Mammography Benefits Outweigh Harms: New European Data

Zosia Chustecka

September 12, 2012

September 12, 2012 —The first comprehensive review of European breast screening programs has found that the benefits of mammography clearly outweigh the harms.

This review shows a greater benefit from screening and a lower risk for harm than some other studies, which have raised concerns about the promotion of mammography.

The review, published in a special supplement of the Journal of Medical Screening, was conducted by the European Screening Network (EUROSCREEN) Working Group and the European Network for Indicators on Cancer (EUNICE).

The teams analyzed data from about 12 million women collected from 26 screening programs in 18 countries from 2001 to 2007.

The review authors conclude that European population-based mammography programs are contributing to the reduction in breast cancer mortality in the European Union and are achieving a mortality benefit at least as great as that seen in randomized clinical trials.

"Furthermore, the estimated number of breast cancer deaths avoided by screening clearly exceeds the estimated number of cases of overdiagnosis," they conclude.

The authors estimate that for every 1000 women who undergo screening every 2 years from the age of 50 to 68 or 69 years, 7 to 9 lives would be saved and 4 women would be overdiagnosed.

"Lives saved by screening outweigh overdiagnosed cases by a factor of 2 to 1," noted coauthor Stephen Duffy, MSc, professor of cancer screening at the Wolfson Institute of Preventive Medicine, Queen Mary University of London, United Kingdom.

These figures show a much greater benefit and far less harm than some other estimates. For instance, data from Norway suggested that for every 1000 women screened, 1 life would be saved but 5 to 15 women would be overdiagnosed and treated unnecessarily (N Engl J Med. 2010:363: 1203-1210).

These data have been used by some critics of mammography, including Peter Gøtzsche, PhD, from the Nordic Cochrane Centre in Copenhagen, Denmark, and colleagues, who argue that the benefits of mammography screening have been oversold; they have argued than the chance of a woman being overdiagnosed is about 10 times higher that the chance that her life would be saved (BMJ. 2009;338:b86). Their criticisms led to a very public controversy in the United Kingdom, which resulted in a government-issued leaflet being withdrawn and redesigned.

Greater Benefit in European Review

The estimates in the European review show a greater benefit, Duffy told Medscape Medical News in an interview.

"There are a number of reasons for this," he said. "Our results are based only on the women who underwent screening, not on all those who were invited for screening." This increases the size of the benefit seen, because obviously any benefit comes from actually undergoing the screening, he pointed out. The invitation itself does not confer any protection, and some of the women invited for screening do not turn up — about 70% do in the United Kingdom, but it is less in France and more Sweden and Finland, he said.

Another reason the absolute benefit was large was the long time period involved, he explained. "We measured the total number of deaths that were prevented in a period of 30 years (from the ages of 50 to 79), following screening over a period of 20 years (from ages 50 to 69)."

In addition, the estimates of harm in our review were lower than in some previous studies. We were very careful to account for lead time, and took into account the underlying trends of increasing incidence in breast cancer, which need to be "disentangled" from the effects of screening, Duffy noted.

"If you screen a population for the first time, you will find very early cancer that otherwise would have remained undetected for a few more years, so there is an artificial increase in incidence due to what is known as lead time," he explained. "You bring forward in time the diagnosis."

To counter this lead-time issue, their review included only studies that separated out early diagnosis from overdiagnosis of breast cancer, he said.

Overall, the review found that for every case of breast cancer that would be overdiagnosed, 2 lives would be saved, Duffy said.

Another coauthor of the review, Eugenio Paci, MD, director of the Cancer Prevention and Research Institute in Florence, Italy, said in a statement that "there has been quite a lot of discussion recently over the worth of breast cancer screening, and for this reason it is timely that the international group of experts has assessed the impact of population-based screening in Europe and has found that it is contributing to the reduction in deaths from the disease."

"Most Organized" Screening

Mammography is one of the most established and fully organized screening programs for cancer in the general population, Duffy noted. Although the program for cervical cancer was started earlier, it was not as well organized, he added. Population-based screening programs were established after evidence from randomized clinical trials showed that mammography reduced breast cancer mortality and a meta-analysis of these trials confirmed the finding (BMJ. 1991;302:845-846).

Most European countries now have such programs.

"This is the only comprehensive review of the results of breast screening services in Europe. It reports results from screening millions of women, and confirms that the screening services are delivering the benefits expected from the research studies conducted years ago," Duffy said in a statement.

J Med Screen. 2012;19(Suppl 1):5-13. Abstract