Colorectal cancer is a leading cause of cancer
deaths and the most common gastrointestinal malignancy.
The lifetime risk of colon and rectal cancer is 6-7%
and it increases sharply after age 50. Many women
are not aware that their chance of getting colorectal
cancer is about 5 times greater than of getting ovarian
cancer. For someone interested in living a long life,
it would be wise to know how to be screened to pick
up colorectal cancer in its earliest stages.
In a recent continuing medical education article,
Early DS: Colorectal cancer screening: An overview
of available methods and current recommendations.
South Med J. 1999; 92:258-265, we are presented with
a good overview of how to go about screening for this
devastating cancer.
How does cancer of the colon develop?
The skin lining the colon (mucosa) sometimes undergoes
a transition from normal to glandular hyperplasia
(adenomatous polyps) to cancerous tissue in a stepwise
fashion over a 5-15 year time period. Nearly all colon
cancers arise from these adenomatous polyps. The opposite
is not true, however, that all polyps develop into
cancer. In fact most polyps of the colon do not become
cancerous, but their existence is a risk factor. There
is one exception to this normal-adenomatous polyp-cancer
progression and that is in ulcerative colitis in which
the cancers are preceded by a different process, a
dysplasia or non-polypoid abnormal growth pattern.
Thus in most cases, if adenomatous polyps can be detected
and removed, that may help prevent colon cancer.
What different diagnostic tests are available
to detect colon or rectal cancer?
The basic tests available to screen for colorectal
cancer are flexible sigmoidoscopy, checking the stool
for hidden blood (fecal occult blood), dye and air
in the colon (air contrast barium enema), and looking
beyond the lower part of the colon (sigmoid) into
the transverse and ascending colon using a scope called
colonoscopy.
Approximately 50-65% of colon cancers occur in the
lower part of the colon that is visualized with a
flexible sigmoidoscopy. Studies have shown that sigmoidoscopy
is associated with a 59-79% reduction in cancer mortality.
Checking the stool for blood is considered a cancer
test because a polyp would need to be fairly large
sized in order to bleed. In other words, looking in
the colon will pick up polyps sooner than waiting
for them to grow big enough to bleed. Fecal occult
blood tests have been shown to reduce colon cancer
by about 33%. Air contrast barium enema can pick up
more lesions than sigmoidoscopy but is not as good
as colonoscopy. If the barium enema is abnormal, a
colonoscopy has to be done anyway so this is not as
cost effective as a screening test. Colonoscopy is
a more sensitive test to find polyps and cancer but
it is more expensive and occasionally patients have
serious complications (3-17/1000) or even death (2/10000).
It is used for screening women who are at high risk
or who have a positive fecal occult blood test.
What are the screening tests recommended
for a woman at average risk of colon cancer?
Starting at age 50, it is recommended that women have
a yearly fecal occult blood screen and a flexible
sigmoidoscopy every 5 years or a total colon exam
( barium enema or colonoscopy) every 5-10 years.
What would make a woman at higher than normal
risk for colorectal cancer?
If a woman has had a history of having adenomatous
polyps or a past colon cancer, a history of inflammatory
bowel disease (ulcerative colitis, regional enteritis),
or a strong family history of colorectal cancer or
adenomatous polyps, then she should be considered
at high risk.
Having had a colorectal cancer, there is an increased
risk of a second colorectal cancer of about 6% over
an 18 year period. Over a third of women in whom colon
polyps have been detected and removed have a recurrence
of the polyps. If there is a family history of polyposis
or colon cancer, there may be a genetic tendency toward
colon cancer. This needs to be investigated and sometimes
genetic testing will have to be performed even though
a genetic tendency only explains about 5% of colorectal
cancers.
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