Background: Breast cancer remains the most common cancer and the leading cause of cancer-related mortality among women worldwide. Regular mammography screening reduces breast cancer mortality and organised screening (Or-SC) promotes equity compared with opportunistic screening (Op-SC). In Switzerland, the staggered and incomplete implementation of organised programmes across cantons has resulted in long-standing coexistence with Op-SC, potentially contributing to regional disparities in breast cancer detection and stage at diagnosis.
Objectives: To compare prognostic characteristics of breast cancers in Switzerland by method of detection, with a focus on organised versus opportunistic screening, and to analyse stage-specific trends between 1999 and 2022 while accounting for the timing of programme introduction.
Methods: We included all women aged 40–79 years diagnosed with in situ or invasive breast cancer in Switzerland between 1999 and 2022 (n=98,330), using nationwide data from the National Agency for Cancer Registration. Tumours were classified by stage, grade, and detection method. Screening intensity was assessed independently using data from the 2022 Swiss Health Survey. Missing stage information was imputed using TNM-based rules and an Extreme Gradient Boosting algorithm. Age-standardised incidence rates were analysed using Joinpoint regression on a time scale aligned to programme introduction.
Results: Among women aged 50–69 years, screen-detected breast cancers were more frequently diagnosed at an early stage and with lower histological grade than symptom-detected cancers. These differences were more pronounced for Or-SC than Op-SC and less marked in women aged 40-49 years, who are not covered by Or-SC. Regions with higher screening intensity showed lower proportions of advanced-stage cancers. Following programme introduction, in situ and early-stage incidence rates showed a transient significant increase, followed by a five-year decrease that persisted only for in situ cancers. Advanced-stage incidence rate declined modestly, and this trend appeared largely independent of programme introduction. Patterns were stronger in cantons with long-standing programmes and were not observed in the 40-49 age group.
Conclusions: Screening programmes in Switzerland have increased screening use and favourably altered stage-specific breast cancer incidence over the subsequent two decades. These incidence trends are overall compatible with the expected impact of screening, albeit the effect of programme introduction on the long-term reduction in advanced-stage incidence rate was less noticeable. Organised screening produced a more sustained stage shift than opportunistic screening, and the impact was greatest among women aged 50-69 years. These findings support the long-term benefits of organised screening while highlighting persistent regional inequalities in access and stage at diagnosis, underscoring the need to expand organised programme coverage.