THOUSANDS of lives are saved by breast cancer screening programmes, leading epidemiologists told New Scientist this week. Their views flatly contradict claims by Danish researchers that were widely publicised last week.
Peter G酶tzsche and Ole Olsen of the Nordic Cochrane Centre in Copenhagen examined eight surveys of the effectiveness of screening programmes, which followed half a million women in Canada, Scotland, Sweden and the US. The surveys compared the death rates from breast cancer for women who had regular X-rays with those who did not. G酶tzsche and Olsen discarded six of the studies as being unsound. From the two that remained鈥攐ne in Canada and one in the Swedish city of Malm枚鈥攖hey concluded that screening had no effect (The Lancet, vol 355, p 129).
In the six studies they dismissed, G酶tzsche and Olsen found the ages of the screened populations were slightly different from the ages of the controls. They say this shows women were not divided randomly. 鈥淚f there is a significant difference in age in several studies, something has gone extremely wrong with randomisation,鈥� says Olsen.
Advertisement
But Stephen Duffy of the University of Cambridge, who helped analyse data from the Swedish surveys, says small age differences are common. The statistics were corrected to take into account the fact that older women have higher rates of breast cancer. 鈥淚t is standard to correct results for such age imbalances,鈥� he says.
Duffy rejects any implication that small age differences introduce a bias. In one Swedish trial, he says, the women who were screened were slightly older than the control group, so they were likely to have a higher death rate. Yet their death rate was 29 per cent lower than the control group.
The two studies that the Danes did not discard have their own problems, say experts. The Canadian study suffered from poor-quality X-rays in the 1980s. And although the Malm枚 study showed no benefit when first published in 1988, after another seven years of the trial there were 20 per cent fewer breast cancer deaths among women who had been screened. The Danes do not mention this, although the results were published in 1995.
Countries that have screening programmes have lower death rates. Harry de Koning of Erasmus University in Rotterdam wrote in The Lancet last week that screening has clearly cut deaths in the Netherlands, Finland and Britain.
Nils Bjurstam of the University of Gothenburg in Sweden, who designed one of the Swedish trials, says that death rates from breast cancer in Sweden, which screens women, are half those in Denmark, which does not screen women鈥攅ven though the incidence of breast cancer is similar in the two countries. 鈥淒enmark has the highest breast cancer mortality rate in Europe, and probably the world,鈥� says Bjurstam. Some Danish medical organisations oppose screening, arguing that the money is better spent on curing cancer.
Richard Peto of the University of Oxford suspects that screening helps to cut breast cancer deaths. But he says that to assess the effect of screening 鈥渆vidence from all trials should be included鈥�.