A cancerous tumour starts from one abnormal cell. In the case of colorectal cancer, it is a cell in a polyp which is the most common site for a cancerous cell to develop. The exact reason why a cell becomes cancerous is unclear. It is thought that something damages or alters certain genes in the cell. This makes the cell abnormal and multiply 'out of control'.

Risk factors

Although colorectal cancer can develop for no apparent reason, there are certain 'risk factors' which increase the chance that colorectal cancer will develop. These include:

• Ageing. Colorectal cancer is more common in older people. Most cases are in people over the age of 50.
• If a close relative has had colorectal cancer (there is some genetic factor).
• If you have familial adenomatous polyposis or hereditary non-polyposis colorectal cancer. These are rare inherited disorders.
• If you have ulcerative colitis (a condition of the colon) for more than 8-10 years.
• Obesity.
• Lifestyle factors: little exercise, eating a lot of red meat, drinking a lot of alcohol, smoking.

Protective factors

There is a reduced risk of developing colorectal cancer in:

• Women who take hormone replacement therapy (HRT).
• People who eat a lot of fruit and vegetables.
• People who take anti-inflammatory medicines for other conditions. For example, aspirin.

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