News from the US, I can scarcely believe it: The New York Times reports that the American Cancer Society now accepts that screening for breast and prostate cancer is not only inefficient, but frequently inaccurate and alarmist. It has realised that such programmes – designed to detect cancer early – can do damage too, because they often detect cancers or pseudocancers that were never going to maim or kill.

That is the bit I can believe. After all, these are evidence-based observations, and none is particularly new. A recent paper in the Journal of the American Medical Association (Jama) also highlighted the weaknesses of screening. What I have difficulty with is that paper’s conclusion: “To reduce morbidity and mortality from prostate cancer and breast cancer, new approaches for screening, early detection, and prevention for both diseases should be considered.” The problem with screening and even early detection is that because these two elements sound useful, we have great difficulty in believing it when the evidence tells us they are not.

The Jama paper states that, after 25 years of screening, “conclusions are troubling: Overall cancer rates are higher, many more patients are being treated, and the absolute incidence of aggressive or later-stage disease has not been significantly decreased”. The authors also say that screening comes at significant cost, including overdiagnosis and overtreatment. The complications of therapy are likely to get worse as the population ages. Not only that, but treatments for relatively indolent disease may in themselves do harm.

Is screening for these disorders so ineffective, relative to its complications, that we should rethink it entirely? Not only has the rate of diagnosed aggressive breast cancers failed to decline, but ductal carcinoma in situ now accounts for up to 30 per cent of breast cancer diagnoses at screening. Before widespread screening, this potential pre-cancer was rarely diagnosed. Now it is treated as though it were cancer, but the actual progress of this lesion is far from certain and it does not always do harm. Similarly, in screening for prostate cancer, difficulty in determining an accurate cut-off point where normal blood levels of prostate-specific antigen (PSA) became abnormal has made it hard to decide what the test means. Indeed, as the paper’s authors state: “There was no level of PSA below which cancer was not found.” In other words, the test couldn’t even reliably exclude cancer.

Why isn’t there more openness about the problems of screening? Is it because we have created a screening industry, both in and outside the NHS?

Margaret McCartney is a GP in Glasgow
margaret.mccartney@ft.com

For lively discussion of the latest medical issues go to Margaret McCartney’s blog at www.ft.com/healthblog