A Norwegian study, published online in the British Medical Journal, found that although mammography screening may reduce deaths from breast cancer by 28 per cent, for every 10,000 women invited to screening only about 27 deaths from the disease might be avoided during their lifetime.
In an accompanying editorial, two American experts said the research confirmed the “modest” benefits of screening and called for women to be given more balanced information, including the potential for overdiagnosis, psychological stress and the high healthcare costs.
The research comes after a number of other studies also suggested that while helping to reduce deaths, screening for breast cancer could also lead to women having treatment they did not need by identifying tumours which would never have caused them harm.
Information given to women invited for screening in the UK has since been changed to outline the potential for overdiagnosis so they can make an informed choice about attending.
In the latest study, the researchers from the Norwegian University of Science and Technology in Trondheim set out to evaluate the effectiveness of 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, during which time the Norwegian mammography screening programme was gradually being implemented.
The researchers found that 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 and area of residence, 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.
They went on to estimate 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.
The researchers said: “In our study, the estimated benefit for breast cancer mortality (28 per cent) associated with invitation to mammography screening indicates a substantial effect.”
But they said that evolving improvements in treatment “will probably lead to a gradual reduction in the absolute benefit of screening”.
In their editorial, Joann Elmore, from the University of Washington School of Medicine, and Russell Harris, from the University of North Carolina School of Medicine, wrote: “The Norwegian study largely confirms what is already known: the benefits of screening mammography are modest at best.
“While the benefits are small, the harms of screening are real and include overdiagnosis, psychological stress, and exorbitant healthcare costs.”
The experts said women were “rarely presented with balanced information” on screening.
“While the results of complex, imperfect science do not easily translate into memorable slogans, campaigns to promote mammography do often catch women’s attention,” they added.
“Many individuals and groups actively promote mammography screening. Doctors discussing mammography with patients are more likely to mention the potential benefits than harms of screening.
Charities stressed screening did save lives and more balanced information was now available.
James Jopling, director for Scotland at Breakthrough Breast Cancer, said: “Breakthrough Breast Cancer believes that screening saves lives but we understand that this is a confusing area and it is essential that women have access to balanced information about the risks and benefits of screening.”
Baroness Delyth Morgan, chief executive at Breast Cancer Campaign, said: “This study adds to existing evidence that confirms that breast screening saves lives.
“Diagnosing breast cancer quickly is vital, as the earlier breast cancer is diagnosed and treated, the greater the chances of survival.”