The researchers estimated that, when one fraternal twin was diagnosed with any cancer, the co-twin's risk of getting cancer was 37 per cent. (Representational Image)
Having a twin sibling diagnosed with cancer poses an excess risk for the other twin to develop any form of the disease, a large new Harvard study has found.
Among the 23 different types of cancer studied, an excess familial risk was seen for almost all of the cancers, including common cancers such as breast and prostate cancer, but also more rare cancers such as testicular cancer, head and neck cancer, melanoma, ovarian and stomach cancer.
The study is the first to provide family risk estimates for these and other rarer cancers.
It also showed, for the first time, that in twin pairs where both developed cancer, each twin often developed a different type of cancer - which suggests that, in some families, there is a shared increased risk of any type of cancer.
"Prior studies had provided familial risk and heritability estimates for the common cancers - breast, prostate, and colon - but, for rarer cancers, the studies were too small, or the follow-up time too short, to be able to pinpoint either heritability or family risk," said Lorelei Mucci, associate professor of epidemiology at Harvard T H Chan School of Public Health in Boston.
Familial risk of cancer is a measure of the cancer risk in an individual. The study also looked at heritability of cancer, a measure of how much of the variation in cancer risk of populations is due to genetic factors.
"Findings from this prospective study may be helpful in patient education and cancer risk counselling," said Jaakko Kaprio, from the University of Helsinki.
The researchers looked at more than 200,000 twins, both identical and fraternal, in Denmark, Finland, Norway, and Sweden, who participated in the Nordic Twin Study of Cancer and were followed over an average of 32 years between 1943 and 2010.
Overall, one in three people in the study developed cancer over the course of a lifetime. Cancer was diagnosed in both twins for 3,316 of the pairs, in whom the same cancer was diagnosed among 38 per cent of the identical twins and 26 per cent of the fraternal twins.
The researchers estimated that, when one fraternal twin was diagnosed with any cancer, the co-twin's risk of getting cancer was 37 per cent; among identical twins, the risk jumped to 46 per cent. One of the strongest familial risks was observed for testicular cancer.
They found that a man's risk of developing this disease was 12 times higher if his fraternal twin developed it, and 28 times higher if his genetically identical twin developed it.
Researchers also found that the heritability of cancer overall was 33 per cent. Significant heritability was found for skin melanoma (58 per cent), prostate cancer (57 per cent), non-melanoma skin cancer (43 per cent), ovarian cancer (39 per cent), kidney cancer (38 per cent), breast cancer (31 per cent), and uterine cancer (27 per cent).
The study appears in the journal JAMA.