The findings are important for cancer risk counselling so that more cases can be detected and treated at an early stage.
Twin studies can give information about how much of the cancer risk is due to genetic factors and how much is due to environmental factors.
In a large study, researchers from the Nordic countries and the United States collected data from the major Nordic twin registries in Norway, Sweden, Denmark and Finland. The twins in the study were followed for 32 years on average. Their findings are published in the Journal of the American Medical Association (JAMA).
The results show that twins generally have the same cancer risk as the general population. The researchers have calculated the cumulative incidence rate to be approximately 32 per cent for people who live to be 100 years and approximately 25 per cent up to 80 years of age.
Genetic factors become apparent
However, one subgroup of twins appears to have a greater cancer risk than the general population. These are the twins whose twin sibling has had cancer. Compared to the general population, the risk is 14 per cent higher for identical twin siblings and 5 per cent higher for fraternal twin siblings of the same sex.
The increased risk may be linked both to genetic factors and common environmental factors. The higher risk for identical twins with a twin sibling with cancer suggests that genetic factors play an important role.
The increased risk for fraternal twins will be similar to ordinary siblings if they grow up under the same conditions.
Different cancer types
In more than two thirds of cases, when both twins in a pair developed cancer, each twin often developed a different type of cancer. This suggests that some families have a greater general risk of developing cancer. The genetic factors may be related to having cancer, or to a risk factor for cancer, for example an increased genetic risk of obesity or nicotine dependency.
Researchers found that genetic differences between people are important for explaining why some people develop certain cancers, including prostate, breast, skin, ovarian, kidney and uterine cancers.
“Although the study confirmed the importance of genetic factors in explaining the liability to develop cancer, the findings also illustrate that genetic explanations alone are not sufficient to explain why some people develop cancer and others do not, says Professor Jennifer Harris who leads the Norwegian research team at the Norwegian Institute of Public Health.
About the study
A large international research group of researchers was involved in the study, including researchers from the Norwegian Institute of Public Health, other Nordic institutes and universities and Harvard University, USA.
Data were collected from over 80,000 identical and 123,000 fraternal twins, of which about 24,000 were Norwegian. They had all consented to be included in the Nordic twin studies or registries. Only fraternal twins of the same sex were included in this study. The twin pairs were followed for on average 32 years, until 2008 - 2010.
During this follow-up period, approximately 24,000 of the participants developed cancer, and 6, 600 of these had a twin who was also affected.
Mucci, LA, Hjelmborg, JB, Harris, JR. Czene, K., Havelick, DJ, Scheike, T., Graff, RE, Holst, K., Möller, S., Unger, RH, McIntosh, C., Nuttall, E, Brandt, I, Penney, KL, Hartman, M., Kraft, P., Parmigiani, G., Christensen, K., Koskenvuo, M., Holm, NV, Heikkilä, K. Pukkala, E, Skytthe, A., Adami, HO, Kaprio, J. Nordic Twin Study of Cancer (NorTwinCan) Collaboration. Familial Risk and Heritability of Cancer Among Twins in Nordic Countries. JAMA. 2016 Jan 5;315(1):68-76. doi: 10.1001/jama.2015.17703.