Dr John P Wright
Updated: 22 May 2007
Colonoscopy is generally accepted by the public, the profession and the funders to be the optimum method to screen patients for adenomas, enable their removal and therefore reduce the incidence of colorectal cancer. Even where occult blood testing is recommended colonoscopy remains the final verification of polyp/no-polyp. This rosy picture may not be entirely true.
The main problem area is the proximal colon as opposed to the recto-sigmoid and left colon. Here the Norwegian 1999 Telemark Polyp Study has shown that flexible sigmoidoscopy can reduce the incidence of colorectal cancer by 80%. This may well be too optimistic as the mortality rate was not changed and other studies from Norway have not yet confirmed this benefit.
Full colonoscopy, the preferred modality for colon cancer prevention was shown in the American 2007 National Polyp Study to lead to a reported 80% reduction in the incidence of colorectal cancer in patients who had undergone colonoscopy and polypectomy. Other studies supported these results. The 80% reduction has become the Holy Grail in colorectal cancer protection and hence the international recommendation of screening colonoscopy as part of any disease prevention program.
Other studies have not confirmed a 80% benefit and some have shown no benefit at all.
The reasons for this wide variation in outcome in colorectal cancer prevention studies has been debated and analyzed. Some of the problem is simply related to differences in study design. Entry and exit criteria, length of follow up and number of colonoscopies performed varies widely. A further observation is that cancers found in the first 3 years of follow up may well represent missed lesions at the primary / entry examination. That these possibly missed cancers tend to be proximal suggests that colonoscopic technique may be a central factor in colorectal cancer prevention. The risk of missed proximal colonic cancers is a theme that keeps recurring in the colorectal cancer prevention literature.
From the above it is reasonable to conclude that colonoscopic screening and polypectomy will prevent colorectal cancers. The problem is in the success of the procedure. There are two main areas of potential problems one is the patient preparation and the other is the expertise of the colonoscopist.
Taking preparation for colonoscopy on the day before the procedure has been shown to be inferior to that taken on the day of the procedure. The right colon is the main area to be obscured by poor preparation. The split method of some preparation on the day before and some on the day of, may be a reasonable alternative. The inferiority of the day before preparation is particularly important in the detection of right sided lesions which, as was indicated above, is the main problem area.
The passage of the colonoscope through to the caecum needs to be assured at all colonoscopies. It is recommended that photographic evidence be collected to confirm visualization of the caecal pole. Without this the examination cannot be assumed to be complete.
There is an inverse correlation between the polyp detection rate and the time taken to withdraw the colonoscope. While speed of execution is a peculiarly male measure of expertise it is alsoinappropriate in this context.
A third factor in the success of colonoscopy in colorectal cancer prevention is the data given to the patient. The patient needs to be informed on the adequacy of the examination. If the quality of preparation is sub-optimal and/or the full colon not visualised the patient needs to know. The next examination date taking this into account needs to be discussed clearly with the patient.
Finally the current recommendations for screening colonoscopy are:
It is a strange, almost Biblical fact, that many patients with a family history are given too many colonoscopies while those without have none.