BMJ editorial questions the effectiveness of mammography

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Perhaps more than any other disease, breast cancer has received a helluva lot of social and political attention over the last decade. And a big part of the trust around raising awareness of the disease is the ‘need’ for women to be screened for breast cancer. The most commonly used technique for this is mammography ” essentially and x-ray of the breast. The idea here is that earlier detection though mammography and more timely treatment will reduce the likelihood that a woman will succumb to breast cancer.

While this makes sense, we now seem to have a enough data to make a decent assessment at just how effective mammography is in terms of saving lives. A round-up of the available science was presently earlier this month in a British Medical Journal editorial. I think the data makes for some interesting reading.

The data presented shows that in women aged 50-69, about 9 in every thousand will be expected to die from breast cancer over a 10-year period if the population is NOT screened with mammography. However, if screening is applied to that population, the number of women expected to die falls to 6 per 1000. Thus, mammography appears reduce risk of death of breast cancer over a 10-year period by a third.

This looks good, but it’s perhaps important to bear in mind that, in real terms, this means screening causes risk of dying from breast cancer over a 10-year period from about 0.9 % to 0.6 % – a reduction in risk of just 0.3 %.

The figures for younger women are even less impressive. Mammography appears to cause deaths in women aged 40-49 to drop (again, over a 10 year period) from 3.3 to 2.5 per thousand. This translates into a real risk reduction of 0.08 per cent. Put another way, for each 1000 women aged 40-49 who undergo mammography, less than 1 life will be saved as a result of this procedure.

In addition, one thing that is sometimes forgotten about mammography is that it is far from foolproof. There is the risk, of what are known as ‘false positive’ results. Essentially, this means results that are suspicious of cancer that turn out not to be cancer but nonetheless require a woman to go through further testing and investigation. The editorial in the BMJ reckons that for every 1000 women screened, 100 – 500 (10 ” 50 %) will end up with a false positive result.

And there’s one other thing. Even in women diagnosed with real breast cancer, the treatment the end up having may in fact be ‘unnecessary’. This may seem counterintuitive on first reading, but hear me out. Imagine a woman develops breast cancer but dies of something else before that cancer is diagnosed. Now imagine this women is screened, has her cancer diagnosed, and then is treated. In reality, this treatment has not helped her because she was not destined to die from breast cancer anyway. However, she may still have had potentially disfiguring surgery, as well as radiotherapy and/or chemotherapy.

The problem, of course, is that when breast cancer is diagnosed, doctors are fresh out of crystal balls and cannot tell for whom treatment will be relevant. This means that more-or-less everyone gets treated. And that means, whether we like it or not, that some women will be treated unnecessarily.

The BMJ editorial also presented data on women who appear to have had an unnecessary diagnosis and treatment for breast cancer.� In women aged 40-49 it is 2-5 women per 1000 (you might want to compared that to the less than 1 life that is saved through mammography). In women aged 50-69, it rises to 3-9 per 1000.

My experience is that mammography is generally touted as a ‘must’ for women of a certain age, but its potential downsides (including the risk of unnecessary diagnosis and treatment) rarely get an airing. The authors of the BMJ editorial make this point, and go on to suggest that women should not be told what to do with regard to mammography, but be encouraged to make a decision that is right for them.� What this means, inevitably, is giving women the best data we have on both the benefits and pitfalls of mammography. That way, they can make a truly informed choice about whether to undergo this test.


Schwartz LM, et al. Participation in mammography screening. BMJ. 2007 13;335(7623):731-2.

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