The investigators explain that measuring the effect of breast cancer screening on mortality is not easy - the main problem being having access to valid comparison groups. Using historical control subjects does not factor in awareness and treatment changes that occur over time.
This study reveals that even though mammograms do reduce mortality (risk of dying), the benefit is surprisingly unexceptional. Women who underwent screening were found to have a 10% lower risk of dying from breast cancer; however, only one third of that 10% was because of the screening. To save one life, 2,500 women would have to undergo mammography screening regularly for over a decade, an accompanying editorial in the NEJM points out.
A breast-cancer screening program started in Norway in 1996 and spread throughout the country during the subsequent nine years. Females aged from 50 to 69 years were offered mammogram screening every two years.
The researchers compared mortality rates in four groups of women:
- Women living in counties with screening - the screening group (1996 to the end of 2005)
- Women living in counties without screening - the non-screening group (1996 to the end of 2005)
- Two groups of women from the same areas as the two groups above, ten years before the screening program started
Team leader, Dr. Mette Kalager of Oslo University Hospital and team, with collaboration from Harvard University and the Dana-Farber Cancer Institute examined the medical documents of 40,075 women with breast cancer.
Breast cancer mortality dropped by 7.2 per 100,000 for the women in the screening group, compared to the women in the same area ten years before the screening program was launched.
Mortality in the non-screening group dropped by 4.8 per 100,000 when compared to women in the same area ten years before the launching of the screening program.
The real impact of screening was consequently:
7.2 minus 4.8 = 2.4 fewer deaths per 100,000 women.
Norway's screening program did not include women over 70 years of age. When studying breast cancer mortality among this age group, the researchers found that mortality had dropped 8% compared to the previous decade - as this improvement had not been due to screening, it was most likely the result of better therapy, the authors believe.
The authors wrote in the NEJM, as a conclusion:
The availability of screening mammography was associated with a reduction in the rate of death from breast cancer, but the screening itself accounted for only about a third of the total reduction.
Breast cancer is the most common cancer for women. About one in every nine women will develop breast cancer in her lifetime. 99% of all breast cancers are diagnosed in women, 1% affect men. However, men with breast cancer tend to have poorer outcomes than women.
Globally, breast cancer represents 10.4% of all female cancer incidence, making it the most common type of non-skin cancer in females, as well as the fifth most common cause of cancer death.
"Effect of Screening Mammography on Breast-Cancer Mortality in Norway"
Mette Kalager, M.D., Marvin Zelen, Ph.D., Frøydis Langmark, M.D., and Hans-Olov Adami, M.D., Ph.D.
N Engl J Med 2010; 363:1203-1210September 23, 2010