Repeated measurements of organochlorine exposure and breast cancer risk (Denmark)

Objective: To prospectively evaluate if repeated measurements of organochlorine exposure provide a more precise measure of breast cancer risk.Methods: In the Copenhagen City Heart Study (CCHS) participants donated blood twice, in 1976–1978 and 1981–1983. Information on breast cancer risk factors was obtained through standardized questionnaires. A cohort nested case–control study of 155 cases and 274 matched breast cancer-free controls who had participated in both CCHS examinations was conducted. The average serum organochlorine concentration over the course of the two examinations was used, testing a possible association between organochlorine exposure and breast cancer risk.Results: A high serum concentration of p,p-DDT over the course of the two examinations was associated with a more than three-fold significantly increased risk of breast cancer, and a dose–response relationship was apparent. Furthermore, the risk of breast cancer increased with increasing serum concentrations of PCB congener 118 and 138 and the total amount of DDT isomers (DDT), but the trends were not significant.Conclusion: This study provides new evidence of the adverse effect of some organochlorines on breast cancer risk. Furthermore, repeated assessment of exposure during a relevant time period may provide a more precise risk estimate than a single measurement.