Early bowel cancer (benign polyps which cause obstruction) is easily curable by surgical resection. Detection and diagnosis of colorectal cancers is usually delayed, however, because often the only clue to their presence are the vague signs of an irritated colon -- constipation, diarrhoea, blood or mucus in the faeces. (Rectal bleeding, for example, affects 20% of the population in any one year, and is usually due to haemorrhoids).
Cells that line the colon are very active, constantly dividing and creating buds, known as polyps. Most polyps are small, benign growths that eventually stop growing. A tiny percentage of these polyps keep growing, sometimes for 10 years or more. Various genetic mutations can transform them into cancerous tumours. As these tumours grow larger, they gather more mutations and begin to burrow deeper and deeper into the muscle wall that surrounds the colon. Once the cancer invades the blood and lymph systems, malignant cells can break off and spread to other organs, such as the liver, lungs and stomach.
Colon-rectal cancer usually affects people in their 60s and 70s. Nearly 1 million people worldwide were diagnosed with the disease in 2000.
Colorectal cancer is the second most common fatal cancer in the UK, with over 20,000 deaths a year; the average risk of dying from it is one in 50. The exceeding high rates of bowel cancer in Scotland in the 1950s have dropped, probably as a result of dietary changes.
In 1997 colon-rectal cancer was the third most common cancer in the USA. It presents the highest cancer risk for non-smokers (smokers' risk of lung cancer is higher). It strikes up to 150,000 Americans a year and kills more than 50,000, the vast majority over the age of 50. The American Cancer Society projects that more than 135,000 Americans will be diagnosed with the disease in 2001. A genetic mutation associated with colon cancer is twice as common in Jews as in non-Jews, making the risk of colon cancer 1 in 17 for American Ashkenazi Jews.
Diets low in fibre and high in fat raise the chance of an individual developing colon cancer.