Colorectal cancer is the third commonest malignancy in
the United Kingdom, after lung and breast cancer, and kills about
20 000 people a year. It is equally prevalent in men
and women, usually occurring in later life (at age 60-70 years). The incidence
of the disease has generally increased over recent decades
in both developed and developing countries. Despite this trend,
mortality in both sexes has slowly declined. This decrease
in mortality may reflect a trend towards earlier diagnosis—perhaps
as a result of increased public awareness of the disease.
Surgery remains the mainstay of treatment for colorectal
cancer, but early diagnosis makes it more likely that the tumour can be
completely resected and thereby improves the chance of cure
Most colorectal cancers result from malignant change in
polyps (adenomas) that have developed in the lining of the bowel
10-15 years earlier. The best available evidence suggests
that only 10% of 1cm adenomas become malignant after 10 years.
The incidence of adenomatous polyps in the colon increases
with age, and although adenomatous polyps can be identified in
about 20% of the population, most of these are small
and unlikely to undergo malignant change. The vast majority (90%)
of adenomas can be removed at colonoscopy, obviating
the need for surgery. Other types of polyps occurring in the colon—
such as metaplastic (or hyperplastic) polyps—are usually
small and are much less likely than adenomas to become malignant.
Colorectal cancer is therefore a common condition, with
a known premalignant lesion (adenoma). As it takes a relatively
long time for malignant transformation from adenoma to
carcinoma, and outcomes are markedly improved by early
detection of adenomas and early cancers, the potential
exists to reduce disease mortality through screening asymptomatic
individuals for adenomas and early cancers.
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