Copenhagen study confirms mammography screening benefit

Critics of population-based mammography screening regularly pick holes in studies claiming to show its value. Now researchers from Denmark have produced a watertight case, showing that mass screening really can cut breast cancer deaths.

Their assessment of population-based screening includes several controls for differences that might otherwise taint its conclusion. The evaluation of the first 10 years of mammography screening in Copenhagen, published in the British Medical Journal, excludes patients with breast cancer known prior to their first screening invitation and compares mortality rates against Danish counties with no such screening program.

The researchers also used two historical control groups to assess breast cancer mortality trends in the 10 years before population-based screening began in Copenhagen (BMJ 2005;330:220).

The analysis focuses on a target group of women, aged 50 to 69, who were invited for mammography screening between April 1, 1991, and March 31, 2001. Participants received repeat invitations every two years. The team concluded that introduction of the screening program reduced breast cancer mortality by 25% in this group. This figure lines up with previous estimates drawn from data on breast screening in Sweden.

"In women who have actually attended for screening, this reduction is even higher - 37%," said coauthor Dr. Ilse Vejborg, a radiologist at University Hospital Copenhagen.

The study succeeds in avoiding statistical pitfalls that earlier works were criticized for, said Dr. Stephen Duffy, an epidemiologist with Cancer Research U.K., who has studied the effects of mammography screening on mortality. But he acknowledged that more such investigations are needed to clarify the impact of population-based screening programs.

"A major task for public health research now is to conduct high-quality observational studies of the service screening programs that have been springing up worldwide in recent years. This study is a good example, incorporating both temporal and geographical control. It shows conclusively that the Copenhagen program is saving lives from breast cancer," he said.

The new data from Copenhagen shows the drop in deaths is primarily due to the introduction of population-based mammography and not to therapy differences, said Dr. Daniel Kopans, director of breast imaging at Massachusetts General Hospital.

"You introduce mammography screening in the general population, and the death rate goes down," he said. "This is what the Swedish studies, the Netherlands study, and now the Copenhagen study show. It has also happened in the U.S."

For more information from the Diagnostic Imaging archives:

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Parallel processing gives boost to mammography

Is 'mortality paradox' real or chance finding?

Confounding factors make mammography less accurate