The 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 represented women with breast cancer in counties where screening wasn't available. The remainder study population consisted of two historical-comparison groups from 1986 through 1995 that mirrored the current groups.
The rate of death was reduced by 7.2 deaths per 100,000 person-years in the screening group (when compared with the historical screening group) and by 4.8 deaths per 100,000 person years in the nonscreening group. The difference in the reduction in mortality between the current and historical groups 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 a third of the mortality reduction in the 20-year period was associated with the screening program, the researchers said. The remaining two-thirds can be linked to enhanced breast cancer awareness, [and] improved diagnostic and treatment for breast cancer," Kalager said.