What is the bowel?

The bowel is a 6-metre-long tube made of muscle, with a lining similar to the inside of the cheek. It is part of the digestive system and winds from the stomach to the rectum and anus. There are two parts of the bowel - the small bowel and the large bowel. 

Food and liquid are broken down in the stomach and then passed into the small bowel to be digested. From there, the nutritional parts of food are absorbed into the bloodstream and the remains pass into the large bowel.

The large bowel is made up of two parts - the colon and the rectum. The colon is the first one-and-a-half metres of the large bowel, and the rectum is the last 12 to 15cm, ending at the anus. The colon removes liquid from digested food, which is turned into solid waste. The rectum holds this solid waste until it is expelled as a bowel motion.

What is bowel cancer?

Bowel cancer is called cancer of the colon, cancer of the rectum, or colorectal cancer. This cancer occurs when the cells in some part of the bowel grow abnormally and form a lump or tumour. 

Most bowel cancers are in the large bowel, which is made up of the colon and the rectum. Occasionally the cancer is in the small bowel, but this is quite unusual.

Usually bowel cancer starts in the cells of a polyp, a mushroom-like growth that occurs inside the bowel (colon or rectum). Not all polyps develop into cancer (only about 5%) but wherever possible all polyps should be removed to ensure they don't become cancerous.

How common is bowel cancer?

Each year in NZ more than 2,000 people develop bowel cancer. It is one of the most common cancers among both men and women in New Zealand. Non-Maori men and women have a higher risk of developing cancer of the colon than Maori men and women. It is much more common in people over the age of 40.

Causes of bowel cancer

Scientists are still unsure about the causes of bowel cancer, which usually starts as a benign (not cancerous) polyp that turns cancerous.

Risk factors include:

• having a parent, brother, sister or child who has had bowel cancer. However, bowel cancer is not catching. You cannot pass it on to your partner, family or anyone else.
• having the rare genetic condition Familial Adenomatous Polyposis (FAP).
  families where there is a genetic condition called Hereditary Non-Polyposis Colorectal Cancer (HNPCC).
• having had Crohn's Disease or ulcerative colitis for more than ten years.

Scientists believe a diet high in animal fats and low in fruit and vegetable fibre may contribute to the development of bowel cancer.

Does bowel cancer run in families?

Bowel cancer occurs more often in people with a parent, brother, sister or child with bowel cancer, especially if they were under the age of 50 when they were diagnosed.

People with close relatives who have bowel cancer may need to be checked regularly by their doctor. Guidelines for screening family members are currently being developed. For further information contact your doctor.

Related topics

How is bowel cancer diagnosed? See Bowel cancer diagnosis

How is bowel cancer treated? See Bowel cancer treatment

What is cancer and how does it form? For more information on cancer, see What is cancer?  What causes cancer?

What does that term mean? See Cancer glossary

Original material provided by the Cancer Society, 2002. Reviewed by everybody, January 2005.