Endoscopic view
of typical Colon Cancer |
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Screening has been determined to be appropriate for colorectal cancer (CRC). Adenomatous polyps (benign tumours of the colon and rectum) are known pre-malignant conditions, which can be treated thus preventing cancer from developing.
Screening tests have been found to achieve accurate detection of polyps or early stage CRC. There is evidence that removal of adenomatous polyps reduces incidence of CRC. It has been shown that detection of early stage CRC reduces mortality.
The various methods of screening for CRC will now be discussed. |
Faecal Occult Blood Test (FOBT)
The principle of this test is that large polyps and cancers bleed more than normal mucosa (lining of the colon). Two thirds of cancers are believed to bleed in the course of a given week. 90% of cancers will be detected with repeated testing over the years.
The advantages of FOBT are that early testing, with full colonoscopy for positive result, has been shown to reduce risk of death from CRC. The test is simple to carry out, low cost and relatively convenient to perform.
However, there are disadvantages of FOBT. The sensitivity is only 72-78% and inevitably some polyps and cancers will be missed. The test is affected by tumour location. Tumours in the transverse colon and descending colon are harder to detect. Also, unless a very specific immunologically based test is used, a strict diet needs to be followed to reduce false positive and false negative results. Blood from any source can give a positive test, e.g. bleeding gums, nosebleed, stomach diseases etc. A positive test on any occasion will lead to a colonoscopy since the colon will need to be thoroughly examined. Since there are many false positive tests, many normal people will end up with colonoscopy anyway.
Barium Enema
This is an x-ray test in which dilute barium sulphate suspension is introduced into the rectum and allowed to flow upwards to coat the entire colon. Air is then introduced to provide finer definition in the x-ray images. The patient is then turned in different directions on the x-ray table to obtain good views of the entire colon.
The advantage of barium enema is that it can examine the entire colon up to the terminal ileum. Large polyps and cancers can be detected. It is also somewhat safer that colonoscopy (although the risk of colonoscopy is already very small).
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The disadvantages of barium enema are firstly the large dose of x-radiation given to a patient. The procedure is very long and many patients find it uncomfortable. Small polyps and cancers may be missed and on the other hand artefacts (such as small adherent lumps of stools) may give false positive results. Even if a lump is seen, no biopsy is possible. In the end, any hint of abnormality leads to a colonoscopy anyway to confirm its presence and to biopsy it. Finally there are no studies that show that barium enema used alone will reduce mortality of CRC. |
| Barium enema showing typical 'apple core' colon cancer |
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