Long-term incidence of breast cancer by trial arm in one county of the Swedish Two-County Trial of mammographic screening

BACKGROUND: This study estimated the excess incidence (overdiagnosis) of breast cancer associated with starting mammographic screening at an earlier age, by using data from the Dalarna County component of the Swedish Two-County Trial of breast cancer screening. METHODS: In Dalarna County, Sweden, 38,589 women aged 40 to 74 years were randomized to invitation to regular mammographic screening (active study population [ASP]) and 18,582 women to usual care (passive study population [PSP]). After 3 screening rounds (6-8 years after randomization), the PSP was invited to screening. The cumulative incidence of breast cancer was calculated in the ASP and PSP from randomization to 29 years later. In addition, cumulative incidence was calculated for invasive cancers, advanced invasive cancers (≥2 cm in maximum diameter or node-positive), and nonadvanced cancers (<2 cm and node negative). RESULTS: There was no excess of cancers in the ASP at 29 year follow-up (relative risk, 1.00; 95% confidence interval, 0.92-1.08). Cumulative incidence in the 2 arms approximately equalized at the conclusion of the first round of screening of the PSP. There was an excess of nonadvanced cancers and a deficit of advanced cancers in the ASP, both of which persisted to 29 years. CONCLUSIONS: There was no additional breast cancer incidence associated with 100,000 additional screens in the ASP. Results suggest that overdiagnosis is small and largely confined to the prevalence screen.