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Real Risks of Cancer Are Not in Environment but in Ourselves

Links to external causes go without proof; more likely triggers are genetic susceptibility, diet and obesity.

June 08, 1998|BRIAN E. HENDERSON and SUSAN PRESTON-MARTIN | Brian E. Henderson and Susan Preston-Martin are professors of preventive medicine at the USC School of Medicine

In the last 25 years, ever since President Nixon declared a war on cancer, one of the primary assumptions investigated by researchers has been that cancer is "caused" by some external exposure to some "unnatural" element. It was a theory supported, in an indirect way, by overwhelming scientific evidence that smoking leads to lung cancer or sun exposure leads to melanoma. It is a perception that continues to live, in the form of beliefs that "leading scientists" regard environmental toxins as a significant cause of rising cancer rates in the developed world.

Yet despite an enormous number of studies conducted over the past three decades examining many possible putative relationships between environmental factors and cancer, few correlations have been found. From electromagnetic fields and pesticides to microwaves and smog, credible studies have shown little evidence of links between environment and cancer risk.

Millions of dollars poured into researching environmental exposures in groups at highest risk--farmers and pesticide exposure, for example, or residents who live in proximity to chemical stockpiles--have found few direct links between exposures and cancer. For example, a major study in the New England Journal of Medicine last fall by researchers from Harvard and Mount Sinai Hospital in New York found no evidence of an increased breast cancer risk among women with high levels of organochlorines from pesticides and industrial chemicals such as DDT and PCB in their blood, the chemicals most likely to pose a risk. Likewise, studies investigating a possible link between smog and lung cancer have turned up no evidence--although they established a link between smog and asthma.

In addition, international studies of developing countries, where exposures to environmental toxins are in many cases much greater than in developed countries, find no corresponding increase in cancer rates.

Confusing the issue is the fact that even though cancer incidence overall is declining, some cancers are indeed on the rise, seemingly correlated to changing factors in the environment. While stomach cancer--which was the No. 1 cancer in the U.S. 40 years ago--is now down to seventh, the rates for endogenous hormone-related cancers like breast and prostate cancer climb.

Explaining the reason that some cancers are increasing is complicated. For example, with childhood brain tumors, measuring the absolute incidence of a rare tumor is difficult enough, and there are data suggesting that the rates of brain tumors at older ages are not increasing beyond what might be expected from changes in diagnostic technology like magnetic resonance imaging. Indeed, even if one assumes that childhood brain tumors could somehow be related to nitrite-containing foods, the amount of nitrites in foods has been steadily decreasing during the same period when childhood brain tumors were increasing. Similarly, if one assumes increased breast cancer is due to pesticides, it would follow that the agricultural heartland would boast the highest rates. Yet they are among the lowest, while urban areas where educated women delay their childbearing are among the highest--suggesting sociocultural changes that have affected women's lifetime hormone exposure are responsible.

Meanwhile, for the first time since Nixon's declaration, we are learning enough about cancer to make intelligent decisions in regard to treatment. Mortality rates, even for breast cancer, are declining. We have even reached a point of designing effective prevention, such as treating women at high risk for breast cancer with tamoxifen. But contrary to the initial assumptions of an external cause, what most researchers are finding--and what is the foundation for the most dazzling advances in cancer--is that the majority of cancers arise from a complex set of combined factors: a genetic predisposition, internal chemistry such as hormone levels, which are often genetically predetermined, and to a certain extent lifestyle decisions that impact those predispositions.

Unfortunately, by pushing for more study of environmental risks, the public perception of this environmental problem becomes a straw man for the real issues in cancer such as smoking, dietary habits that aren't healthy like consuming excessive fat and weight gain. Studies have shown that the U.S. population's increasing obesity is more of a cancer risk than any environmental exposure. Studies also have shown that women who delay childbearing until after 30, who have early menarche and fewer pregnancies, who get limited exercise, particularly in their teens and 20s and who take estrogen replacement therapies are more at risk for breast cancer because their lifetime exposure to estrogen is greater.

Instead of throwing out a pesticide wild card, we need rational public education of the real issues in cancer. It is a harder message, however, to say that the cause of cancer is not from an external exposure but may be from within you, your own genetic susceptibility combined with things you do like diet and exercise.

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