Epidemiology
Colorectal cancer is the third most common cancer and second most common cause of cancer death in the UK. Occurrence is strongly related to age, with almost 85% of cases occurring in people aged 60 years and over. It is significantly more common in the developed world, occurring most frequently in New Zealand, Canada, USA and UK. There is no gender bias.
Aetiology
Colorectal cancer is generally considered to be an environmental disease, with the majority of cases being linked to lifestyle and advancing age, and only a minority occurring as a result of genetic predisposition.
Many of the lifestyle factors thought to encourage development of colorectal adenocarcinoma are diet-related and increased consumption of red meat, alcohol and a high calorie intake play an important role. Other risk factors include smoking, sedentary lifestyle and obesity. Colorectal cancer occurs more commonly in patients with inflammatory bowel disease and risk increases with duration of disease and severity of bowel inflammation.
Approximately 5% of all colorectal cancer cases occur as a consequence of genetic syndromes, the most common of which is hereditary non-polyposis colorectal cancer (HNPCC or Lynch syndrome type I). HNPCC develops as a consequence of germ-line mutations in one of several DNA mismatch repair genes and carries a 40% lifetime risk of developing colorectal cancer. The condition is transmitted in an autosomal dominant pattern. Patients with HNPCC typically develop colorectal cancer in the fourth decade of life. Other malignancies such as ovarian, endometrial, gastric, urinary and hepatobiliary cancer may occur and these are recognised as Lynch syndrome type II (see Chapter 8).
Familial adenomatous polyposis (FAP) is a more rare type of hereditary colorectal cancer. It is inherited in an autosomal dominant pattern and arises as a result of germ-line mutations in the tumour suppressor gene APC
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