New paper on mortality reductions due to mammography screening in Ireland

New analysis using data from the Registry and the Central Statistics Office finds that the rate of breast cancer mortality in women eligible for the Irish breast screening programme (BreastCheck) was approximately 9% lower in areas where screening started earlier compared with those areas where screening was rolled out at a later stage. 

Mammography machine

BreastCheck began in 11 eastern counties in early 2000 after a pilot scheme. It was extended to 12 other counties in late 2007. Treatment for breast cancer has improved considerably in the last few decades, so it is important to measure how much of an impact breast screening alone was having on the numbers dying of breast cancer. The overall aim of the paper was to investigate how many fewer breast cancer deaths there were between 2000 and 2013 in the first 11 counties (region 1) compared with the counties that started more recently (region 2).

The analysis was conducted by Katie O'Brien in collaboration with Jim Hanley from McGill University in Canada and Ailish Hannigan from the University of Limerick.

The authors made comparisons between region 1 and region 2 to emphasise the delay before the reductions in mortality become evident, and to show that the average up to now is going to underestimate what the reductions will be in each region in the long term. The screening effects are not immediate like those of vaccines, so the 9% reduction that was observed is an underestimate of what the long term reduction will be.

The counties in region 2 have been in the screening programme for almost half of the period 2000-2013, and the longer the time period of the study, the difference in the death rates will naturally narrow, and the opportunities to exploit this ‘natural experiment’ diminish.

The estimated 9% reduction in mortality that was computed is an average over the 70% or so of women who take up the invitation to participate in screening and the 30% who do not. For those who take up the invitations, mortality may be reduced by as much as 13%; while those that don’t participate in screening will have no reduction.

Mortality reductions are not the only benefits of earlier detection and treatment. Others include less extensive treatments and less morbidity.

Even if the resources being expended each year on screening ultimately result in mortality reductions of 20% to 25%, these and other benefits must also be balanced against the cost of overdiagnosis. Like the number of deaths averted, the number of cases of over-diagnosis is a statistical estimate obtained by subtraction. Nevertheless, overdiagnoses have real personal consequences, and real resource implications.

The British Medical Journal (BMJ) has written an article about the paper in their research news. The article is available here.

The article is published in PLoS One and available to download here.

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