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Colon cancer and rectal cancer are kinds of cancer that grow in your intestines (also called your bowels). They are serious conditions, but there are good treatments. You might not get any symptoms to start with, so finding out that you have cancer can be a shock.

Each year, about 150,000 Americans get colon or rectal cancer.¹ The cancer starts because some cells in the bowel start growing out of control and damage the surrounding cells. You can get treatment to try to remove or kill the cancer and stop it from spreading to other parts of your body.

If you've been told you have colon or rectal cancer, you'll probably have lots of questions about your illness and what might happen to you. Learning about your disease may reassure you and help you cope.

• When you first get colon or rectal cancer, you might not have any symptoms.
• Most people need surgery to get rid of the cancer.
• You might also need radiation therapy or chemotherapy.
• Anyone can get colon or rectal cancer, but it's more common in people over 50.
• Screening tests can help spot the cancer early on. To read more, see Colon and rectal cancer screening.

You may also hear the phrases colorectal cancer or bowel cancer to describe these kinds of cancer.

People who have colon or rectal cancer say that when they have gotten over the shock, they find their illness has helped them focus on the things that mean a lot to them, such as their family and other important relationships.²

Your intestines are part of your digestive system. This is the group of organs that break down the food you eat. The part right below your stomach is called the small intestine or small bowel. It's a narrow tube about 20 feet long (rather like a garden hose).

Your small intestine leads into your large intestine (also called the large bowel), which is about 7 feet long and much wider.

Your colon is the first 6 feet of your large intestine, and your rectum is the last 8 inches to 10 inches.

Cancer usually grows in the large intestine. It is rare in the small intestine.

See More about your colon and rectum to learn more.

Your colon and rectum are made mostly of muscle. The inner lining is a thin layer of cells called the mucosa. Cells in this lining have a short life. They grow, divide in two, then die and get replaced. Your body carefully controls this activity, so that cells are replaced every five days or so.

But sometimes things go wrong. Cells keep dividing when they don't need to, and old ones don't die. If this happens, you can get a cluster of extra cells called a tumor. Some tumors are made up of normal cells and they aren't dangerous. Doctors call them benign tumors. They stay put and don't cause any problems. But other tumors are made up of cancer cells. Doctors call them malignant tumors.

Cancer cells are different from healthy cells in several ways:

• They grow faster
• They don't grow in an orderly way
• They stay "immature"
• They don't stop growing when they touch other cells
• They grow in the wrong place.

Cells in tumors can break away and move to other parts of your body. These breakaway cells are carried in your bloodstream or in your lymphatic system. When the cancer cells arrive at a different part of your body, they can start growing into another tumor. When cancer spreads like this, it's called metastasis. Blood from your bowel goes to your liver, so this is where colon and rectal cancer cells might travel.

It's hard to say why some people get colon or rectal cancer and others don't. Some kinds of cancer run in the family, but there are many other things that make it more or less likely that you'll get this kind of cancer.

See Risk factors for colon and rectal cancer to find out more.

At least two-thirds of colon and rectal cancers start inside lumps of cells called polyps.³ These are small growths on the inside of the bowel wall.

A polyp is a harmless, fleshy lump (a benign tumor), but it can turn into cancer (a malignant tumor). Some doctors think that taking these polyps out makes it much less likely that you'll get colon or rectal cancer.^{4 5}

To read more, see Colon and rectal cancer screening.

Here's what we know about polyps.

• It can take as long as 10 years for cancer to grow in a polyp. This seems to happen more quickly in the rectum than in the colon.⁶
• Some polyps are more likely than others to turn into cancer. Polyps that are less than a quarter of an inch wide are very unlikely to turn into cancer. But about half the polyps that are more than an inch wide turn into cancer.⁷ The more polyps you have, the more likely it is that you'll get cancer.
• Between 15 percent and 30 percent of Americans will get polyps.³ But less than 1 in 10 of these polyps will grow into cancer.⁸

Your doctor will need to find out how advanced your cancer is. This is called staging your cancer. The doctor will check how big your cancer is and how far it has spread. These tests help doctors choose the best treatment.

Your doctor will also look at a few of the cancer cells under a microscope. This is called grading your cancer. The more "normal" the cells look, the less severe your cancer. If the cancer cells look very unlike normal cells, it can mean the tumor is growing quickly.

See What stage is your cancer? to read more.

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.