Jacky Luiten

18 | Chapter 2 Materials and methods Study population All women who attended the breast cancer screening program between July 2005 and July 2015 at four specialized screening units in a southern part of the Netherlands, were included in the study. A total of 484, 422 screens were obtained during this 10‐year inclusion period. All women included in this study gave written informed consent with the permission to use their screening and follow‐up data for evaluation purposes. Three women who refused to give this permission were excluded from the study. Seventeen patients were excluded due to incomplete or missing tumor characteristics (four initial screens and thirteen subsequent screens). An initial screen was defined as the first time a woman attended the breast cancer screening program. This group consisted of women aged approximately 50 years. The abnormalities detected at initial screening mammography reflect the prevalence of asymptomatic DCIS or invasive carcinoma. The women with suspicious mammographic findings detected at subsequent screening were aged 51‐75, with a median age of 62 years. According to the Dutch Central Committee on Research involving Human Subjects (CCMO) ethical approval was not required for the current study. We separated all included women in three groups; two SFM‐only cohorts (July 2005‐June 2007 and July 2007‐June 2009), the transition cohort (July 2009‐June 2011) and two FFDM‐only cohorts (July 2011‐June 2013 and July 2013‐June 2015). Screening procedure The Dutch nation‐wide breast cancer screening program offers biennial screening mammography for all women aged 50‐75 years. The details of the screening program have been described previously. 2,10,11 In summary, all mammograms were obtained by certified screening mammography radiographers and all screens were double read by a team of 16 certified screening radiologists, each of them evaluating at least 3000 screens per year. In case of a subsequent screen, the previous screens were available for comparison. Each screening mammography was classified according to the breast imaging reporting and data system (BI‐RADS). 12,13 Women with a screening BI‐RADS 0, 4 or 5 were recalled to a dedicated breast unit. At screening, mammographic abnormalities were classified according to the following categories: suspicious mass, suspicious calcifications, suspicious mass in