London: Modern mammography screening may reduce deaths from breast cancer by about 28 per cent, a new long-term study suggests.
The study published in the British journal, BMJ said this means that for every 10,000 women invited to screening, about 27 deaths from breast cancer might be avoided during their lifetime.
An accompanying editorial said the study largely confirms what is already known - that the benefits of breast screening "are modest at best" - and calls for women to be given balanced information including the screening harms of overdiagnosis, psychological stress, and high healthcare costs.
Randomised trials from the 1970s and 80s suggested that mammography screening prevents deaths from breast cancer. But the methods used by some of these studies have been criticised, and this has raised doubts about the validity of the findings.
Researchers in Norway set out to evaluate the effectiveness of modern mammography screening by comparing the effects on breast cancer mortality among screened and unscreened women.
They analysed data from all women in Norway aged 50 to 79 between 1986 and 2009 - the period during which the Norwegian mammography screening programme was gradually implemented.
They compared deaths from breast cancer among women who were invited to screening with those who were not invited, making a clear distinction between cases of breast cancer diagnosed before (without potential for screening effect) and after (with potential for screening effect) the first invitation for screening.
Based on more than 15 million person years of observation, breast cancer deaths occurred in 1,175 of the women invited to screening and in 8,996 of the women who were not invited.
After adjusting for factors such as age, area of residence, and underlying trends in breast cancer mortality, the researchers estimated that invitation to mammography screening was associated with a 28 per cent reduced risk of death from breast cancer compared with not being invited to screening.
The screening effect persisted, but gradually declined with time after invitations to screening ended at 70 years of age.
Using the simulation model, they also estimated that 368 women aged 50-69 would need to be invited to screening every two years to prevent one death from breast cancer during their lifetime.
"In our study, the estimated benefit for breast cancer mortality (28 per cent) associated with invitation to mammography screening indicates a substantial effect," the study authors said.
But evolving improvements in treatment "will probably lead to a gradual reduction in the absolute benefit of screening," they said.