Cancer of the large bowel (the colon and the rectum) is the second leading cause of cancer deaths in the USA. Several screening methods are used to try and detect it early, so that treatment is more likely to be effective. Testing small samples of feces for blood is the simplest method. More complex is a barium enema, where the mixture contains a contrast material that shows on X-ray. The other screening method used today is sigmoidoscopy -- an instrument is passed up into the lower part of the colon, allowing the physician a view of the bowel from the inside. All these methods have their advantages and disadvantages. Perhaps the most serious disadvantage is that they do not reveal all the tumors that there may be in the large bowel. The fecal tests for blood and barium enema are not sensitive enough, and sigmoidoscopy does not reach "round the band" to allow the physician to see the whole of the colon. Now two studies have shown that by using a thin, flexible viewing instrument called a colonoscope it is possible to visualize the entire length of the colon, showing all the areas where tumors may occur.

In the first study colonoscopy was done on over 3000 people attending US Veteran Affairs medical centers. The bowel was cleaned out ("prepared") by having the subject drink a large amount of a special salt solution the night before the procedure, which led to complete emptying - this in itself is probably the most unpleasant part of the whole examination. Colonoscopy was done using a fairly strong sedative given by IV injection.

The average age of the subjects was 63, and 97% of them were men. One in eight of them had one or more polyps - these are benign outgrowths of the lining of the colon, which may turn into tumors after some years. Just over a third of the subjects had one or more tumors of the large bowel - almost all of these were a benign, slow-growing tumor (adenoma), but 1% (30 subjects) had invasive cancer (carcinoma).

The investigators classified the tumors according to whether they were situated in the upper part of the colon (the ascending and transverse colon), or in the lower part (the descending and the sigmoid [or S-shaped] colon, and the rectum). The boundary between the upper and lower parts was the splenic flexure, a point high under the left rib cage where the transverse colon bends to become the descending colon. The most serious lesions for the two parts of the colon were identified separately for each subject. 7.3% of the subjects had a tumor in the lower part of the colon, and 4.1 percent had a tumor in the upper part. It was found that those who had benign tumors in the lower part of the colon were 2-3 times more likely to have a tumor in the upper part of the colon.

The second study described the results of colonoscopy done in 2,000 employees of a company. Fifty of them were found to have tumors in the upper part of the colon which would not have been found if only the lower part had been examined by sigmoidoscopy ¹ .

The findings of these two studies will obviously encourage the gastroenterologists who would like to recommend that all people over 50 should have a colonoscopy. At present, the screening recommendations (testing for blood in the feces, barium enema, sigmoidoscopy) are actually carried out in less than 30% of those who qualify for such tests. This is probably because the examinations are distasteful and unpleasant, and are not considered by the patient to be important enough.

The high death rate from cancer of the large bowel shows the risks of not having screening tests. Obviously, education of everybody concerned (patients and their health professionals) needs to be intensified, to overcome the poor numbers of persons actually having the screening tests performed. However, the findings of these two studies suggest that more complete examination (i.e. colonoscopy) is necessary if one wants to detect all tumors before they become malignant.

The costs and the small risks associated with colonoscopy have led to its being reserved, so far, for persons who have had a positive result from one of the screening tests. Now it is time to promote both the importance of screening for this disease, and to encourage the wider use of colonoscopy. As this procedure is more widely used, the costs will decrease and familiarity with it will remove much of the apprehension.

Source

Use of colonoscopy to screen asymptomatic adults for colorectal cancer
DA. Lieberman, DG. Weiss, JH. Bond,  et al., N Engl J Med, 2000, vol. 343, pp. 162--168

Footnotes

1.

Risk of advanced proximal neoplasms in asymptomatic adults according to the distal colorectal findings.
T. Imperiale, DR. Wagner, CY. Lin,  et al., N Engl J Med, 2000, vol. 343, pp. 169--174