Effect of organised mammography screening on stage-specific incidence in Norway: population study

Background: We aimed to estimate the effect of organised mammography screening on breast cancer stage distribution by comparing changes in women eligible for screening, based on birth cohort, to the concurrent changes in younger, ineligible women. Methods: In an open cohort study in Norway, which introduced national mammography screening county-by-county from 1995 to 2004, we identified women (n = 49 883) diagnosed with in situ or invasive breast cancer (ICD10 codes: D05 or C50) during the period 1987-2011 and born between 1917 and 1980. We estimated relative incidence rate ratios (rIRRs) comparing the development in the screening vs historic group to the younger vs younger historic group. Results: Including the compensatory drop, eligible women experienced a 68% higher increase in localised cancers (rIRR = 1.68, 95% confidence interval (CI): 1.51-1.87) than younger women, while the increase in incidence of advanced cancers was similar (rIRR = 1.11, 95% CI: 0.90-1.36). Excluding the prevalence round, eligible women experienced a 60% higher increase in localised cancers (rIRR = 1.60, 95% CI: 1.42-1.79), while the increase in incidence of advanced cancers remained similar (rIRR = 1.08, 95% CI: 0.86-1.35). Conclusions: Introduction of organised mammography screening was followed by a significant increase in localised and no change in advanced-stage cancers in women eligible for screening relative to younger, ineligible women.