Every few months I receive a computerized invitation from my doctor asking me to have a colonoscopy to screen for polyps in my bowel. I always tell myself that I am too busy just now, I will have it another time. But really I don’t want to have it at all, and I know that when the next invitation comes I will be too busy then as well.
I am also eager to find a rational reason, or at least a rationalization, for my refusal. I thought I found it in a paper from Norway in a recent edition of the New England Journal of Medicine.
The authors examined the death rate from colorectal cancer in Norway among the 40,826 patients between 1993 and 2007 who had had polyps removed at colonoscopy in that country (the records are more or less complete). They compared the number of deaths in that population with the expected death rate from the disease in the population the same age as a whole. The paper reports that 398 deaths were expected and 383 deaths were observed.
This small difference does not mean that colonoscopy does not work in preventing death from colorectal cancer, of course. This is because the relevant comparison is with people who had polyps not removed by colonoscopy rather than with the population as a whole.
The 40,826 patients who had polyps removed at colonoscopy, however, were not a random sample of the adult population because Norway does not have a screening program for colonic polyps. The patients had colonoscopy in the first place because they were symptomatic, for example bleeding per rectum. They were therefore much more likely to suffer from polyps or cancer in the first place than the rest of the population.
Colonic polyps are not all of equal significance. Small and histologically normal polyps are precursors to cancer much less frequently than large and histologically disordered ones. But the authors found that removal of low-risk polyps reduced the risk of death from colorectal cancer by 25 percent compared with the general population, while those who had high-risk polyps still had a death rate 16 percent higher than that of the general population, though presumably it would have been higher still had it not been for colonoscopy.
The excess number of deaths from colorectal cancer of those with high-risk polyps removed was 33; but perhaps most surprising, at least to me, was that the excess of deaths from all causes in this group was 2222. In other words, colonoscopy saved the lives of only 1.5 percent of those in whom high-risk polyps had been found.
Another very interesting fact revealed in the paper was that mortality from colorectal cancer was 37 percent lower for those who had their polyps (of whatever kind) removed in the 2000s compared with those who had them removed in the 1990s. Why was this? The paper does not say; perhaps the technique of colonoscopy had improved and polyps were removed more thoroughly in the later period than in the earlier; perhaps the treatment of the cancer itself had improved; it is even possible that the nature of the disease itself had changed.
When you put everything together, including the fact that colonoscopy is itself not without risks, I think my avoidance of my doctor’s invitation might not be quite as absurd as it appears. It is true that, if everyone had colonoscopy, many but not all deaths from colorectal cancer might be prevented; but I am only one person, as is everyone else, and so the chances of it saving my life, or the life of any individual person, are very small.
As for repeat colonoscopies, no one knows how often, or whether, they are advisable.