Copenhagen study confirms mammo screening benefit

The true value of widespread mammography screening is a contentious issue, with claims for its success difficult to disentangle from coexisting factors. Now, research from Denmark has produced the strongest evidence to date that screening produces significant re-ductions in breast cancer mortality.

Arguments in favor of population-based breast screening can appear cut and dried. Randomized controlled trials have shown that mammography screening produces a statistically significant benefit, and studies of the impact of population-based programs have confirmed a drop in breast cancer deaths in the target population.

Critics of mass screening, however, have always found fault with the statistical methodology. Evaluations of screening in the U.K., the Netherlands, and Sweden, for example, have failed to separate out breast cancers diagnosed before screening began. The effects of advances in therapy further muddy the waters.

The latest assessment of population-based screening controlled for differences that might otherwise taint its conclusion. An evaluation of the first 10 years of mammography service screening in Copenhagen excluded patients with known breast cancer prior to their first screening and compared mortality rates against counties with no such program. The researchers 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 researchers' analysis focused on a target group of women, aged 50 to 69, who were invited for mammography screening between 1991 and 2001. Those participating in the program received repeat invitations every two years. The team concluded that the program reduced breast cancer mortality by 25% in this group. This figure correlates with previous estimates based on data from Sweden.

The Danish study succeeds in avoiding statistical pitfalls, said Dr. Stephen Duffy, an epidemiologist with Cancer Research U.K.

"A major task for public health research now is to conduct high-quality observational studies of service screening programs. This study shows conclusively that the Copenhagen program is saving lives," Duffy said.

The new data from Copenhagen show that the decrease 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.

"Introduce mammography screening in the general population and the death rate goes down," he said. "This is what the Swedish, Netherlands, and Copenhagen studies show."

The study's positive conclusions should encourage more Danish women to take advantage of service screening where offered, said Dr. Ilse Vejborg, a radiologist at University Hospital Copenhagen and coauthor of the study. The breast screening acceptance rate in the target population in Copenhagen is just 62%.

"There is a heated debate in Denmark about mammography screening. Many women may not have attended for screening because they were confused," Vejborg said.

The hard statistical evidence may also help speed plans to expand breast screening availability in Denmark, she said. The Danish parliament voted in 1999 that regional councils should offer mammography screening to women aged 50 to 69. To date, only three of the country's 16 administrative regions have established a service.

"Perhaps this paper will make it happen sooner," Vejborg said.