Trends in breast cancer stage distribution before, during and after introduction of a screening programme in Norway

Background: Screening is intended to advance diagnosis thereby shifting the stage distribution towards more locally confined stages. Consequently we aimed to estimate trends in stage-specific breast cancer in relation to the introduction of population-based screening. Methods: From the Cancer Registry of Norway we retrieved cancer stage, age and year of diagnosis on all women aged 20 or older diagnosed with breast cancer during the period 1987-2010 in Norway (approximate source population: 1.8 million). Three calendar-time periods were defined: before (1987-95), during (1996-2004), and after (2005-10) screening was introduced; and two age groups: women eligible for screening (50-69 years) or younger (20-49 years). Poisson regression was used to estimate the incidence of localized (stage I) and more advanced cancer (stages II+), respectively, and logistic regression to estimate the proportion of localized cancer. Results: The annual incidence of localized breast cancer among women aged 50-69 years rose from 63.9 per 100 000 before the introduction of screening to 141.2 afterwards, corresponding to a ratio of 2.21 (95% confidence interval: 2.10; 2.32). The incidence of more advanced cancers increased from 86.9 to 117.3 per 100 000 afterwards, corresponding to a 1.35 (1.29; 1.42)-fold increase. Advanced cancers also increased among younger women not eligible for screening, whereas their incidence of localized cancers remained nearly constant. Conclusion: Incidence of localized breast cancer increased significantly among women aged 50-69 years old after introduction of screening, while the incidence of more advanced cancers was not reduced in the same period when compared to the younger unscreened age group.