What is colon cancer?

Clinical Oncologist Katherine Aitkens explains bowel cancer (which includes colon cancer), giving an overview of bowel cancer symptoms, diagnosis and treatment.


The colon is part of the large bowel, which is part of the digestive system. Colon cancer is more common in older people. In the UK, almost 6 in 10 bowel cancer cases (58%) each year are diagnosed in people aged 70 and over.

We have more information about other types of bowel cancer including:

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Booklets and resources

Symptoms of colon cancer

Some people may start by seeing their GP because they have symptoms. Symptoms of colon cancer can include blood in your poo (stools), bleeding from the back passage and a change in your normal bowel habit. If you are worried about colon cancer, we have more information about bowel cancer symptoms.

Other people may be diagnosed by:

  • Having NHS bowel screening

    Bowel cancer screening is a way of finding bowel cancer at an early stage before it causes symptoms.

  • Going to hospital with a problem, such as bowel obstruction

    This is when part of the bowel becomes blocked. It may cause symptoms such as tummy pain, nausea and vomiting, and constipation.

If you have any symptoms or notice anything that is unusual for you, see your GP straight away.

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Causes of colon cancer

The exact cause of colorectal cancer (colon or rectal cancer) is unknown. But, things called risk factors can increase the chance of a person developing it. Having one or more risk factors does not mean you will definitely get colon cancer. And if you do not have any risk factors, it does not mean you will not get colon cancer.

Diagnosis of colon cancer

If you have symptoms, you will usually begin by seeing your GP, who will examine you. If they think that your symptoms could be caused by cancer, they will refer you to a specialist doctor.

At the hospital

The doctor will ask you about your general health and any previous medical problems you have had. They will also ask whether you have a family history of bowel cancer.

The doctor will examine you and you will usually also have:

  • Rectal examination

    A rectal examination is when the doctor places a gloved finger into your back passage to feel for any lumps or swelling.

  • Blood test

    You will usually have a blood test to check your level of red blood cells. If you have a low number of red blood cells, this is called anaemia. You will also have blood tests to check whether your liver and kidneys are working normally.

You may then have:

  • Colonoscopy

    A colonoscopy looks at the inside of the whole length of the large bowel. You can usually have this test as an outpatient. It takes about an hour.

  • Virtual colonoscopy

    In a virtual colonoscopy a computerised tomography (CT) scanner takes a series of x-rays. This builds up a three-dimensional picture of your bowel.

  • Sigmoidoscopy

    A sigmoidoscopy looks at the inside of the rectum and the part of the colon closest to the rectum (the sigmoid colon). You can usually have it as an outpatient.

Waiting for test results can be a difficult time, we have more information that can help.

Further tests for colon cancer

If any of your biopsies show that there is cancer in the colon, you will have more tests. The results will help you and your doctor decide on the best treatment for you. Some tests may be repeated during and after treatment. You may have any of the following tests:

  • More blood tests

    Your blood may be tested for a protein called carcinoembryonic antigen (CEA). Some people with bowel cancer have higher levels of this protein.

  • CT scan

    CT (computerised tomography) scan uses x-rays to build a three-dimensional (3D) picture of the inside of the body.

  • MRI (magnetic resonance imaging) scan

    An MRI scan uses magnetism to build up a detailed picture of areas of your body.

  • PET-CT scan

    PET-CT scan is a combination of a CT scan, which takes a series of x-rays to build up a three-dimensional picture, and a positron emission tomography (PET) scan. A PET scan uses low-dose radiation to measure the activity of cells in different parts of the body.

Staging and grading of colon cancer

The results of your tests help your doctors find out more about the size and position of the cancer and whether it has spread. This is called staging.

A doctor decides the grade of the cancer by how the cancer cells look under the microscope. This gives an idea of how quickly the cancer might grow or spread.

Knowing the stage and grade helps your doctors plan the best treatment for you.

Treatment for colon cancer

A team of specialists will meet to discuss the best possible treatment for you. This is called a multidisciplinary team (MDT).

Your cancer doctor or specialist nurse will explain the different treatments and their side effects. They will also talk to you about things to consider when making treatment decisions.

Surgery is the most common treatment for colon cancer. But the treatment you have depends on the stage of the cancer and where it is in the colon. It also depends on your general health and preferences. Find out more about treatment options for colon cancer

  • Surgery

    Surgery is often used to remove cancer or areas affected by the cancer. It may also be used to help control symptoms.

  • Chemotherapy

    Chemotherapy uses anti-cancer drugs to destroy cancer cells. You may have it as your main treatment or with other cancer treatments such as surgery.

  • Targeted therapies

    Targeted therapy drugs target something in or around a cancer cell that is helping it grow and survive. Targeted therapies are sometimes used to treat bowel cancers (colon or rectal) that have spread to other parts of the body.

You may have some treatments as part of a clinical trial.

Most people have changes in how their bowel works after treatment for colon cancer. These effects usually improve over time. We have more information about managing bowel changes caused by treatment.

After colon cancer treatment

Follow-up after treatment for colon cancer

The aim of follow-up care is to make sure everything is going well and to find out if you have any concerns. The appointments are usually every few months in the first year.

We have more information on follow-up care after treatment.

You may get anxious between appointments. This is natural. It may help to get support from family, friends or a support organisation. 

Macmillan is also here to support you. If you would like to talk, you can: 

Sex life

Colon cancer and its treatment can affect your sex life. Changes may get better over time, but sometimes they are permanent. If you have any problems or are worried about this talk to your doctor or nurse. They can explain what to expect and there are often things that can help.


Some cancer treatments can affect whether you can get pregnant or make someone pregnant. If you are worried about this, it is important to talk with your doctor before you start treatment.

Well-being and recovery

Even if you already have a healthy lifestyle, you may choose to make some positive lifestyle changes after treatment.

Making small changes such as eating well and keeping active can improve your health and wellbeing and help your body recover.

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About our information

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Chief Medical Editor, Professor Tim Iveson, Consultant Medical Oncologist.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

Date reviewed

Reviewed: 30 April 2020
Next review: 30 April 2023

This content is currently being reviewed. New information will be coming soon.

Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

This means it is easy to use, up-to-date and based on the latest evidence. Learn more about how we produce our information.