Letter to the Editor Mammography Screening and Breast Cancer Mortality—Response Suzie J. Otto 1 , Jacques Fracheboud 1 , Andr e L.M. Verbeek 2 , Rob Boer 1 , Jacqueline C.I.Y. Reijerink-Verheij 3 , Johannes D.M. Otten 2 , Mireille J.M. Broeders 2,4 , and Harry J. de Koning 1 , for the National Evaluation Team for Breast Cancer Screening We thank Drs. Autier and Boniol for their comment raised on the outcome of our case–control study on the impact of mammography screening on the risk of death from breast cancer, but we feel that the metho- dologic limitations inherent to the case–control design are small. Aarts and colleagues (1) indeed showed that the par- ticipation rate among women of high socioeconomic sta- tus (SES) is somewhat higher than those of low SES (87% vs. 79%), in the Netherlands mostly coinciding with women of non-Western ethnic descent. The attendance rate is lower in this group of women (2), in which breast cancer incidence is also considerably lower (3) as well as the risk of death from breast cancer than in the native Dutch population (4). Aarts and colleagues further report that low SES women are diagnosed with prognostically less favorable breast cancers. However, these differences in tumor stage and overall survival were observed among nonpar- ticipants as well as participants, either screen-detected or symptomatically diagnosed interval breast cancer (1). Therefore, it is incorrect to deduce that participants and nonparticipants present with genuine differences in risk factors associated with dying from breast cancer or from other causes. Risk differences associated with SES groups would reflect only very partially in risk differ- ences between participants and nonparticipants. Case– control studies that could adjust for SES showed no effect of this correction on the estimated ORs (5). Autier and Boniol question the validity of the factor used for correction of self-selection bias, which is calcu- lated as the relative risk (RR) of death from breast cancer among nonparticipants compared with uninvited wom- en. We used individual data on breast cancer mortality in nonparticipants from the study period 1990 to 2003 and, due to privacy regulations, aggregated data on uninvited women from the prescreening period (1986–1989). Our RR of 1.11 was remarkably similar to the RR of 1.08 from another Dutch study (6) that used data on contempora- neous groups of nonparticipants and uninvited women in the implementation period for screening (1990–1995) from the same region. If breast cancer mortality after 1990 has been decreasing among nonparticipants (numerator of the RR) due to better treatment, then this would also apply for not invited women (denominator of the RR). The RR would then approach unity and results in a higher effect of screening on the risk of breast cancer mortality, adjusted for self-selection. There is no evidence of differential treatment. As screening is fully implemen- ted in the Netherlands, estimation of a correction factor for more recent years is hampered, but given the stable attendance rate in the Netherlands, there is little reason to believe that this will change considerably. Thus, in organized breast cancer screening programs, self-selec- tion appears to be relatively minor. In our case–control study, we minimized the biases inherent to an observational study design (e.g., identifi- cation and selection of cases and controls, equal access to screening during the exposure period, definition of expo- sure, source population). We showed that breast cancer screening resulted in a 49% reduced risk of dying of breast cancer for women invited and attended mammography screening. Observational study designs are crucial for the evaluation of the effect of mammography screening in the actual female population. Disclosure of Potential Conflicts of Interest No potential conflicts of interests were disclosed. Received February 22, 2012; accepted February 23, 2012; published OnlineFirst March 8, 2012. References 1. Aarts MJ, Voogd AC, Duijm LE, Coebergh JW, Louwman WJ. Socio- economic inequalities in attending the mass screening for breast cancer in the south of the Netherlands–associations with stage at diagnosis and survival. Breast Cancer Res Treat 2011;128:517–25. 2. Vermeer B, van den Muijsenbergh METC. The attendance of migrant women at the national breast cancer screening in the Netherlands 1997–2008. Eur J Cancer Prev 2010;19: 195–8. Authors' Affiliations: 1 Department of Public Health, Erasmus MC, Uni- versity Medical Center Rotterdam; 2 Department of Epidemiology, Biosta- tistics and HTA, Radboud University Nijmegen Medical Centre; 3 Cancer Screening Organization for Southwest Netherlands, Rotterdam; and 4 National Expert and Training Centre for Breast Cancer Screening, Nijme- gen, the Netherlands Corresponding Author: Suzie J. Otto, Erasmus MC, University Medical Center Rotterdam, P.O. Box 2040, Rotterdam 3000 CA, the Netherlands. Phone: 31-10-7043446; Fax: 31-10-7038474; E-mail: s.otto@erasmusmc.nl doi: 10.1158/1055-9965.EPI-12-0235 Ó2012 American Association for Cancer Research. Cancer Epidemiology, Biomarkers & Prevention Cancer Epidemiol Biomarkers Prev; 21(5) May 2012 870 on June 13, 2020. © 2012 American Association for Cancer Research. cebp.aacrjournals.org Downloaded from Published OnlineFirst March 8, 2012; DOI: 10.1158/1055-9965.EPI-12-0235