A twin sibling diagnosed with cancer poses an excess risk for the other to develop any form of the disease, a new Harvard study has found.
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 ones such as testicular cancer, head and neck cancer, melanoma, ovarian and stomach cancer.
It also showed, for the first time, that in twins where both developed cancer, each twin often developed a different type of cancer - suggesting 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 common cancers, but for rarer cancers the studies were too small, or the follow-up time too short, to pinpoint either heritability or family risk," said Lorelei Mucci, associate professor of epidemiology at Harvard.
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 the population is due to genetic factors.
The researchers looked at more than 2,00,000 twins, both identical and fraternal, 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% of identical twins and 26% 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%; among identical twins, the risk jumped to 46%. 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.