Women with breast cancer are more likely to show high serum concentrations of organochlorine pesticides, such as DDT, which are known oestrogenic compounds, a study has found ( Occupational and Environmental Medicine 2003;60:348-51).
The researchers, at the Sart Tilman Hospital in Liège , Belgium , tested 159 women who had had positive diagnoses of breast cancer, while 250 controls were randomly selected from women without known cancers who were attending for routine vaginal cytological examinations. At least one control was matched to each woman with breast cancer, for factors such as age, reproductive status, and menopause. The proportion of smokers was similar in the two groups, as were the proportions of urban dwellers and of breast feeders.
The women were tested for two organochlorines, dichlorodiphenyltrichloroethane (DDT) and hexachlorobenzene (HCB). The total burden of DDT was calculated by adding up all isomers of DDT and its metabolite. The mean concentration of DDT was significantly higher in the women with breast cancer than in controls (3.94 versus 1.83 parts per billion; P<0.0001). The same was true of HCB (0.79 versus 0.09 parts per billion; P=0.0005).
Patients with levels of DDT above 0.5 parts per billion in their blood were five times more likely to be cases than controls (95% confidence interval 1.81 to 17.65), while for those with more than than 0.5 parts per billion of HCB the risk was increased by a factor of nine (95% CI 2.84 to 29.41).
While the findings were significant, the authors were cautious about drawing the obvious conclusion that organochlorine pesticides increase the risk of breast cancer.
Since the postulated mechanism is oestrogenic, it might be expected that women with tumours that tested positive for oestrogen receptor status would have the highest levels of pesticide exposure. However, no such correlation was found. Neither was there a correlation between tumour size and organochlorine burden. Several studies have looked at organochlorines and breast cancer in the past, but the only meta-analysis concluded that no link was proved.
Richard Sullivan, head of clinical programmes at Cancer Research UK , said the study had merit but had neglected to control for important factors such as family history and hormone replacement therapy. “In a sense the number of factors that can be controlled for is limited by the study’s size. It’s certainly not the definitive study that proves the case. In the pantheon of environmental carcinogens oestrogen still ranks pretty low.”
DDT has been banned in Europe and North America for decades but can remain in human tissue for up to 50 years. The highest concentration detected in this study was 20 parts per billion, much lower than in some previous studies. “This kind of environmental research remains essential,” said Dr Sullivan, “but fortunately this particular study is more a lesson from history than a warning of a future threat.”