Inherited genetic factors have a minor role in causing cancers, but the environment is the main contributor to the majority of cases, according to Scandinavian researchers.
In a study of almost 90,000 identical and non-identical twins, data from the Swedish, Danish and Finnish twin registries were combined to assess the risks of cancer at 28 anatomical sites for the twins of persons with cancer.
The study found that even among identical twins the risk of developing the same type of cancer was generally below 10 per cent. The research, published in the New England Journal of Medicine (2000;343:78), showed an increased risk among the twins of persons affected by stomach, colorectal, lung, breast and prostate cancer.
For prostate cancer, 42 per cent of the risk could be attributed to heritable factors, 35 per cent in the case of bowel cancer and 27 per cent in breast cancer. The researchers conclude that, apart from prostate cancer, environmental factors account for at least 65 per cent of the risk.
In an accompanying editorial (ibid, p135) Dr Robert Hoover (National Cancer Institute, Rockville, US) comments that the study should dispel the "fatalism of the general public about the inevitability of genetic effects."
The low probability that a cancer will develop in a person whose identical twin has the same type of cancer should be instructive to those who believe that it will be possible to predict accurately who will contract a disease and who will not, he adds.
Dr Lesley Walker (director of cancer information, Cancer Research Campaign) told The Journal on July 18 that, because of the large numbers of twins studied, the research was able to generate "nice figures" confirming what was already known. She added: "We accumulate genetic damage as we go through life and this can be related to occupation, environment and diet." Dr Walker commented that the study also showed the importance of the inherited component of risk, including "low-risk genes" which were likely to interact with other genes and the environment.
The message that should be taken away from the study was the importance of a healthy lifestyle in reducing cancer risk, she added.