Using mammography to screen women for breast cancer may be less effective in reducing death rates than previously estimated

For immediate release: Wednesday, September 22, 2010

Boston, MA — A new study led by Harvard School of Public Health (HSPH) researchers has found that a breast cancer screening program in Norway, which made mammographic screening available to women between the ages of 50 and 69, resulted in a 10% decrease in breast cancer deaths in that age group.

“The observed reduction in death from breast cancer after introduction of the mammography screening program was far less than we expected,” said lead author Mette Kalager, a visiting scientist at HSPH and a surgeon at Oslo University Hospital in Norway. “The results showed that other factors, such as enhanced breast cancer awareness and improved treatment, actually had a greater effect on reducing mortality from breast cancer.”

The study appears in the September 23, 2010 issue of The New England Journal of Medicine.

Each county in Norway was required to establish multidisciplinary breast cancer management teams and breast units before enrollment in the national screening program. Among women older than 70 years, who were all treated by multidisciplinary teams and not invited to undergo mammography screening, the study found an 8% reduction in death from breast cancer. “This can be explained by treatment of multidisciplinary teams of highly specialized radiologists, radiologic technologists, pathologists, surgeons, oncologists and nurses that managed the care of the patients,” say the authors. “Thus, the 10% reduction we found among women aged 50 to 69 years old being invited to mammography screening can be attributed to both the mammograms and management by multidisciplinary teams.”

The authors analyzed data from 40,075 women diagnosed with breast cancer who participated in the Norwegian breast cancer screening program, which began in 1996. The researchers compared the rates of death from breast cancer among four groups of women. One group of women (ages 50-69) lived from 1996 through 2005 in Norwegian counties where mammographic screening was provided every two years; a second group from the same time period represented women with breast cancer in counties where screening wasn’t available. The remaining study population consisted of two historical-comparison groups from 1986 through 1995 that mirrored the current groups.

The results showed that the rate of death was reduced by 7.2 deaths per 100,000 person-years in the screening group as compared with the historical screening group and by 4.8 deaths per 100,000 person-years in the nonscreening group as compared with the historical nonscreening group, for a relative reduction in mortality of 10% in the screening group. Thus, the difference in the reduction in mortality between the current and historical groups that could be attributed to screening alone was 2.4 deaths per 100,000 person-years, or about a third of the total reduction of 7.2 deaths.

“Only one third of the mortality reduction we observed in the 20-year period was associated with the screening program. The other two thirds can be attributed to enhanced breast cancer awareness, improved diagnostic and treatment for breast cancer,” said Kalager.

Kalager and her colleagues say the most accurate diagnosis and best treatment can be achieved by specialized doctors working together in multidisciplinary teams.

Other HSPH authors include senior author Hans-Olov Adami, Professor of Epidemiology, and Marvin Zelen, Lemuel Shattuck Research Professor of Statistical Science.

Support for the study was provided by the Cancer Registry of Norway and the Research Council of Norway.

“Effect of Screening Mammography on Breast-Cancer Mortality in Norway,” Mette Kalager, Marvin Zelen, Frøydis Langmark, Hans-Olov Adami, The New England Journal of Medicine, September 23, 2010

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Todd Datz

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