Cancer of the colon or rectum (large bowel) is a common form of cancer. After the lung, the colon is the most common site of cancer affecting both men and women. It is also the second most common cause of death from cancer. About one person in 27 develops a colorectal (large bowel) cancer. The disease becomes more common with age and is rare before the age of 40. It occurs most frequently in the age group 60 to 75.

Colorectal cancers start as little mushroom-like bodies called polyps. Many people have polyps that remain non-malignant, but a small proportion of polyps go on to form cancers. There are rare disorders that run in families, in which large numbers of polyps develop in the colon. People in these families are very liable to develop colorectal cancers. In 90% of cases, however, there is no strong family history.

About 40 per cent of colon cancers occur in the lowest part of the colon and in the rectum.

Causes

There is some evidence that diets low in fibre (roughage) and high in animal protein and fat make you more likely to develop colorectal cancer. The reasoning is that this kind of diet results in slower movement of contents through the intestine than occurs with a diet high in roughage. Slow movement allows the cancer-producing factors to remain longer in contact with the inner lining of the intestine, where cancers always start.

There may also be a family history and inherited syndromes such as hereditary non-polyposis colorectal cancer (HNPCC).

Treatment

Colorectal cancer is treated by a surgical removal of the affected segment of the bowel, together with the associated lymph nodes (glands). Generally the cut ends of the bowel can be joined back together. However, if the rectum has to be removed, the end of the bowel is brought to the skin of the abdomen to drain into a external bag (colostomy).

In many cases bowel cancer can be cured by surgery, but this depends almost entirely on the stage the cancer has reached at the time when it is diagnosed. Chemotherapy is often used, and radiotherapy is used for rectal tumours.