BREAST CANCER: ENVIRONMENTAL INFLUENCES

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BREAST CANCER: ENVIRONMENTAL INFLUENCES

One of the most disconcerting things about breast cancer is that there seems to be so little we can do to reduce our chance of getting it. The established risk factors include a family history of the disease, early menarche, late menopause, and delayed childbearing or having had no children.

Moreover, the incidence of breast cancer appears to be rising. A decade ago one in 10 women developed it during her lifetime; today that figure has risen to one in eight.

This apparent increase has puzzled medical scientists. Some have speculated that it is merely an artifact -- that it only seems as though there are more cancers because they are being detected earlier Others have suggested it may be the product of a trend toward postponing pregnancy until later in life.

In the last few years some groups have been pursuing another possibility -- the effects of environmental agents. This theory has been bolstered by recently reported data from the Surveillance, Epidemiology, and End Results Program of the National Cancer Institute (NCI). The statistics indicate that the rate of malignancies not linked to smoking is on the rise and that breast cancer is no exception. A woman born in the 1940s is twice as likely to have breast cancer as an age-matched counterpart who was born a half-century earlier. The NCI researchers are speculating that the rise could to be due a number of environmental changes during that period.

The list of candidates includes several compounds introduced into drinking water, vehicle exhaust, chemicals formed in high-temperature cooking, and low-frequency electromagnetic fields. Among the prime suspects are the pesticide DDT, which was banned in 1972, insulating fluids called PCBs (the polychlorinated biphenyls), and another group of pesticides called PBBs (polybrominated biphenyls).

The latter group -- DDT, PCBs, and PBBs -- have received special attention because they accumulate in body fat and take a long time to eliminate. Moreover, all are known carcinogens and act somewhat as estrogen does in the body.

One line of reasoning assumes that these chemicals have been stored in breast tissue of women who were exposed to them during the 1950s and 1960s, where they have stimulated cell proliferation. Proponents of this theory cite supporting circumstantial evidence. They point to an increased breast cancer incidence of 32% in women over age 50 between 1973 and 1980 -- a period when there was only an 8% increase in incidence among women under 50. They note that the older women had a greater cumulative exposure to DDT and PBCs in the previous decades than did the younger women. They also cite a 1990 Israeli study indicating a sharp decline in breast-cancer cases among younger women a decade after DDT was banned.

The most convincing implication of DDT has been provided by the New York University Women's Health Study. The study investigators reported in the April 21, 1993 issue of The Journal of the National Cancer Institute that blood extracted from 58 women who developed breast cancer contained significantly higher levels of DDE -- a metabolic product of DDT -- than did blood from 171 women who were free of the disease. While the women with cancer also had higher levels of PCBs, that difference was not significant. The investigators had eliminated confounding factors by matching each cancer patient with at least two other women who were the same age and at similar risk for developing breast cancer.

Last April the NCI announced that it had undertaken five major studies in regions of high exposure to environmental toxins. Most will compare levels of carcinogens in blood and breast tissue from women who have developed breast cancer and those who haven't. If these investigations indicate links to breast cancer, additional research will be needed to establish carcinogenic doses of each compound. Thus, a definitive answer is still years away.

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