Should Canadians be offered systematic screening for colorectal cancer?

Absolute risk of mortality

In public health, it is never appropriate to launch a program to prevent early mortality due to a specific cause in segments of the population already affected by high mortality due to all sorts of causes. It is a consequence of aging that larger numbers of people will die from an increasing number of causes, which is to say that the contribution that a specific cause makes to general mortality quickly becomes negligible. In this context, focusing on prevention of mortality caused by a specific disease becomes futile, even when a validated test is available. That is the first problem with colorectal cancer screening. Even if this screening were applied correctly, it would have no effect on the absolute risk of dying. This phenomenon was in fact noted by the researchers in Minnesota who were the first to determine that it was possible to reduce mortality from colorectal cancer with the fecal occult blood test.

It is estimated that, in 2007, 72 700 Canadians died of cancer; of these, 8700 (12%) died from colorectal cancer. About 86% of deaths from colorectal cancer occur in the seventh, eighth, and ninth decades of life (7450/8700).2 Mortality due to all other causes is also increasing steadily in all of these age groups such that even if screening for colorectal cancer were widely accepted by the population and applied meticulously, it would not be possible to obtain a measurable reduction in the absolute risk of dying. Because this is an absurd eventuality, the principles framing public health practices should prohibit us from organizing this screening.

Test limitations

It gets worse. Although the test recommended for this screening, the fecal occult blood test, is capable of reducing mortality from colorectal cancer by 30%, it is a bad test because of the high number of false-positive results it produces. Think of the suffering that all these false-positive results create in a population for whom the mere possibility of cancer is a tragedy in and of itself. Some health professionals have taken heart from the fact that methods of validating positive results have become more reliable and more widely available. But this seems to me to be a rationalization after the fact. The methods for confirming a diagnosis are neither pleasant nor completely safe. The National Committee estimates that, during the first 10 years of a widely used screening program, these methods will cause 75 deaths and 611 perforations.1 This is an exorbitant price to pay for a reduction in general mortality so minuscule that it cannot be measured.

Appropriate use of resources

And what of the considerable increase in the use of health services that such a program would no doubt cause? Given the bottlenecks that are already affecting too many health care sectors, how can we justify preparing for what could be a veritable tsunami of requests for gastroenterology services? And what of the work that will fall to family physicians in managing the follow-up from this screening, which will be weighted toward the very great majority of those who receive positive test results? It is truly unfortunate that all the energy that family physicians will devote to supporting these patients, whom they have sent on ill-fated diagnostic journeys, will result in painful experiences that will probably ruin the quality of life of a substantial portion of their patients’ remaining days.