Beneficial effect of repeated participation in breast cancer screening upon survival.

Abstract

552 Background: The benefit of mammography screening in reducing population mortality from breast cancer is well established. However, the effect of repeated participation at scheduled screens on case survival is less well documented. Methods: In this retrospective analysis of breast cancer survival, we analyzed incidence and survival data on 37,079 women ages 40-69 from nine Swedish counties who had between one and five invitations to mammography screening and were diagnosed with breast cancer between 1992 and 2016. Of these, 4,564 subsequently died of breast cancer. For each breast cancer diagnosed in each county’s period of observation, we obtained data on previous screening history, and subsequent death (or not) from breast cancer. Formal comparisons of survival with respect to numbers of screening exams attended prior to a diagnosis of breast cancer were carried out using Cox proportional hazards regression with time-varying covariates, ie, cumulative numbers of screens, yielding hazard ratios and 95% confidence intervals (CI). We corrected for potential self-selection using the method of Duffy et al, which gives a corrected estimate based on the proportion of participants, the uncorrected estimate, and the relative risk of breast cancer death for non-participants compared to an uninvited population. Results: Depending on number of invitations, 58-73% (average 65%) participated in all scheduled screens, and 73-96% (average 91%) participated in at least one examination. There was successively better survival among women with increasing numbers of screening exams. For a woman with five previous screening invitations who participated in all five screening exams, the hazard ratio was 0.28 (95% CI 0.25-0.33, p<0.0001), a 72% risk of dying from breast cancer, compared to a woman attending none. Following a conservative adjustment for potential self-selection factors, the hazard ratio was 0.34 (95% CI 0.26-0.43, p<0.0001). Conclusions: For those women who develop breast cancer, regular prior participation in mammography screening confers significantly better survival. Most women will not develop breast cancer in their lifetime. These results indicate that for those who do, regular participation in screening considerably improves the probability of surviving it. The distinction between the general, population benefit of mammography screening, and the benefit of regular participation in mammography screening should be clearly articulated in breast cancer screening messaging and decision aids. [Table: see text]