Breast cancer screening with mammography: overview of Swedish randomised trials

Abstract

Despite encouraging results from screening trials the efficacy of mammography in reducing mortality remains somewhat controversial. Five studies have been done in Sweden. This overview, based on 282 777 women followed for 5-13 years in randomised trials in Malmö, Kopparberg, Östergötland, Stockholm, and Gothenburg, reveals a 24% (95% confidence interval 13-34%) significant reduction of breast cancer mortality among those invited to mammography screening compared with those not invited. To avoid the potential risk of differential misclassification causes of death were assessed by an independent end-point committee after a blinded review of all fatal breast cancer cases. The mortality reduction was similar, irrespective of the end-point used for evaluation ("breast cancer as underlying cause of death" or "breast cancer present at death"). There was a consistent risk reduction associated with screening in all studies, although the point estimate of the relative risk for all ages varied non-significantly between 0·68 and 0·84. The cumulative breast cancer mortality by time since randomisation was estimated at 1·3 per 1000 within 6 years in the invited group compared with 1·6 in the control group. The corresponding figures after 9 years are 2·6 and 3·3 and after 12 years 3·9 and 5·1. The largest reduction of breast cancer mortality (29%) was observed among women aged 50-69 at randomisation. Among women 40-49 there was a non-significant 13% reduction. In this younger age group cumulative breast cancer mortality was similar in the invited and control group during the first 8 years of follow-up. After 8 years there was a difference in favour of the invited women. There was no evidence of any detrimental effect of screening in terms of breast cancer mortality in any age group. Among women aged 70-74 years screening seems to have had only a marginal impact.