Colon
Cancer ScreeningAt this point there is no reliable specific blood test,
which would allow mass screening for colon and rectal cancer. However, a combination
of the following tests are useful o come to a diagnosis (Ref. 1,2 and 3): 1.
Test stool for occult blood: As mentioned above, we know that 85%
of cancers develop from polyps, which can and do erode and shed microscopic amounts
of blood into the stool. This cannot be seen, but it can be tested for. There
are very sensitive screening tests available ("stool for occult blood"-
testing). Three different stool samples are spread onto a special cardboard container.
One of them that is very popular is by Immunostics,Inc., 3505 Sunset Ave., Ocean,
New Jersey 07712. The lab or the physician's office can then add a developer substance,
which colors the stool sample, if it contains blood. It fails to color it, if
there is no blood in it. There are a small number of false positives and false
negatives, but it is the best test that we have at the present time. 2.
Rectal examination: Quite a few rectal cancers or polyps can be
reached this way and this test can be done by the examining physician.3.
Flexible sigmoidoscopy: This test procedure is easily available
by the gastroenterologist and reaches 60% of colorectal cancers, which almost
can replace the rectal exam. 4. Colonoscopy: A
full length colonoscope is used to do
a colonoscopy of the entire colon. Most colon cancers in the past used
to be in the rectum or sigmoid colon. But in the last decades more and more cancers
are found also in the transverse and in the ascending colon, which is only accessible
by a full length colonoscopy, where the entire rectum and colon is visualized. Screening
recommendation: High risk family members with any of the genetic
syndromes with multiple colonic polyps should have a colonoscopy
done in regular intervals from age 35 onwards. Other people ideally
should have this test from age 50 or 55 onwards as a baseline.
Testing for occult blood should be started at age 40 for everybody
and then done on a yearly basis. This is the only method apart from a colonoscopy
to know whether or not there are polyps present. Unfortunately different health
care plans are not always supportive of this screening procedure, but they will
pay for taking care of your cancer when it is too late. In future
there likely will be a screening test available that would involve monoclonal
antibodies, which will test for a group specific tumor antigen for
rectal/colon cancer (similar to the PSA test for prostate cancer). At the present
time the carcinoembryonic antigen (=CEA
) can be tested as a test for this is available, but unfortunately
only a minority of colon cancer patients are positive for this. Another
line of research is very promising (see Ref. 9). Dr. Haili Wang, a surgeon at
the University of Alberta Hospital, presented a paper from a study involving metabolites
in urine to distinguish between healthy subjects and those who are carriers of
colonic adenomatous polyps. The urine samples were tested with nuclear
magnetic spectroscopy (NMR) and analyzed with software from Chenomx Inc of Edmonton.
The urine test had a specificity of 80.6% and sensitivity of 81%. The researchers
concluded: "This is the first study to demonstrate that NMR urine metabolomics
has the ability to distinguish normal healthy subjects from patients with adenomatous
polyps with far superior accuracy than that of current fecal based screening tests". Occasionally
the doctor might order a Barium enema test, but this has largely been replaced
by colonoscopy.
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