Breast cancer incidence, by stage at diagnosis, and mortality in 21 European countries in the era of mammography screening: an international population-based study.

Background: Mammography screening programmes have been widely implemented across European countries over the past 40 years with the main aim to detect breast cancer earlier and thereby reduce breast cancer mortality. This study aimed to analyse and compare changes over time in breast cancer incidence, by stage at diagnosis, and breast cancer mortality across countries in relation to the timing of screening implementation and age at diagnosis. Methods: In this population-based study conducted in 21 European countries, data from cancer registries covering over 3 million female patients diagnosed with breast cancer, along with data from national statistical offices from 1978 to 2019 were analysed. Annual age-standardised breast cancer incidence rates (by stage and age at diagnosis) and age-standardised breast cancer mortality rates were calculated. Average annual percent changes (AAPCs) in these metrics within 10 years before and 10 years after screening implementation were estimated. Findings: Overall, breast cancer incidence rates increased over the study period, with the largest increases observed in the first two decades (1978-1987 and 1988-1997), and AAPCs of up to 4.55 (95% confidence interval, CI, 2.56-6.57). In contrast, breast cancer mortality rates decreased most predominantly in the last two decades (1998-2007 and 2008-2018/19), with AAPCs down to -5.40 (95% CI, -9.70 to -0.89). The largest increases in incidence were seen for in situ and stage I cancers (AAPCs ranging from non-significant to 12.03 (95% CI, 7.40-16.86) following screening implementation). Incidence of stage IV cancer declined or remained stable in most countries, with AAPCs down to -6.16 (95% CI, -8.28 to -4.00) after screening introduction. These trends were particularly pronounced among age groups targeted by screening (mostly 50-69 years). Breast cancer mortality rates declined by up to 3 percent annually after screening initiation (lowest AAPC estimate -3.29 (95% CI, -6.26 to -0.23); yet, AAPCs as low as -2.54 (95% CI, -3.15 to -1.93) were also observed before introduction of screening programmes in countries where implementation occurred later, in the 2000s. Interpretation: This study suggests that mammography screening has influenced trends in breast cancer incidence and mortality in European countries. The results point to the contribution of mammography screening, alongside advances in diagnostics and treatment, to the observed reductions in breast cancer mortality. Funding: There was no funding source for this study.