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CION Cancer Clinics
Why does tumour side change the drug choice in colon cancer? | CION Cancer Clinics
Where a colon cancer started, on the right or left side of the colon, can change which drug is added to chemotherapy once it has spread. Left-sided cancers with normal RAS and BRAF genes tend to respond better to cetuximab or panitumumab. Right-sided cancers usually get bevacizumab instead. This page explains why the side matters, when it does not, and who this does not apply to. At CION Cancer Clinics, a multidisciplinary team plans colon cancer treatment for your stage, from surgery to chemotherapy and follow-up.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed October 2026, next review due October 2027
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On this page
- Why does the side of the colon change the drug choice?
- What makes right-sided and left-sided cancers different?
- How does the usual first choice differ by side?
- In what order does the team weigh the side?
- What do people often misunderstand about tumour side?
- What this page cannot tell you
- Common questions about tumour side and drug choice
The short answer
Why does the side of the colon change the drug choice?
Because cancers from the right and left sides of the colon behave like slightly different diseases. They tend to carry different gene changes, and they respond differently to the same drugs. Once colon cancer has spread, this matters most when a targeted drug is added to chemotherapy. EGFR-blocking drugs such as cetuximab work far better in left-sided tumours. Bevacizumab is usually preferred for right-sided ones.
Which side is which
The right side is where the colon begins, low on the right of the belly, and runs up as the ascending colon. The left side is the descending and sigmoid colon, which ends at the rectum. The transverse colon crosses the top between them, and teams sometimes classify it differently. The exact part of the colon is named on your colonoscopy and CT reports.
When the side actually matters
The side tips the drug decision when three things are true together. The cancer has spread, the RAS genes are normal, and BRAF is normal. Then the team is choosing between two drug families, and the side decides which one goes first. In most other situations, the side changes the operation and leaves the drug choice alone.
It is where the cancer started that counts. A left-sided cancer that has spread to the liver is still treated as left-sided.Two different diseases
What makes right-sided and left-sided cancers different?
The two halves of the colon grow from different parts of the developing baby, and the cancers that form in them keep some of that difference.
Different beginnings
The right colon develops alongside the small bowel, and the left colon alongside the rectum. They have separate blood supplies and lymph drainage, which is why the operation differs too. A right-sided cancer usually needs a right hemicolectomy, meaning removal of the right half of the colon.
Different gene changes
Left-sided cancers more often depend on the EGFR growth signal, which is the very signal cetuximab and panitumumab block. Right-sided cancers more often carry other changes that bypass it.
More common on the right
- MSI-high tumours, with a faulty DNA repair system
- A BRAF V600E change
- Mucinous cancers, which make a lot of mucus
Different response to the same drug
In trials, people with RAS-normal left-sided cancers did better when an EGFR-blocking drug was added to chemotherapy. People with RAS-normal right-sided cancers gained little from it, so bevacizumab is usually chosen for them instead.
Where the difference fades
The side only tips the choice when the gene tests leave two options open. A RAS change, a BRAF change or an MSI-high result decides the drug on its own, wherever the tumour sits.
Not sure whether this applies to you?
Ask an oncologistSide by side
How does the usual first choice differ by side?
Left-sided: descending and sigmoid
Right-sided: caecum and ascending
RAS and BRAF normal: chemotherapy with cetuximab or panitumumab is usually favoured
RAS and BRAF normal: chemotherapy with bevacizumab is usually favoured
RAS changed: chemotherapy with bevacizumab
RAS changed: chemotherapy with bevacizumab, the same as the left
MSI-high: immunotherapy is usually considered first
MSI-high: immunotherapy is usually considered first, and this result is more common here
After surgery for stage 2 or 3: chemotherapy alone
After surgery for stage 2 or 3: chemotherapy alone, the same as the left
How the team decides
In what order does the team weigh the side?
Confirm where the cancer started
The colonoscopy, the CT scan and, after an operation, the pathology report pin down the original site. For a tumour near the middle of the colon, the tumour board agrees how to classify it.
Read the molecular results
RAS, BRAF and MSI results come first. These tests can settle the drug choice before the side is even considered.
Apply the side
If RAS and BRAF are normal and the tumour is not MSI-high, the side guides the choice of targeted drug. Cetuximab or bevacizumab is the usual question at this point.
Weigh the goal and your health
If the aim is to shrink liver spots enough for surgery, or if skin side effects or blood pressure are a concern, the team may adjust. What you want from treatment counts here too.
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Commonly believed
What do people often misunderstand about tumour side?
"My cancer is right-sided, so the good drugs are not for me."
