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    Original Research

    Swedish Two-County Trial: Impact of Mammographic Screening on Breast Cancer Mortality during 3 Decades

    Published Online:Sep 1 2011https://doi.org/10.1148/radiol.11110469

    Abstract

    The results of the Swedish Two-County Trial of mammographic screening are qualitatively the same at 29-year follow-up as when they were first published: A substantial and significant reduction in breast cancer mortality was associated with an invitation to screening.

    Purpose

    To estimate the long-term (29-year) effect of mammographic screening on breast cancer mortality in terms of both relative and absolute effects.

    Materials and Methods

    This study was carried out under the auspices of the Swedish National Board of Health and Welfare. The board determined that, because randomization was at a community level and was to invitation to screening, informed verbal consent could be given by the participants when they attended the screening examination. A total of 133 065 women aged 40–74 years residing in two Swedish counties were randomized into a group invited to mammographic screening and a control group receiving usual care. Case status and cause of death were determined by the local trial end point committees and, independently, by an external committee. Mortality analysis was performed by using negative binomial regression.

    Results

    There was a highly significant reduction in breast cancer mortality in women invited to screening according to both local end point committee data (relative risk [RR] = 0.69; 95% confidence interval: 0.56, 0.84; P < .0001) and consensus data (RR = 0.73; 95% confidence interval: 0.59, 0.89; P = .002). At 29 years of follow-up, the number of women needed to undergo screening for 7 years to prevent one breast cancer death was 414 according to local data and 519 according to consensus data. Most prevented breast cancer deaths would have occurred (in the absence of screening) after the first 10 years of follow-up.

    Conclusion

    Invitation to mammographic screening results in a highly significant decrease in breast cancer–specific mortality. Evaluation of the full impact of screening, in particular estimates of absolute benefit and number needed to screen, requires follow-up times exceeding 20 years because the observed number of breast cancer deaths prevented increases with increasing time of follow-up.

    © RSNA, 2011

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    Article History

    Received March 22, 2011; revision requested April 14; revision received May 2; accepted May 3; final version accepted May 5.
    Published online: Sept 2011
    Published in print: Sept 2011