The environment, not genes, has the biggest impact on who will get cancer, say researchers at the Karolinska Institute in Stockholm and elsewhere.

Paul Lichtenstein and colleagues studied 44,788 pairs of twins from Sweden, Denmark, and Finland. They found at least one cancer in 10,803 people among 9,512 pairs of the twins. By comparing the incidence of cancer in identical twins (who share all the same genes) to fraternal twins (who, like most siblings, share an average of S0 percent of their genes), the researchers estimated what percent of the cancers were hereditary.

The risk of being diagnosed with colorectal, breast, or prostate cancer by the age of 75 ranged from 11 to 18 percent for someone whose identical twin already had the disease. For someone whose fraternal twin already had one of the three cancers, the risk was three to nine percent.

Using that data, the scientists estimated that genes account for 27 percent of the risk of breast cancer, 35 percent of the risk of colorectal cancer, and 42 percent of the risk of prostate cancer.

Genes play an equal or smaller role in leukemia or cancers of the stomach, lung, pancreas, ovary, and bladder, they concluded. That means that the environment--diet, smoking, lifestyle, pollution, etc.--accounts for more than half of the risk of getting most cancers. "The fatalism of the general public about the inevitability of genetic effects should be easily dispelled," said Robert Hoover of the National Cancer Institute in an editorial published along with the study.

New Eng. J. Med. 343: 78, 135, 2000.


By Bonnie Liebman

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