Minor influence of self-selection bias on the effectiveness of breast cancer screening in case-control studies in the Netherlands

Abstract

Background Self-selection bias is considered to be a problem when evaluating the effectiveness of breast cancer service screening in case-control studies. Objective Using the incidence-based mortality method (IBM), a correction factor for the potential influence of self-selection can be derived from a group of non-screened women and a group of notinvited women. Methods Breast cancer patients, diagnosed in 1990 -1995 and between the ages of 50 to 70, were selected from the Netherlands Cancer Registry and five screening regions in the Netherlands. Person-years were calculated for non-screened and not-invited women by using population data available on the number of women invited, the number of women screened and the total population. Incidence-based breast cancer mortality rates according to screening status were calculated for the five screening regions. Results Between 1990-1995, 15,541 patients were diagnosed with breast cancer. An analysis of ten-year follow-up after diagnosis resulted in 3903 breast cancer deaths, of which 2631 were notinvited and 345 were non-screened. Poisson regression analysis showed heterogeneity between the regions with a range of IBM ratios from 0.64 (95% CI: 0.46 -0.90) to 1.08 (95% CI: 0.82 -1.43). Conclusions Heterogeneity between the regions stresses the importance of a country-and/or regionspecific estimate of self-selection. Adjusting for self-selection bias in the regional case-control studies would not change the breast cancer mortality reduction in three regions and would result in an even larger effect in two regions. Looking at the range of IBM ratios the overall influence of selfselection in the Netherlands is minor.

Mammography Screening and Breast Cancer Mortality—Response

“…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)(1987)(1988)(1989). Our RR of 1.11 was remarkably similar to the RR of 1.08 from another Dutch study (6) that used data on contemporaneous groups of nonparticipants and uninvited women in the implementation period for screening (1990)(1991)(1992)(1993)(1994)(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).…”

Case–control Studies on the Effectiveness of Breast Cancer Screening

“…these differences could partly be explained by the fact that the trial estimates do not take the pseudo-diagnosis date into account. 10,14 in the current population, the risk in non-attenders appeared to be lower than the risk in uninvited women, which resulted in correction factors smaller than one. the non-attender groups for the case-control and the trial estimates may thus slightly differ.…”

“…For the ever/never-estimate, the person-years are therefore based on non-attendance during the entire trial period, which may result in an increased screening opportunity for women who are not diagnosed with breast cancer. 10 in both the trial and the case-control analyses, no marked association between breast cancer screening and breast cancer mortality in women ages 40 to 50 years was observed. Despite the differences, the results from both methods seemed to suggest a similar or smaller breast cancer mortality risk in non-attenders compared with uninvited women.…”

Is the incidence of advanced‐stage breast cancer affected by whether women attend a steady‐state screening program?

“…Given that women with a low SES were more likely to be diagnosed with advanced cancer in this study, we corrected for SES in the multivariable analysis. Furthermore, another study has shown that the overall influence of bias due to self‐selection was minor on the effectiveness of the Netherlands screening program …”