Followup after 11 years – update of mortality results in the Stockholm mammographic screening trial

Abstract

Results from several randomised mammography screening trials haveshown that it is possible to reduce mortalityin breast cancer by mammographic screening at leastfor women above 50 years of age. Thepurpose of this article is to present dataon mortality in breast cancer in study andcontrol groups of the Stockholm trial after 11years of followup, to analyse which age groupbenefits most from screening. In March 1981, 40,318women in Stockholm, aged 40 through 64 years,entered a randomized trial of breast cancer screeningby single view mammography alone, versus no interventionin a control group of 20 000 women.Two screening rounds were performed and the attendancerate was over 80% in the two rounds.During 1986 the control group was invited onceto screening. Totally 428 and 217 cases ofbreast cancer were diagnosed in the study andcontrol groups respectively. After a mean follow-up of11.4 years a nonsignificant mortality reduction of 26%was observed for the whole study group, witha relative risk (RR) of death in breastcancer of 0.74 (CI(confidence interval)=0.5–1.1). Forwomen aged 50–64 years a significant 38% mortalityreduction was observed with a RR of 0.62(CI=0.38–1.0). For women aged 40–49 yearsno effect on mortality was found, with aRR of death in breast cancer of 1.08(CI=0.54–2.17). The breakpoint for benefit inthis study seemed to be at 50 yearsof age when 5-year age groups were analysed,but this tendency is uncertain because of thelow statistical power in the analysis of theyounger age groups. Long screening intervals, the useof single-view mammography, and the fact that morethan 50% of the women in age group40–49 years were still below 50 years ofage when the study was closed, were allfacts that could have influenced the results inage group 40–49 years. Larger studies are neededto answer the question whether mammographic screening canbe successful in younger age groups.