Page 257

Because most reports have been based on U.S. or European populations, a publication from Mexico, where DDT is still used for malaria control, should reflect the breast-cancer experience in a more highly exposed population (Lopez-Carrillo et al. 1997). A case-control study based on 141 newly diagnosed cases and equal numbers of controls from three referral hospitals in Mexico City between 1994 and 1996 produced findings very similar to those reported in other recent studies. Mean serum DDE levels (ng/mL) were 4.75 in cases and 4.07 in controls, not a statistically significant difference. When DDE levels for all subjects were divided into tertiles, the multivariate adjusted odds ratio for breast cancer for the highest relative to the lowest tertile was 0.76 and for the second versus the first tertile was 0.60. Confidence intervals of 95%, including unity, surrounded each estimate. The unexpected and not fully explained finding in this study was the relatively low blood levels of DDE among all study subjects. The levels were similar to those reported in the Nurses Health Study, which encompasses many geographic areas in the United States and the Wolff et al. (1993) study from New York City.

The small hospital-based case-control study by Schecter et al. (1997) examines DDT/DDE concentrations in the serum of women in Vietnam where DDT has been heavily used in the recent past. The p,p' DDE levels were more than twice as high (16.7 ng/mL in controls) as those characteristic of United States or European women. Little can be said about the lack of case-control differences given the small sample size.

The recent report by Høyer et al. (1998) was based on the population of women in the Copenhagen City Heart Study. From the sample of 10,317 women identified in 1967, bloods were drawn and stored on 7,712. This cohort of women was followed until 1993 through linkage to the Danish Cancer Registry to identify the occurrence of breast cancer. The 268 women who developed invasive breast cancer provided the cases for a nested case-control study in which two controls were matched to each case on age, date of examination, and vital status at the time of the case diagnosis. Serum samples were available for 240 cases and 477 controls (Høyer et al. 1998).

Using gas chromatographic techniques, serum samples were analyzed for 18 different pesticides or their metabolites and 28 different PCB congeners. The data analysis provided odds ratios estimated by conditional multiple logistic regression, with the reference group based on the lowest quartile of the frequency distribution of the particular compound in the controls. Data were presented on four of the 46 individual compounds that were subjected to laboratory analysis in addition to total PCBs and total DDT. There were no associations between any of the specific DDT isomers or metabolites or any of the individual PCB congeners and breast cancer. Of the 46 compounds analyzed, however, only dieldrin showed a positive association with breast cancer risk: odds ratios of 1.96 and 2.05 in the top two quartiles of the distribution of values. Whether this is a biolgically significant or a chance occurrence, given the 46 different compounds analyzed and the multiple compari-soft