Medical Oncologist
CION Cancer Clinics
Right-sided vs left-sided colon cancer: what the difference means for you | CION Cancer Clinics
Right-sided colon cancer starts in the first part of the colon and left-sided cancer in the last part. They are the same disease, but they tend to cause different symptoms, show different results on tumour tests and need different operations. In advanced disease the side can also change the choice of targeted medicine. This page sets the two side by side and explains what the side does and does not tell you. At CION Cancer Clinics in Hyderabad, our oncologists explain colon cancer in plain words and plan care around you and your family.
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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
- What is the difference between right-sided and left-sided colon cancer?
- How do right-sided and left-sided cancers usually differ?
- Does the side change how the cancer is found and treated?
- Four things families assume about the side, and what is true
- What this page cannot tell you
- Common questions about right-sided and left-sided colon cancer
The short answer
What is the difference between right-sided and left-sided colon cancer?
Right-sided colon cancer starts in the first part of the colon, on the right of your belly. Left-sided colon cancer starts in the last part, on the left. Both are colon cancer, but the two halves of the colon develop differently, so cancers on each side tend to cause different symptoms and carry different gene changes. In advanced disease the side can also change which medicines your oncologist chooses.
Where the dividing line falls
The right side covers the caecum, the ascending colon, the bend under the liver and roughly the first two-thirds of the transverse colon. The left side covers the rest of the transverse colon, the bend near the spleen, the descending colon and the sigmoid. If those names are new to you, the page on the parts of the colon walks along the tube one section at a time.
Why two halves of one tube behave differently
Before birth, the right colon grows from one part of the developing gut and the left colon from another. They keep separate blood supplies and separate nerve and lymph routes for life. The cells lining each side also handle damage in slightly different ways. That is why doctors now treat the side of the tumour as a real feature of the cancer, not just an address.
The rectum is not part of this comparison. Rectal cancer is staged and treated differently and has its own hub at rectal cancer.Side by side
How do right-sided and left-sided cancers usually differ?
These are tendencies seen across large groups of people. Any one person's cancer can break the pattern.
Right-sided colon cancer
Left-sided colon cancer
Fed by the superior mesenteric artery, the main artery to the small bowel
Fed by the inferior mesenteric artery, a separate branch lower down
Hidden blood loss, anaemia (a low haemoglobin) and tiredness are common first signs
Visible blood, a change in bowel habit and cramps are common first signs
Often larger when found, because the wide tube rarely blocks
More likely to narrow the bowel and cause a blockage
More often MSI-high (a failed DNA spelling-check), BRAF-changed or mucinous (jelly-like)
More often stable on MSI testing, with other gene changes
Removed by a right hemicolectomy
Removed by a left hemicolectomy or a sigmoid colectomy
More common in women and in older people
More common in men and in younger adults
Where the side counts
Does the side change how the cancer is found and treated?
In four places it does. In others, such as whether you need chemotherapy after an operation, the stage matters far more.
How it shows itself
On the right, stool is still liquid and the tube is wide, so a growth can bleed slowly for months without anyone seeing blood. Many people first hear about it when a blood test shows anaemia, a low haemoglobin, with no obvious cause. On the left, stool is firmer and the tube narrower, so bleeding is more visible and the bowel habit changes sooner.
Read next
The operation
Surgery removes the affected part of the colon along with its blood supply and lymph nodes, the small glands that cancer can travel to first. A right-sided tumour means a right hemicolectomy. A left-sided tumour means a left hemicolectomy or a sigmoid colectomy. CION's surgical oncologists plan the operation, and it is carried out at an accredited partner hospital.
What the tumour tests show
Every colon cancer is tested for MSI, a sign that the cell's spelling-check system has failed. MSI-high results are more common on the right, and so are BRAF changes and mucinous, jelly-like tumours. These results guide treatment far more precisely than the side alone.
An MSI-high result can also point to an inherited cause, so your team may suggest a genetics referral.Medicines in advanced disease
When cancer has spread and the RAS genes are normal, medicines that block a signal called EGFR, such as cetuximab, tend to help more for left-sided tumours. For right-sided tumours your oncologist may lean towards a different partner drug with chemotherapy. CION gives these treatments at its own centres.
Read next
Not sure whether this applies to you?
Ask an oncologistCommonly believed
Four things families assume about the side, and what is true
"It is on the right, so it must be the worse kind."
The side is one feature among many. Stage, lymph node involvement and the tumour test results say far more about what lies ahead. Some right-sided cancers are MSI-high, and in advanced disease those can respond very well to immunotherapy.
"There was no bleeding, so it cannot have been the colon."
