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Thanks to national breast cancer screening, breast abnormalities can already be detected before they cause symptoms. The earlier breast cancer or possible pre-stages are detected, the better the prognosis for the woman. In addition to breast cancer, screening also detects abnormalities that are not breast cancer or are only a pre-stage (DCIS), which can sometimes, but not always, develop into breast cancer. This thesis provides important new insights into the usefulness of national breast cancer screening: by filtering out high-grade DCIS in particular, less high-grade/more aggressive breast cancer can be found through screening. On the other hand, improved digital screening techniques have also led to an increase in diagnosing breast abnormalities of which the biological behaviour remains mostly - but not always - benign (premalignant abnormalities). Diagnosing and treating these abnormalities leads to overtreatment, which is an unintended but unavoidable consequence of screening. This dissertation also describes the trends in referral, additional studies and treatment of these premalignant abnormalities.

Background: Trends in the detection of suspicious microcalcifications at mammography screening and the yield of these lesions after recall are unknown.Purpose: To determine trends in recall and outcome of screen-detected microcalcifications during 20 years of mammography screening.Materials and Methods: The authors performed a retrospective analysis of a consecutive series of 817 656 screening examinations (January 1997 to January 2017) in a national breast screening program. In 2009-2010 (transition period), screen-film mammography (SFM) was gradually replaced by full-field digital mammography (FFDM). The recalls of suspicious microcalcifications from all radiology reports and pathologic outcome of recalled women with 2-year follow-up were analyzed. Screening outcome in the era of SFM (1997-2008), the transition period (2009-2010), and the era of FFDM (2011-2016) were compared. Trends over time and variations between the SFM and FFDM periods were expressed by using proportions with 95% confidence intervals (CIs). In cases where the analysis based on the CI confirmed clear periods (eg, before and after introduction of FFDM), pre- and postchange outcomes were compared by using chi(2) tests.Results: A total of 18 592 women (median age, 59 years; interquartile range, 14 years) were recalled at mammography screening, 3556 of whom had suspicious microcalcifications. The recall rate for microcalcifications increased from 0.1% in 1997-1998 to 0.5% in 2015-2016 (P,.001). This was temporally associated with the change from SFM to FFDM. The recalls yielding ductal carcinoma in situ (DCIS) increased from 0.3 per 1000 screening examinations with SFM to 1.1 per 1000 screening examinations with FFDM (P <.001), resulting in a decrease in the positive predictive value for recall for suspicious microcalcifications from 51% to 33% (P <.001). More than half of all DCIS lesions were high grade (52.6%; 393 of 747). The distribution of DCIS grades was stable