tumour is MSS or pMMR?
You are in the majority, and you still have established treatment. Combination chemotherapy is the backbone of treatment for advanced MSS colorectal cancer, chosen on your fitness, prior treatment and pattern of disease. Other molecular results such as RAS, BRAF and HER2 continue to guide targeted treatment choices, so a molecular report is never wasted. Where spread is limited, surgery or local treatment may still be considered alongside systemic treatment. Combination approaches for MSS disease are being studied in clinical trials, and that information is given so you can ask about it, not as an offer of enrolment.
How is MSI or MMR status tested, and how long does it take?
It is tested on tumour tissue already taken at biopsy or surgery, so a fresh procedure is usually not needed. Immunohistochemistry stains for the four mismatch repair proteins and reports dMMR or pMMR. PCR or next-generation sequencing measures microsatellite instability directly and reports MSI-High, MSI-Low or MSS. Turnaround is typically several working days once the laboratory has the tissue block, though it varies by laboratory and method. NCCN guidance recommends this testing for every patient diagnosed with colorectal cancer, so if your report does not mention it, ask for it rather than assuming it was not indicated.
Why are hepatitis B and C tests done before immunotherapy?
It is standard protocol before starting immunotherapy, not a comment on your history. Immune-related side effects are sometimes treated with steroids or other immune-suppressing medicines, and a previous hepatitis B or C infection can reactivate when the immune system is suppressed. Screening beforehand lets the team plan monitoring, or preventive antiviral treatment, instead of reacting to a problem later. Baseline thyroid, liver, kidney and blood-count tests are taken at the same time for the same reason: later changes can then be recognised as immune-related effects rather than guessed at.
This page is general patient-education information, not a substitute for the written guidance an oncology team gives based on your own pathology report, MMR/MSI result and treatment plan.