Breast cancer mortality after implementation of organized population-based breast cancer screening in Norway.

Abstract

Background We estimated breast cancer (BC) mortality reduction associated with invitations to a nation-wide population based screening program and changes in treatment, in Norway.

Material and methods BreastScreen Norway started in 1996 and became nationwide in 2005. It invites women aged 50–69 to biennial mammographic screening. We retrieved individual-level data for 1,340,333 women from national registries. During 1996–2014 (screening window), women contributed person-years (PY) in non-invited and invited periods. We created comparable periods for 1977–1995 (pre-screening window) by dividing the follow-up time for each woman into a pseudo-non-invited and pseudo-invited periods. We estimated BC mortality for the four periods, using the so-called evaluation model: counting BC deaths in each period for all women diagnosed within the period, counting BC deaths and person-years after screening-age only for women diagnosed within screening-age. We used a multivariable flexible parametric survival model to estimate hazard ratio (HR) for the effect of invitation and improved treatment.

Results There were 5818 BC deaths across 16,533,281 PY. Invitations to screening reduced BC mortality by 20% (HR: 0.80, 95%CI: 0.70-0.91) among women ≥50 years old and by 25% (HR: 0.75, 95%CI: 0.65-0.86) among screening-aged women. The treatment effect was 23% (HR: 0.77, 95%CI: 0.65-0.92) for women ≥50 years old, and 17% (HR: 0.83, 95%CI: 0.74-0.94) for screening-aged women.

Conclusion We observed a similar reduction in BC mortality associated with invitations to screening and improvements in treatment among women ≥50 years old, during 1977–2014.