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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.