Real-world impact of organised mammography screening on breast cancer mortality: A population-based case–control study

ObjectiveTo investigate the real-world impact of organised mammography screening on breast cancer mortality in metropolitan areas with widespread opportunistic imaging.MethodsWe conducted a population-based incidence-density case-control study among women aged 50-74 years resident in Milan, Italy. We matched 900 breast cancer deaths (diagnosed 2008-2022; died 2012-2023) to 3600 controls by age (±3 months), postcode and citizenship. Individual screening histories were reconstructed by deterministic linkage across five administrative databases. Conditional logistic regression estimated odds ratios (ORs) for breast cancer death by invitation and participation. Opportunistic mammography was modelled, and self-selection bias was corrected using a factor of 1.28.ResultsInvitation was associated with lower mortality (adjusted OR 0.71, 95% CI 0.58-0.85). Participation in at least one programme screen yielded an adjusted OR of 0.44 (0.36-0.52), which attenuated to 0.64 (0.53-0.77) after self-selection correction. Adjustment for opportunistic mammography had minimal impact on programme estimates.ConclusionsIn a large metropolitan setting characterised by intensive opportunistic imaging, organised screening confers a significant mortality reduction, chiefly among participants. Policies discouraging opportunistic substitution and enhancing adherence may increase population benefit.