Much ambiguity (why do people die?)
There's one more wrinkle I must throw in here. These calculations were based on the assumption that researchers want to detect changes in the number of deaths due to a specific cancer. That's fine, so long as you are confident that the testing and early treatment process never leads anyone to die from something else.
But I'm not especially confident of that. Awoman who has part of her lung removed after an early lung cancer is found may be "cured" of cancer but die of pneumonia six months later. The surgery made pneumonia more likely, but it won't be counted as a lung cancer death. Similarly, a man who has part of his colon removed after a colon cancer is found may be "cured" of cancer but die of intestinal obstruction six months later.
The surgery made the obstruction more likely, but it won't get counted as a colon cancer death. Other treatments for cancer may slightly increase the long-term chance of death from a secondary cause for example, radiation seems to encourage heart disease, while chemotherapy may stimulate second cancers22 yet these deaths are not counted as deaths from the target cancer either. The thing is real: researchers reported that the number of deaths in cancer patients due to something other than cancer was 37% higher than in men and women of similar age. Because these excess noncancer deaths occurred shortly after diagnosis, they concluded that a large proportion of them were due to cancer treatments.
And there are all the people who don't put on cancer but are nonetheless affected by the screening process. A few of the radiological studies and all the biopsies done as part of screening have risks. Sometimes doctors stumble onto other things along the way and start treatments they wouldn't have otherwise. Any death during the screening process won't get counted as cancer death, because these patients never had cancer to begin with.
Sure, such negative consequences are rare. But so are the benefits. Let me provide some context. The randomized trial showing the largest benefit of breast cancer screening, for example, found a difference of 22 breast cancer deaths (22 fewer among women screened than among those not screened) among a total of 1,000 deaths overall; the trial showing the largest benefit of colon cancer screening found a difference of 40 colon cancer deaths among a total of 6,000 deaths overall. All it would take is a few deaths somehow related to screening and the positive effects would diminish, or even disappear.
Before we set about persuading healthy people to go looking for cancer, I think we owe it to them to be damn sure that cancer detection in fact lowers overall death rates. I'd rather not worry about how somebody somewhere decided why someone died. I don't want to worry that we are trading off one cause of death for another. And I believe a lower chance of death is what most people expect we mean when we say things like "screening works" or "screening can save lives."
But to be certain we are lowering overall death rates means we have to study even more people. As you saw above, to detect a 25% reduction in colon cancer deaths requires studying 120,000 people. But colon cancer represents at most 4% of all deaths. Looking for a 25% reduction in deaths from a disease that is responsible for 4% of all deaths means we'd like to reliably detect a 1% drop in total death rate (25% × 4%). The cost of this power? Astudy involving over a million people.
Many researchers involved in early cancer detection say simply, "It can't be done"; "It's just too big." What they are really saying is, "We can't be sure whether a strategy of early cancer detection truly saves lives." I don't know what the right answer is, but I think we have one of two choices: either do the study or publicly acknowledge that we cannot be sure whether early detection lengthens, shortens, or has no effect on how long people live. And you should be clear that if it takes so many people to find out for sure, then the benefit must be, at best, small.
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