Researchers from a respected Cochrane centre have controversially concluded that screening with mammography for breast cancer is a waste of time. After reanalysing data from previously published trials, Dr Peter Gçtzsche and Mr Ole Olsen of the Nordic Cochrane Centre in Copenhagen, stated that “there is no reliable evidence that screening decreases breast cancer mortality” (Lancet 2000;355:129-34).
The investigators re-examined the evidence on which national breast cancer screening programmes are based—eight randomised trials comprising half a million women. They said that in six of the trials (in Edinburgh, New York, and areas of Sweden) the randomisation procedures were inadequate and the groups of women compared were not strictly comparable because the groups differed substantially in age or other risk factors.
Another problem was that the exact numbers of women in each group were uncertain in four of the trials. According to Dr Gçtzsche and Mr Olsen, only two trials (those carried out in Canada and Malmï, Sweden) were adequately randomised, with the women in the two groups properly matched.
They found that the evidence from the Canadian and Malmï trials showed that screening had no significant effect, whereas the other six trials, in which the randomisation was inadequate, found that screening decreased the risk of death by about 25%However, for death from any cause, these six trials showed a slight increase in risk for screened women.
Dr Gçtzsche and Mr Olsen said that anyone believing that the Swedish trials (other than the Malmï trial) are unbiased must also “accept from the data that screening for breast cancer with mammography causes more deaths than it saves.”
The controversial conclusions have been criticised, however, by several leading cancer researchers. Dr Muir Gray, director of the National Screening Committee in Britain, said that, although the paper is important and will be examined closely by the committee, he could see no reason to change the NHS breast cancer screening policy.
Dr Gray said: “I don't think the reasons for excluding the six studies are particularly strong. For example, one study was excluded because the two groups were not matched in age. How-ever, the difference was just a month between the study and the control group, which I would not regard as statistically significant.”
Dr Gray added that there were problems with the two studies that Dr Gçtzsche and Mr Olsen said were methodologically sound. “It is not as clear cut as they have made out; all studies have some flaws.”
Dr Gray said he suspected that the paper had been rushed and not adequately peer reviewed, as the researchers stated that the meta-analyses were carried out only on 20 December 1999, yet the paper was published on 8 January 2000.
Professor Jack Cuzick, from the Imperial Cancer Research Fund, said that the conclusions reached by the Danish researchers are not supported by all of the available evidence. “Screening detects cancers when they are smaller and more easily treated; cancer rates are reduced following screening; and trials have shown that mortality is reduced by 20-30%,” he said.
Mortality from breast cancer is falling in the United Kingdom—latest figures show a drop of 14%between 1989 and 1998.