Causes of Colon
Cancer
No cancers are fully understood, but Colon cancer is better
understood than most.
About 90 per cent of the risk for Colon cancer is thought to be
due to dietary factors, with the other 10 per cent due to genetic (inherited)
factors.
Dietary factors
Dietary factors that increase Colon cancer risk are not yet
clearly defined. Populations with a high-fibre intake tend to have a low risk
of Colon cancer. However, the results of studies in which people, usually those
who have already developed polyps, have been given high-fibre diets are
disappointing.
It now seems as though the beneficial effect of
fibre is not simply
due to its mechanical effect on helping the Colon to regularly pass faeces.
Evidence suggests that vegetable fibre is more protective that
cereal fibre. Recent studies have also shown that specific chemicals in
vegetables, for example the isothiocyanates, which give brassicas (cabbage,
broccoli, brussel sprouts, cauliflower) their characteristic pungent taste,
might be especially protective against cancer. A high intake of calories and
obesity are both risk
factors for Colon cancer, and a high intake of red meat is also linked with
increased risk.
The best available approaches for a low risk of developing
Colon cancer are:
-
a diet high in green vegetables, particularly cabbage,
broccoli, brussel sprouts or cauliflower.
-
a diet low in red meat. In particular, avoid burnt meat,
which contains cancer-promoting chemicals called cyclic amines.
-
keeping to a normal body weight and taking regular exercise.
-
Although still controversial, it seems that taking
aspirin (eg Disprin) regularly
(300mg per day or more ie one standard tablet) reduces the risk by about 50 per
cent. However, prolonged use of aspirin carries a risk of intestinal ulceration
and bleeding, so whether the benefits would outweigh the risks is unclear at
present.
Genetic factors
Approximately 10 per cent of Colon cancers have a strong
genetic factor. The commonest is hereditary non-polyposis colon cancer (HNPCC
or Lynch syndrome). This condition is caused by mutation in any one of at least
five different genes. These genes carry the instructions for manufacturing
proteins that can repair damaged DNA.
Inheritance of this type of cancer is autosomal dominant,
which means that half the children of someone with HNPCC are at risk of
inheriting the condition. When these genes are defective, DNA repair does not
take place, and damaged (mutant) DNA accumulates within cells, greatly
increasing the cancer risk.
The colon is not the only organ affected. The syndrome also
carries an increased risk of cancers of the stomach, ovaries, breasts and
uterus.
The Colon cancers in affected individuals tend to develop as
flat lesions rather than as polyps. The cancers more commonly affect the
proximal (right sided) colon, whereas other cancers are more common in the
distal colon (nearer the rectum) or rectum. They occur at a younger age and
this condition should be suspected in anyone who develops Colon cancer before
the age of 45.
About 1 per cent of Colon cancers occur in people who inherit
a defect in the gene for familial polyposis coli. These
people develop hundreds of adenomatous (pre-cancerous) polyps in the colon by
the time they are in their teens and almost invariably develop Colon cancer by
middle age unless the colon is removed.
Patients with
ulcerative colitis or
Crohn's disease
of the colon (conditions that together affect about 1 in 800 of the population
in Western countries) have about a five-fold increased risk of Colon cancer.
The risk is greater if the colitis (colon inflammation) seen
in both conditions affects the whole colon, and if it has been present for more
than ten years. The risk is probably reduced by regularly taking mesalazine
(5-aminosalicylic acid), a medication that is widely used to reduce the risk of
relapse in these diseases.
Symptoms
of colon cancer
Because early cancers often cause no symptoms, screening of
symptom-free individuals is being considered. About 85 per cent of people with
Colon cancer are currently not diagnosed until the cancer has penetrated
through the Colon wall or spread to lymph nodes or elsewhere. Cure is
nevertheless still possible. The earliest symptom is often bleeding from the
back passage. Later changes include loss of the normal form of Colon motions
sometimes followed by
diarrhoea.
Constipation can also
occur. If the growth starts to block the Colon then colicky lower abdominal
pain (ie coming in waves each of which lasts for a few minutes) can develop. If
the Colon becomes completely obstructed, severe abdominal pain and vomiting
occurs, followed by complete constipation. This obstruction is a surgical
emergency requiring immediate admission to hospital, since the Colon is at risk
of becoming gangrenous if the obstruction is not relieved.
In the proximal colon, the lumen (the space inside) is larger
(about 4 or 5cm in diameter) and less likely to become obstructed. Cancers of
this part of the colon, including the caecum, tend to show themselves very
subtly as
iron-deficiency
anaemia, due to loss of small amounts of blood over a long period of
time. The anaemia can lead to symptoms of pallor, shortness of breath or simply
tiredness.
Cancers of the rectum typically cause rectal bleeding, which can
easily be mistaken for bleeding
haemorrhoids (piles).
Other symptoms include the feeling that you haven't fully emptied your
Colon accompanied by a need to frequently empty the Colon. Loss of appetite and
weight loss tend to be late features in Colon cancer.
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