1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene and polychlorinated biphenylsand breast cancer: Combined analysis of five US studies

Background: Environmental exposure to organochlorines has been examined as a potential risk factor for breast cancer, in 1993, five large U.S. studies of women located mainly in the northeastern United States were funded to e valuate the association of levels of 1,1-dichloro-2,2-bis(p-chlorophenyl) e thylene (DDE) and polychlorinated biphenyls (PCBs) in blood plasma or serum with breast cancer risk. We present a combined analysis of these results t o increase precision and to maximize statistical power to detect effect mod ification by other breast cancer risk factors, Methods: We reanalyzed the d ata from these five studies, consisting of 1400 case patients with breast c ancer and 1642 control subjects, by use of a standardized approach to contr ol for confounding and assess effect modification. We calculated pooled odd s ratios (ORs) and 95% confidence intervals (CIs) by use of the random-effe cts model. All statistical tests were two-sided, Results: When we compared women in the fifth quintile of lipid-adjusted values with those in the firs t quintile, the multivariate pooled OR for breast cancer associated with PC Bs was 0.94 (95% CI = 0.73 to 1.21), and that associated with DDE was 0.99 (95% CI = 0.77 to 1.27). Although in the original studies there were sugges tions of elevated breast cancer risk associated with PCBs in certain groups of women stratified by parity and lactation, these observations were not e vident in the pooled analysis. No statistically significant associations we re observed in any other stratified analyses, except for an increased risk with higher levels of PCBs among women in the middle tertile of body mass i ndex (25-29.9 kg/m(2)); however, the risk was statistically nonsignificantl y decreased among heavier women. Conclusions: Combined evidence does not su pport an association of breast cancer risk with plasma/serum concentrations of PCBs or DDE. Exposure to these compounds, as measured in adult women, i s unlikely to explain the high rates of breast cancer experienced in the no rtheastern United States.