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Breast screening: balancing risk

Jonathan Wood

A new, dispassionate analysis is needed of all the available data on breast cancer screening programmes and their effectiveness, says an Oxford University researcher in an article today in the BMJ medical journal.

Although it saves lives, the benefits and harms of breast cancer screening are perhaps more evenly balanced than many of us might naively imagine.

This has led to increasingly heated debates among specialists in the field who take one side or the other, says Professor Klim McPherson in his BMJ article. These polarised arguments are not helping women to make an informed decision about screening, he argues.

It can be a very difficult thing to assess the benefits of cancer screening programmes – something OxSciBlog has looked at before.

You have to be clear that picking up breast cancer early on through screening leads to improved outcomes (fewer deaths) for those women. You also have to be wary of false positives, or treating cases where perhaps there was no need – the type of cancer involved may not have progressed, for example. This is known as over-diagnosis and over-treatment, and can potentially be harmful.

The BMJ’s press release today also highlights the number of women that need to take part in a screening programme before lives are saved:

'A recent US report on screening for breast cancer estimated that the mortality reductions attributable to breast screening are 15% for women aged 39–49, 14% for those aged 50–59, and 32% for those aged 60–69. Worse still, estimated numbers of women needed to be invited to a US screening programme in order to save one life are high. For the younger group it is nearly 2,000 while in those aged 60–69 it is still nearly 400. In the UK, the figure is around 1,600 for women aged 40–55.'

Klim McPherson, visiting professor of public health epidemiology at Oxford’s Nuffield Department of Obstetrics and Gynaecology, tells OxSciBlog that: ‘There has been a debate about whether the actual benefit of mammography in earlier diagnosis justifies the risk of over-diagnosis and over-treatment it may cause.

‘The evidence on breast screening programmes is that the chance of saving a woman's life is small but real, and the chance of unnecessary anxiety and treatment is also low and real – and not properly discussed. The uncertainty is in the relative size of these two risks for a woman.’

This uncertainty does need addressing, he says, and it may be time to look again at the NHS screening programme thoroughly.

The burden of breast cancer is unremitting and we must do anything we can to contain it, he is clear. But screening for a progressive disease is justified only if earlier diagnosis and treatment improve disease progression enough to justify the harms of screening.

‘Breast cancer screening is no panacea, is expensive and it is irresponsible not to properly assess it, so that women can be better informed,’ says Professor McPherson.

‘The problem is that the debate between cynics and enthusiasts is going nowhere and getting more and more heated. It needs to be properly resolved by, for example, the National Institute for Health and Clinical Excellence (NICE) using all the available data dispassionately.’