The risk of breast tumours and lifetime history of oral contraceptive use.
Background: Previous epidemiological studies have inconsistently shown an increased or decreased risk of breast cancer in users of - mainly high estrogen dose - oral contraceptives (OCs). A reduced risk of benign breast tumours was associated to OC use in most of earlier studies. Little is known about the effects of exclusive use of low estrogen dose 0Cs. Methods: The German Cohort Study on Women's Health is an ongoing study analysing the lifedme risk of tumours going back in time in a cohort of volunteers. Self-reported data were collected from women who responded to a call for participation circulated in all German states. The data include information on exposure and time, type and dose of exposure as well as time of occurrence of any outcome. Initial cohort data until 2000 were analysed using logistic regression to determine the association between lifetime OC use and the occurrence of tumours of the breast Results: The cohort currently covers 610,328 women years of observation since birth on 15,374 participants. 308 cases of breast cancer and 1,309 cases of benign breast tumours were observed. The adjusted relative risk (RR) for the occurrence of breast cancer comparing users and never-users of OCs was 0.6 (95 % confidence interval (95% CI): 0.5 to 0.8). The comparison only of ever-users of low estrogen 0Cs versus never-users shows an adjusted RR of 0.8 (95%0 0.5 to 1.2). This was lower in women < 50 years of age (RR 0.2(95% 0 0.1-0.4). Similar risk reductions were found for benign breast tumours in OC users, especially pronounced in younger women. Duration of OC use seems to playa role in benign breast tumours but not for breast cancer. Time since last OC use was not an effect modifier in breast cancer, but plays a role for benign tumours in women over 50. Conclusion: Ever-use of OCs is associated with a markedly decreased risk of developing malignant or benign breast tumours particular in women under 50. In these younger women, the reduced risk associated with lifelong exclusive use of low estrogen OCs may be more pronounced. (author's)