Bevacizumab with chemotherapy is a standard, effective first choice for right-sided cancer. The side steers which drug is used and does not take treatment away. If the tumour is MSI-high, immunotherapy may also be an option.
"Once it has spread to the liver, the side no longer matters."
The side of the original tumour still guides the drug choice. Spots in the liver carry the biology of the place where the cancer began.
"My tumour is left-sided, so I will definitely get cetuximab."
Only if RAS and BRAF are normal and the tumour is not MSI-high. A left-sided cancer with a RAS change is treated with bevacizumab, exactly like a right-sided one.
"The side decides whether I need chemotherapy after surgery."
For stage 2 and 3, that decision rests on the stage and the risk features of the tumour. The usual regimen after surgery is the same on both sides.
Being straight with you
What this page cannot tell you
It cannot tell you which drug is right for you. The side is one input among several, and it never outranks your molecular results, your fitness or what you want from treatment. Your oncologist may have good reasons to step away from the usual pattern.
Who this does not apply to
If your colon cancer was fully removed and has not spread, the side does not change the chemotherapy offered after surgery. If your tumour is MSI-high, immunotherapy usually comes first, whichever side it is on. If the cancer is in the rectum, it is grouped with left-sided cancers for drug choice, but its surgery and radiotherapy are planned differently, so start with the rectal cancer pages.
It cannot read your report for you
Reports sometimes name a part of the colon without saying right or left. The transverse colon and the two bends at either end of it cause most of the confusion. Ask your oncologist which side they have counted your tumour as, and why.
CION's tumour board reviews the tumour site, the molecular results and the scans together before any drug is chosen. You can bring reports from elsewhere for a second look.Questions we are asked
Common questions about tumour side and drug choice
Does the side change the operation as well?
Yes. A right-sided cancer is usually removed with a right hemicolectomy, and a left-sided one with a left hemicolectomy or sigmoid colectomy. CION's surgical oncologists plan the operation, and it is carried out at an accredited partner hospital.
Which side does the transverse colon count as?
It sits between the two sides, and classification varies. Many teams count most of it as right-sided for drug decisions, while others judge it case by case. If your tumour is in the transverse colon, ask how your team counted it and whether that changed the drug.
Can a right-sided cancer ever be treated with cetuximab?
Sometimes, in later lines of treatment, if RAS and BRAF are normal and other options have been used. It is chosen first less often, because the benefit on the right side is smaller. Your oncologist weighs this against whatever else is still available.
Does the side matter for immunotherapy?
Not directly. Immunotherapy depends on an MSI-high or MMR-deficient result, wherever the tumour sits. Right-sided cancers are more often MSI-high, so the question comes up more often there. The immunotherapy pages explain how those drugs are used.
Does the side change chemotherapy after surgery?
No. After surgery for stage 2 or 3 colon cancer, the decision rests on the stage and the risk features of the tumour. The usual regimens, FOLFOX or CAPOX, are the same on both sides, and targeted drugs are not added at this stage.
Is rectal cancer counted as left-sided?
For drug choice once the cancer has spread, rectal cancer is usually grouped with left-sided cancers. Its surgery and radiotherapy are planned quite differently from colon cancer, though, and the rectal cancer pages cover those decisions separately.
What if my report does not say which side?
Ask. The report should name the exact part of the colon, and your oncologist can tell you which side it counts as. It is a fair question, and the answer helps you understand why one drug was chosen over another.
Can CION review a plan made at another hospital?
Yes. A CION medical oncologist can review your scans, pathology and molecular reports, and explain whether the drug choice fits your tumour's side and test results. Call the CION helpline to arrange a second-opinion consultation.
Your Specialists
Meet CION's colon cancer specialists. Bring your colonoscopy, scan or biopsy report.
Our surgical and medical oncologists plan colon cancer care together: which operation, whether chemotherapy is needed, and the follow-up that comes after.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
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Sources
- American Cancer Society — Colorectal Cancer Targeted Therapy
- National Cancer Institute — Colon Cancer Treatment (PDQ), Health Professional Version
- ESMO — Colorectal Cancer: A Guide for Patients
- American Cancer Society — Colon Cancer Treatment, by Stage
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Right-sided vs left-sided colon cancerHow the two sides differ beyond the drug choiceWhen targeted therapy is addedWhen a targeted drug joins chemotherapy at allWhy RAS and BRAF testing comes before treatmentThe tests that outrank the sideCetuximab for RAS wild-type left-sided cancerHow the drug favoured on the left is given
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Not sure why one drug was chosen over another?
Bring your scans, pathology and molecular reports, and a CION medical oncologist will explain how the tumour's side and test results shaped the plan. One helpline serves every CION centre.