What is bowel cancer?
Bowel cancer is named according to where it is found: for example, 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 cancers are in the large bowel (see diagram of the digestive system). Cancer in the small bowel is less common.
The bowel
The bowel is a six-metre-long tube made of muscle, with a lining similar to the inside of the cheek. It is part of the digestive system and extends 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. 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 (faeces).
The digestive system
How common is bowel cancer?
The latest provisional information available from the New Zealand Cancer Registry recorded 2,759 new cases of colorectal cancer in 2008. It is one of the most common cancers among both men and women in New Zealand. It may occur at any age, although 90 percent of cases are found in people over the age of 50.
Causes of bowel cancer
Scientists are still unsure about the causes of bowel cancer, which usually starts as a benign polyp that becomes cancerous. A polyp is a mushroom-like growth that occurs inside the bowel. Only about 5 percent of polyps develop into cancer.
The following factors may increase the risk of developing bowel cancer:
Lifestyle factors
- A diet high in fat and protein and low in fruit and vegetables
- Alcohol consumption
- Weight gain, particularly around the waist
- Low rates of physical activity.
Family history of bowel cancer
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Having a parent, brother, sister or child who has had bowel cancer.
(You cannot catch bowel cancer or pass it on through personal contact.) Most bowel cancer (as much as 90 percent) is not inherited.
Rare genetic conditions
- Hereditary non-polyposis colorectal cancer (HNPCC) — a condition in some families where the tendency to develop cancer is inherited. Up to 5 percent of all bowel cancer is due to HNPCC. About 80 percent of people who have a gene for HNPCC will develop a bowel cancer at some time in their life.
- Familial adenomatous polyposis (FAP) — a condition that causes hundreds of small growths (known as polyps) in the bowel of the person affected. If left untreated, FAP always turns into bowel cancer. Only 1 percent of all bowel cancer is due to FAP.
Other conditions that may increase risk
- Having had Crohn’s disease or ulcerative colitis for more than 10 years.
Should those at increased risk be checked?
In some cases, people who have a higher than average risk of developing bowel cancer are advised to be checked. Advice as to who should be checked, what tests they should have and how often, has been developed by the New Zealand Guidelines Group. This advice is summarised in a leaflet available from your family doctor, your local Cancer Society and the website of the New Zealand Guidelines Group.
Each person has a right to make an individual decision on whether or not to be checked.
Screening programmes
Some countries have a screening programme (for those without symptoms) or are in the process of implementing screening for bowel cancer; for example, Australia, the United Kingdom and Finland. In New Zealand, a bowel cancer screening programme is in the early stages of development. For more information, talk to your doctor or visit the National Screening Unit’s website.