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Cancer screening means looking for signs of cancer in healthy people. Your colon and rectum are part of your intestines (also called your bowels). Colon and rectal cancer screening can find cancer in your intestines early, before it starts causing any symptoms. It can also prevent colon and rectal cancer, by finding small growths called polyps that might turn into cancer if they are not removed.

Cancer of the colon or rectum is one of the main causes of death from cancer in the United States.¹

Colon and rectal cancer is sometimes called colorectal cancer or bowel cancer. Your bowel turns food your body doesn't need into solid waste. The waste then leaves your body when you go to the bathroom.

To find out more, see Colon and rectal cancer.

Doctors use many different tests to screen for colon and rectal cancer. We've described the four most common tests. To learn more about these tests, see What will happen to me during my test?

This is where a sample of your stool is tested for blood. It is called an occult blood test because blood may be hidden in your stools (the word "occult" can mean "hidden"). If you have blood in your stools, it might be because you have a tumor in your bowel.

In this test, a doctor puts a thin, bendy tube into your anus. This allows the doctor to look at your rectum and the lower part of your bowel.

In this test, a doctor puts a thin, bendy tube into your anus. This is pushed up your bowel so the doctor can see all the way along it.

In this test, a thick white liquid is put into your rectum through a tube in your anus. Then X-rays are taken of your bowel.

Screening is used to look for cancer at a very early stage, before it starts to cause symptoms. If your doctor suggests you have screening for cancer, it doesn't mean he or she thinks you have cancer. It means you might benefit from having the screening test, in case you do have cancer.

The earlier colon or rectal cancer is found, the more likely it is to be cured. Studies show that:²

• If your colon or rectal cancer is found after it has started causing symptoms, you have a 50 percent chance of surviving for at least five years
• If your colon or rectal cancer is found before it has started causing symptoms, you have an 80 percent chance of surviving for at least five years.

Screening doesn't just help to spot colon or rectal cancer early. It can prevent it. Some types of screening can show up lumps called polyps in your bowel. These might turn into cancer if you don't have them removed.¹

Polyps are harmless (doctors call them benign) lumps of cells that can grow on the wall of your bowel. You're more likely to get them as you get older.²

Most polyps never become cancerous. But some do, and colon or rectal cancer almost always start from a polyp. Doctors usually remove all polyps when they find them, to be safe. Most can be easily removed during a screening test.³

Screening tests don't exist for all types of cancer. When tests do exist, doctors need to be sure that their benefits are bigger than their risks before offering them to patients.

Regular screening for colon and rectal cancer is now recommended for most men and women over 50. People who are at high risk of colon and rectal cancer can get tested earlier and more often.

To read more, see Who should have screening?

There are some risks with the screening tests used to detect colon and rectal cancer. These include:⁴

• Damage to your colon or rectum. In rare cases, the lining of the bowel can get injured during sigmoidoscopy and colonoscopy tests.
• False negative results. This means you do have cancer, but the test doesn't find it. So you might dismiss symptoms of cancer and miss out on vital treatment
• False positive results. This means you don't have cancer, but the test result says you do. So you get needlessly worried and have more tests or treatment that you don't really need.

Despite these risks, doctors recommend screening for colon and rectal cancer because there is good evidence that it offers two very important benefits:⁵

• It cuts the number of people who die from colon and rectal cancer.
• It cuts the number of cases of colon and rectal cancer. It probably does this by detecting polyps that would have turned cancerous if they had not been removed.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.