Exogenous hormone use and the risk of postmenopausal breast cancer: results from The Netherlands Cohort Study.

The association between the use of exogenous hormones as either oral contraceptives (OC) or hormone replacement therapy (HRT) in relation to postmenopausal breast cancer incidence was examined in the Netherlands Cohort Study (NLCS) among 62,573 women aged 55 to 69 years. Information on these types of exogenous hormone use and other risk factors was collected by mailed questionnaire. During 3.3 years of follow-up, 471 incident breast cancer cases were identified. After adjustment for traditional breast cancer risk factors, the relative risk (RR) of breast cancer was 1.09 (95 percent confidence interval [CI] = 0.79-1.48) for women who ever used OCs cf women who never used OCs. The relative rates (with CIs) for women who used OCs for a period < 5 years, 5-9 years, 10-14 years, and 15+ years were 0.97 (0.61-1.55), 1.20 (0.69-2.07), 1.03 (0.60-1.77), and 1.96 (0.99-3.89), respectively. The test for trend was not significant (P = 0.13). There was no evidence of any association between the number of years between the first and the last use of OCs and breast cancer incidence. In the subgroup of women with first-degree relatives with breast cancer, the RR for breast cancer associated with ever use of OCs was 1.51 (CI = 0.67-3.41), whereas in the remaining women, the RR was 0.97 (CI = 0.73-1.27). Ever-use of HRT compared with never-use was not associated with an increase in breast cancer risk in the multivariate analysis (RR = 0.99, CI = 0.68-1.43).(ABSTRACT TRUNCATED AT 250 WORDS)