| This year 57,100 Americans are expected to
die of a largely preventable disease, yet fewer than half of Americans
at risk undergo preventive screening. That disease is cancer of the
large intestine, better known as colorectal cancer. Only lung cancer
will cause more cancer deaths this year. Breast and prostate cancer
will each claim fewer lives (40,200 and 28,900, respectively). And,
whereas over 90% of women over 50 have undergone mammography and at
least 75% of men have had a PSA test for prostate cancer, screening
for colorectal cancer is far less common.
The low rate of colorectal cancer screening is
particularly troubling because available tests are capable not only
of discovering early, treatable stages of the disease, but of actually
preventing it completely. To understand how, one needs to know about
polyps.
The Polyp Story
Colorectal polyps are abnormal benign growths on the lining of the
large intestine. They are present in about 20% of people over age
50, and about 40% of those over age 60. Most polyps never cause
problems, but some eventually become cancerous over a period averaging
10 years or so. Polyps larger than one centimeter and those possessing
certain tissue characteristics pose the greatest cancer risk.
About 80% of colorectal cancers start out as benign
polyps. (Many experts believe that virtually all colorectal cancers
arise from polyps). This means that by finding and removing polyps,
colorectal cancer can be prevented. Unfortunately, one can't depend
on warning symptoms to detect polyps. Unless bleeding occurs, which
is usually so minor as to be visually undetectable, polyps almost
never cause symptoms. So how are they discovered?
Screening for Blood in the Stool
The propensity of cancers and some polyps to bleed microscopically
is the basis for the fecal occult blood test (FOBT), the simplest
of the available screening measures. In this test one smears samples
of stool from three different bowel movements onto cardboard slides.
Exposing the slides to a chemical agent reveals a bluish color in
the presence of occult (microscopic) blood.
Occult blood in the stool can arise not only from
polyps or cancer, but also from several other causes throughout
the entire gastrointestinal tract. In addition, red meat or certain
plant products can cause a positive test, which is why these foods
should be avoided before and during the sampling period. Nevertheless,
a positive test always requires further investigation, usually with
colonoscopy. It is never wise to repeat a test to see if it becomes
negative, because colorectal growths are notorious for bleeding
intermittently.
The FOBT clearly can save lives, but it also misses
many polyps and even some overt cancers. This has led to the increasing
use of more aggressive screening techniques.
Sigmoidoscopy and Colonoscopy
Sigmoidoscopy entails the insertion of a flexible fiberoptic instrument
that allows direct inspection of as much as 60 centimeters of the
colon lining. This terminal portion of the colon, consisting of
the rectum and the sigmoid colon, harbors roughly half of cancers
and polyps. The rest are beyond the reach of the sigmoidoscope.
Colonoscopy, in contrast, involves using a longer instrument that
can traverse the entire colon.
The advantages of sigmoidoscopy are that is cheaper,
does not require sedation, and can be performed by a non-specialist.
Colonoscopy is more expensive and requires intravenous sedation.
But, besides permitting full visualization of the colon, colonoscopy
has another distinct advantage: polyps can usually be removed completely
during the procedure. As for safety, sigmoidoscopy has the edge,
but the risk of serious complications with either procedure is low.
One additional test that can detect polyps or
cancers is the air contrast barium enema. Barium coats the colon
lining, while the introduction of air during the procedure reveals
surface irregularities detectable on x-ray. Nowadays this procedure
is used much less commonly than colonoscopy.
Which Test is Best?
For the average risk adult, regular screening should begin at age
50. Choosing which test to undergo requires weighing considerations
of safety, cost and effectiveness, thus most authorities recognize
that the decision should be individualized. The commonest recommendation
is to undergo annual FOBT plus sigmoidoscopy every five years. Alternatively,
full colonoscopy can be performed every 10 years, or air contrast
barium enema every five years. More and more experts are leaning
toward colonoscopy screening because of its higher reliability,
and some even advocate shorter screening intervals.
The foregoing recommendations are not applicable
to high risk adults (those with a family history of colorectal cancer
or with certain hereditary polyp syndromes), or to those with a
prior history of polyps or cancer. For these individuals, earlier
or more frequent testing are often required.
Where to get additional information:
American Cancer Society (703) 938-5550 www.cancer.org
National Cancer Institute's Cancer Information Service (800) 4-CANCER
www.cancer.gov
August 2003
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