Jacky Luiten

98 | Chapter 7 following BCS and the use and type of axillary staging. Additionally, we analyzed the risk of invasive local relapse in patients undergoing BCS. Methods Study population In this population based retrospective cohort study, data and records of all newly diagnosed women with DCIS in the Netherlands were retrieved from the Netherlands Cancer Registry (NCR). The NCR contains all new cases of in situ and invasive malignancies and data on patient, tumor and treatment characteristics. 17 Data are available on a national level since 1989. Patients were included in the NCR database, after notification by the nationwide Dutch Pathology Archive of Histo ‐ and Cytopathology (PALGA). 18 Specially trained data managers collected data from patient files in Dutch hospitals. The NCR routinely collected information on the occurrence of invasive relapse and the date of death. Follow ‐ up for these end ‐ points was completed until January 2019. In the Netherlands, the first round of a population ‐ based screening program for breast cancer was implemented during 1989 ‐ 1996, offering free of charge biennial mammography to women aged 50 ‐ 70 years. Since 1999 women aged 70 ‐ 75 years are also invited. Screen ‐ film mammography was replaced by full ‐ field digital mammography in 2009 ‐ 2010. Since digital mammography a two ‐ view mammography (medio ‐ lateral ‐ oblique view and cranio ‐ caudal view) of each breast is obtained by a certified radiographer and the examination is assessed by two screening radiologists. For the current study, all screen ‐ detected and clinically detected DCIS from January 1989 until January 2019 in women aged 50 ‐ 75 years were included. Whether a patient was detected by screening was adequately registered before 2011. For further analysis on subgroups we excluded all two ‐ year cohorts with more than 20% missing data. This meant that for analyses of the type of local treatment and ALND patients treated before 1995 were excluded. For analysis on grade, we only included patients diagnosed from 2001 onwards. And for the analysis on SLNB, patients were included since 2005. Women with sentinel lymph node (SLN) involvement could not be included in our analyses, as in these cases the diagnosis of DCIS was overwritten by invasive breast cancer in the NCR database.