For some years now there has been quite a raging debate regarding the effectiveness of mammography, and the politics surrounding this screening strategy. I’m polarising things a bit here, but in the two camps on either side of the debate have been:
1. those whose view appears to be that mammography is a no-brainer, and that women who don’t have their breasts examined need their heads examined.
2. those who argue that mammography is not very effective at saving lives, and exposes many women to investigations and treatment for cancers that would never have bothered them during the natural course of their lives (termed ‘over-diagnosis’).
I should declare that I am in the latter camp. My real objection to mammography is not so much the procedure itself – it’s the fact that, for the most part, women are generally not given the facts and therefore are disabled in terms of their ability to make an informed choice about whether they have mammography or not.
The controversy around mammography has been swirling long enough and hard enough for an ‘independent review’ to be conducted. Professor Sir Mike Richards (National Cancer Director in England) and Dr Harpal Kumar (CEO of Cancer Research UK) asked Professor Sir Michael Marmot to assemble and chair a panel to review the benefits and harms mammography. The panel took evidence from trials as well as expert opinion. The results are in and were published this week in the medical journal the Lancet [1].
The panel accepted the weaknesses of the evidence and the fact that the studies were done a long time ago, but concluded that (in its opinion) mammography reduces risk of death from breast cancer is reduced by about 20 per cent over time.
This 20 per cent figure represents what is known as ‘relative risk reduction’. But the benefits in real terms (known as ‘absolute risk reduction’) are dependent on how much risk there is in the first place. Relative risk reductions most meaningful when overall risk is high. Fortunately, the review panel crunched the numbers in a way which allows all of us to get a better idea of how effective (or ineffective) mammography is in reality.
According to their calculations looking at a range of evidence, if 10,000 women aged 50 were invited to screening for the next 20 years, 43 would be prevented from dying from breast cancer. 10,000 divided by 43 = 233. So, 233 women would need to be invited for screening for 20 years to prevent one breast cancer death. Not all women attend when invited, and it turns out that 180 women would need to be screened for 20 years to prevent one breast cancer death. Screening in the UK is scheduled every 3 years, so lets multiply 180 by 6 (the number of screenings a woman will have in 20 years) and we have a total of 1,080. So, according to these stats, more than 1,000 mammograms have to be performed for one life to be saved from breast cancer. I suspect most women (or men) if asked would never imagine that mammography is, overall, a very inefficient way to save lives.
It is perhaps worth bearing in mind that some evidence paints an even gloomier picture of the effectiveness of mammography. One study from the respected Cochrane group of researchers published last year estimated (based on what the researchers believed to be the best evidence) that the number of women needed to be invited to screening to prevent one breast cancer death was in the order of 2,000 [2]. Other reviews have found numbers needed to be screened to prevent one death from breast cancer of 720 and 400).
But going with the recent expert panel’s more optimistic data, what this all boils down to is that even after 20 years of screening, only about half a per cent of women benefit from mammography.
Against this, of course, we need to put the risk over-diagnosis. The panel puts this at 129 women per 10,000 invited to screening. That is precisely 3 times the number who, they calculated, have their lives saved.
There are other hazards to mammography, in particular the emotional stress (and discomfort) associated with having the procedure and further investigations. The impact of these things is difficult to quantify, but I suspect tips the balance even further in favour of those in camp 2 (see above).
But one good thing that has come out of the debate and this review is this: it appears there is now going to be most honesty and transparency about the relative ineffectiveness of mammography as well as its very real hazards. That way, women will be able to make much more informed choices about whether to subject themselves to this procedure. Now that more facts will be made available to women, something tells me that growing numbers of women are going to take the view that mammography is not a no-brainer after all.
References:
1. Independent UK Panel on Breast Cancer Screening. The benefits and harms of breast cancer screening: an independent review. Lancet 30 October 2012
2. Gotzsche PC, et al. Screening for breast cancer with mammography. Cochrane Syst Rev 2011;1:CD001877
