British experts have rejected the findings of a study published yesterday claiming that breast screening has had minimal impact on death rates.
Researchers said their careful study in Denmark disproved claims that mammography screening had cut deaths by a quarter.
The findings were published on-line by the British Medical Journal and claim that screening has cut death rates by just one per cent a year.
But Professor Stephen Duffy, of Queen Mary University of London, UK, condemned the research as "less credible" than the original.
Professor Duffy, Cancer Research UK professor of screening, said: "Many independent studies have shown that breast screening saves lives. It can pick up cancers at an early stage when treatment is more likely to be successful.
"This study’s methods don’t produce an accurate representation of the benefits and harms of breast screening. For example, it doesn’t directly compare deaths rates before and after the screening programme was introduced."
Dr Karsten Jorgensen of the University of Copenhagen in Denmark, and colleagues looked into the 25 per cent reduction in breast cancer mortality in Copenhagen after the introduction of screening.
They investigated whether this was due to the screening, by comparing mortality in areas with and without screening over 20 years. Figures came from national death records.
They found a one per cent annual drop in death rates in screening areas, but a two per cent annual drop in non-screening areas among women of the same age (55 to 74 years).
The authors conclude: "We were unable to find an effect of the Danish screening programme on breast cancer mortality.
"The reductions in breast cancer mortality we observed in screening regions were similar or less than those in non-screened areas, and are more likely explained by changes in risk factors and improved treatment than by screening mammography."
Similar results have been found in the UK, they claim. "Our results are similar to what has been observed in other countries with nationally organised programmes. We believe it is time to question whether screening has delivered the promised effect on breast cancer mortality."
Larger effects were seen in screening trials many years ago, but the experts suggest that treatment has progressed, and publicly-available screening programmes "could yield a different effect from that in the trials because of differences in the qualifications of the staff, type of equipment, and uptake rates".
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