Comparative analysis of breast cancer mortality following mammography screening in Denmark and Norway.

BACKGROUND: Denmark and Norway are the best countries to study effects of mammography screening, because they are the only countries with stepwise introduction of nationwide mammography screening, enabling comparative effectiveness studies of high quality. Although Denmark and Norway are countries with similar populations and health care systems, reported reductions in breast cancer mortality (incidence-based) caused by screening differed vastly; 25% in Denmark versus 10% in Norway. This study explores reasons for this difference. PATIENTS AND METHODS: We compared two published studies from the Danish and Norwegian screening programs (Olsen et al., 2005; Kalager et al., 2010) investigating biennial mammography screening for women age 50-69 years. Four comparison groups of women were constructed ('current' and 'historical screening groups'; 'current' and 'historical nonscreening groups') based on county of residence. We calculated incidence-based breast cancer mortality in the current versus the historical period for screening and nonscreening groups, using mortality rate ratios (MRR) in the two countries, accounting for concomitant changes in breast cancer mortality. RESULTS: In the screening groups, similar reductions in breast cancer mortality were found when periods preceding and following start of screening were compared, in Denmark [25%; MRR 0.75; 95% confidence interval (CI) 0.64% to 0.88%] and in Norway (28%; MRR 0.72; 95% CI 0.63% to 0.81%). However, mortality increased in Denmark in the current nonscreening group compared with the historical nonscreening group; for women >59 years, breast cancer mortality increased by 14% (MRR 1.14, 95% CI 1.07-1.22), whereas in Norway a 19% reduction was seen (MRR 0.81, 95% CI 0.72-0.92). This increase accounts for the different relative effect of screening in Denmark and Norway; 25% breast cancer mortality reduction in Denmark, 10% in Norway. CONCLUSIONS: The seemingly larger effect of screening in Denmark may not be solely attributable to screening itself, but to increased breast cancer mortality in women older than 59 years not invited to screening.