Right-sided cancers often bleed in amounts too small to see. The only clue may be tiredness, breathlessness on the stairs or a low haemoglobin on a routine test. Unexplained iron-deficiency anaemia in an adult always deserves a look at the bowel.
"The sigmoidoscopy was clear, so the whole colon is fine."
A sigmoidoscopy looks only at the left end of the colon. It cannot see the right side at all. If symptoms or anaemia point to the right, a full colonoscopy or a CT scan is needed to check the rest of the tube.
"Left-sided cancer always means a bag."
In a planned operation the two ends of the bowel are usually joined straight back together. A stoma, where the bowel opens onto the skin, is more likely after emergency surgery for a blockage, and is often temporary. The page on whether you will need a stoma explains when it happens.
Being straight with you
What this page cannot tell you
It cannot tell you which side a cancer is on, or whether there is one at all. Symptoms are a poor guide to location. Plenty of people with right-sided cancer have no belly symptoms, and most people with left-sided bleeding turn out to have piles or a small tear. Only a colonoscopy and a CT scan, read together, can place a tumour. CION arranges both through its diagnostic partners.
It cannot predict how your cancer will behave
Studies comparing the two sides describe averages across thousands of people. They show differences in outlook, mostly in advanced disease, and much of that difference comes from MSI and BRAF status rather than the side itself. None of it is a forecast for one person. Your own stage and test results, read by your oncologist, are what count.
Who this does not apply to
If your report says rectum or rectosigmoid, this comparison does not fit you. Rectal cancer is planned with an MRI and often treated with radiation before surgery, so read colon versus rectal cancer first. Cancers of the appendix or the anal canal are different diseases again.
If your report names a site and you are unsure which side it counts as, read the site line to the helpline and someone will tell you.Questions we are asked
Common questions about right-sided and left-sided colon cancer
Is right-sided colon cancer more dangerous than left-sided?
Not as a rule. In large studies the side is linked to differences in outlook, mainly once cancer has spread. Much of that comes from MSI and BRAF status, which your tests measure directly. Your stage and your own results matter far more than the side of the colon.
Why was my right-sided cancer found so late?
The right colon is wide and its contents are liquid, so a tumour can grow for a long time before it blocks or causes pain. Its bleeding is usually hidden. Tiredness and anaemia are easy to put down to work, age or diet, which is why so many right-sided cancers surface through a routine blood test.
Does the side decide whether I need chemotherapy after surgery?
No. That decision rests mainly on the stage, especially whether lymph nodes were involved, and on risk features in the pathology report. MSI status can matter in earlier stages. The tumour board weighs all of these together, and the page on the stage 2 dilemma explains the hardest case.
Which side is the transverse colon on?
Both. The transverse colon crosses the upper belly, and doctors usually count roughly its first two-thirds as right and the rest as left. A tumour near the middle may be described either way. Your report and your surgeon will say how yours is being classed.
Why does the side change which targeted medicine is used?
Left-sided tumours with normal RAS genes depend more on the EGFR signal, so blocking it tends to help more. Right-sided tumours often rely on other pathways. This applies only when cancer has spread and only after RAS and BRAF testing, which is done on the tumour sample.
Is an MSI-high right-sided cancer inherited?
Usually not. In older people MSI-high is most often caused by a switch-off that happened inside the tumour alone. Sometimes it points to Lynch syndrome, an inherited condition. Further tests on the tumour and a Lynch syndrome assessment tell the two apart.
Can colonoscopy miss cancers on the right side?
Occasionally. Some precancerous growths on the right are flat and pale, and they are harder to see if the bowel is not fully clean. Good bowel preparation and a scope that reaches the caecum both matter. Your report should say whether the caecum was reached.
Where do I start if my report says right-sided or left-sided?
Find the site line and check the stage and the MSI, RAS and BRAF results if they are ready. Then bring the reports to a consultation, where an oncologist can explain what they mean together. Call the CION helpline if you are unsure which centre to visit.
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 — What Is Colorectal Cancer?
- American Cancer Society — Colorectal Cancer Targeted Therapy
- National Cancer Institute — Colon Cancer Treatment (PDQ), Health Professional Version
- ESMO — Colorectal Cancer: A Guide for Patients
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
Parts of the colon and why location mattersEach section of the colon, named and placedWhy tumour side changes the drug choiceThe side and targeted medicines in advanced diseaseWhich operation for which part of the colonHow the side decides the operationWhat MSI-high status meansThe tumour result that is more common on the right
Talk to us
Your report names a side. What happens next?
Bring your colonoscopy, scan and tumour test reports and a CION oncologist will explain what the location and the results mean for your treatment. You can ask every question your family has before anything is decided. One helpline serves every CION centre.