SCREENING for breast cancer detects many cancers that do not exist or pose no hazard, a study indicates.

This means that many women may be needlessly treated for cancers that will not threaten their lives. Although screening saves many lives — 1,400 a year in Britain, according to the latest estimates — women who have a wrong or needless diagnosis pay the price in worry and unnecessary treament.

A Swedish team has determined that this “overdiagnosis” amounts to about 10 per cent of women screened. Thus, in every ten women who test positive, one gains no benefit because she does not have cancer, the cancer she does have will not spread, or she will die of another cause before it can affect her. The team has been able to arrive at the figure by following up a group of women who took part in a trial of cancer screening in Malmö that began in 1976. The study included more than 42,000 women.

By following up those who were screened and those who acted as controls, the team was able to show that about 10 per cent more cancers were diagnosed in the screened group than in the control group, which was not screened.

The two groups were chosen to have identical risk of the disease, so this suggests that screening leads to overdiagnosis. In plain terms, for every woman saved by screening, two women had a needless diagnosis of the disease, two British specialists have written in a commentary on the study, which appears alongside it in the British Medical Journal.

These two overdiagnosed women pay the price for saving the life of the third woman by becoming breast cancer patients and undergoing treatment they do not need, according to Henrik Møller and Elizabeth Davies, of King’s College London. They say that it is impossible to predict who these three women — one saved, two treated to no benefit — will be. It appears impossible to avoid some degree of overdiagnosis, but until now it has been difficult to quantify.

This risk is not clearly explained to women who are called in for screening, according a second paper in the BMJ. The screening sceptics Karsten Juhl Jørgensen and Peter Gøtzsche, of the Nordic Cochrane Centre in Denmark, write: “Very few women are aware that screening can detect non-progressive cancer, and probably even fewer know that invasive cancer can sometimes regress spontaneously. Many will falsely believe that their lives have been saved by screening when, in fact, they have only been physically and psychologically harmed.”

But Michael Dixon, a consultant breast surgeon, writes in a BMJ editorial that it is time to accept that, for all its limitations, breast cancer screening does save lives.

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