Increased incidence of invasive breast cancer after the introduction of service screening with mammography in Sweden

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Abstract
Screening with mammography has been shown to substantially reduce mortality from breast cancer. The incidence of invasive cancer will increase as screening starts, and it is desirable that it gradually returns to the same level as before screening. Age-specific incidence of invasive breast cancer in 11 Swedish counties, including 463,000 women aged 40–74 years, was analysed before and after the start of service screening with mammography. Incidence, as observed on average during 12.8 years from screening start, was compared to expected incidence based on the incidence during a 15-year period preceding screening start. The height of the incidence peak during the first screening round was increasing with increasing age, compatible with the accumulation in the population of slowly growing tumours by age. All analysed age groups showed an increased ratio between observed stabilised incidence 7–14 years after screening start and expected incidence. When relative risks were adjusted for lead time, the estimates were 1.54 (95% confidence interval [CI] 1.33–1.79) and 1.21 (95% CI 1.04–1.41) for the age groups 50–59 and 60–69 years, respectively. In the age groups 40–49 and 70–74, no change was observed. The findings were further confirmed by the observation of a disappearance in the screened population of the notch in the increasing trend of age-specific breast cancer incidence for the ages after menopause. This notch could indicate hormone-related retardation in tumour growth around menopause. It appears that many of these clinically insignificant, retarded tumours are detected with screening mammography.