Organized mammographic screening has been shown to reduce breast cancer mortality across numerous high-quality studies. However, advancements in treatment, changes in screening techniques and adherence might alter the mortality reduction. In this study, we used data from an organized breast cancer screening program that started 30 years ago to investigate breast cancer mortality among women who were invited and attended the program (screened) versus women who were invited, but who did not attend (unscreened).

This retrospective cohort study included information on invitation to and attendance in BreastScreen Norway, breast cancer diagnosis, emigration, death, and cause of death for women residing in Norway between January 1, 1996, and December 31, 2023. A total of 1,192,148 women aged 50–69 years without diagnosis of breast cancer at first invitation to screening and inclusion in the study were followed until breast cancer death, death of other causes, emigration or end of follow-up. We applied a Poisson regression model to compare incidence-based breast cancer mortality rate ratios (MRR) with 95% confidence intervals (CI) between screened and unscreened women invited to attend the program, adjusting for age, calendar year, time since cohort entry, and self-selection bias using correction coefficients based on Norwegian data, and a coefficient proposed by Duffy in 2002.

The screened cohort comprised 14,223,189 women-years, while the unscreened cohort accounted for 2,474,340 women-years. The crude breast cancer mortality rate was 28.2 per 100,000 women-years in the screened cohort, compared to 51.2 per 100,000 women-years in the unscreened cohort. The resulting unadjusted mortality rate ratio (MRR) was 0.55 (95% CI: 0.52–0.59). After adjusting for self-selection bias, the MRR remained strongly in favor of attending screening, and varied from 0.49 (95%CI 0.46–0.53) to 0.71 (95% CI: 0.66–0.76) when using self-selection bias correction coefficients derived from Norwegian data and was 0.61 (95% CI: 0.56–0.65) when using the coefficient provided by Duffy.

After 27 years of follow-up, breast cancer mortality was 30–50% lower for invited and screened women compared to invited but unscreened women in BreastScreen Norway.