A recent study of twins in Denmark, Finland and Sweden, published in the New England Journal of Medicine, suggests that environmental factors may be a larger contributor to cancer incidence than heredity.
The research team identified 10,803 people who had contracted at least one cancer among 9,512 pairs of twins.
Because genetic makeup of twins is so similar, a particular type of cancer for which genetics plays a critical role would tend to show up in both twins.
If environmental factors are the key determinant, however, the cancer would appear more frequently in only one of the twins.
The study found that genetics were associated with five types of cancer: stomach, colorectal, lung, breast and prostate cancer.
Environmental factors were found to be more important than genetic factors for other illnesses investigated, including non-Hodgkin''s lymphoma, Hodgkin''s disease and cancer of the lip, mouth, throat, thyroid, bone and soft tissue.
While discussion of environmental factors often is focused on exposure to pesticides and other synthetic chemicals, such factors may include tobacco, alcohol, obesity, stress, light, infections and diet.
"Most cancer is caused by your own behavior, not something the government is going to have much effect on," said Dr. Bruce Ames, director of the Center for Environmental Health Sciences at the University of California at Berkeley. "The best way to prevent cancer is to eat a diet with lots of fruits and vegetables and to stop smoking."
Research underway at the Massachusetts Institute of Technology (MIT) may provide additional insights into the cause of cancer.
MIT researchers have compiled U.S. mortality data as far back as 1900 that provides information on whether cancer mortality rates have changed.
Because humans'' genetic makeup likely has remained the same over this time, any increase probably results from environmental factors, according to Dr. William Thilly, who heads the project.
The MIT work suggests that lung, kidney and brain cancers, as well as leukemia and lymphoma, have increased in recent decades.
While the increase in deaths from lung cancer has been associated with smoking, the causes for the other illnesses are less clear.
By contrast, the risk of dying from breast cancer, which the public often associates with exposure to synthetic chemicals, has stayed about the same over the past 100 years, according to MIT researchers.
While the environment may play a role in causing breast cancer death, whatever environmental factor is at work has not changed in 100 years in the United States, suggested Thilly.
by Virginia Sutcliffe