Routine Mammograms Cut Breast Cancer Mortality Modestly

A study reported in the New England Journal of Medicine [2010;363(13):1203-1210] found that routine mammography screening reduced the mortality rate from breast cancer. However, this benefit accounted for only about one-third of the total reduction in breast cancer death rates.

Based on randomized clinical trials, the World Health Organization in 2002 concluded that screening mammography for women aged 50 to 69 years reduced the rate of death from breast cancer by 25%. The use of screening mammography, however, is still debated, primarily due to concern over methodology limitations in clinical trials. The benefit of mammography when implemented in a population-based service program also remains poorly quantified. This study aimed to establish valid comparison groups to quantify the effect of screening mammography on breast cancer mortality in Norway.

Norway’s breast cancer screening program began in 1996 and was expanded 2 years later with staggered enrollment by county over 9 years. Since 2005, all Norwegian women aged 50 to 69 years are eligible for a screening mammography every 2 years.

Data on the Norwegian female population from January 1, 1986, through December 31, 2005, were retrieved from the Cause of Death Registry at Statistics Norway. Data on the women who had received a diagnosis of invasive breast cancer were retrieved from the country’s cancer registry. The incidence-based rates of breast cancer were compared in 4 groups: a screening group of women who from 1996 through 2005 were living in areas with a program; a nonscreening group in counties that did not offer screening; and 2 historical comparison groups from the decade before the screening program began that mirrored the county residence of the current groups.

A total of 40,075 women diagnosed with breast cancer were followed for an average of 2.2 years and a maximum of 8.9 years. During the follow-up period, 4791 (12%) died from breast cancer. Of the women who died, 423 (9%) were diagnosed after the screening program began. Among women aged 50 to 69 years, 6967 were diagnosed with breast cancer between 1986 and 1995, compared with 12,056 diagnosed between 1996 and 2005.

The mortality rate among women aged 50 to 69 years in the screening group was 18.1 per 100,000 person-years, compared with 25.3 per 100,000 person-years among its historical counterparts, for a difference of 7.2 deaths per 100,000 person-years (rate ratio [RR], 0.72; 95% confidence interval [CI], 0.63-0.81; P<.0001)—a 28% relative reduction. In the nonscreening group, the mortality rate was 21.2 per 100,000 person-years, compared with 26.0 per 100,000 person-years among the historical groups, for a difference of 4.8 deaths per 100,000 personyears (RR, 0.82; 95% CI, 0.71-0.93; P<.0001)—an 18% relative reduction. Factoring in the reduction in mortality among women in the nonscreening group with the historical counterparts, the researchers determined that the relative reduction among the screening group was 10% (95% CI, −4 to 24; P=.13). Therefore, the difference in the reduction in mortality between the current and historical groups that can be attributed to the screening group independently was 2.4 deaths per 100,000 person-years (95% CI, −1.7 to 6.5), representing one-third of the total reduction of 7.2 deaths.

The researchers cited 3 study limitations: the maximum follow-up time of 8.9 years may be too short to demonstrate the full potential of the screening program; because the screening program was implemented gradually, diagnoses were more recently in the screening group; and some women in the nonscreening group may have undergone opportunistic mammography screening.

Despite evidence supporting the reduction in mortality from breast cancer, the benefits of mammography appeared modest in Norway’s program. Also, the apparent benefit conveyed by optimized patient care may be overlooked unless breast cancer screening is part of a healthcare system that is accessible to the entire population, the researchers concluded.