A study of twins with 22 980 individuals suggested that the familial risk for testicular cancer may be dramatically underestimated.1

Researchers looked at 23 types of cancer and found an excess familial risk for almost all of the cancers. However, a man’s risk of developing testicular cancer was 12 times higher if his fraternal twin developed it and 26 times higher if his genetically identical twin developed it.

“The results of this study have clinical implications for counseling patients and their families on familial risk of cancer,” said co-lead study author Lorelei Mucci, MPH, ScD, who is an associate professor of epidemiology at Harvard T.H. Chan School of Public Health, Boston, MA.

“For 20 of the 23 cancers, we found an excess cancer risk for a twin if his or her twin also developed cancer. For some of the cancers studied, the excess familial risk was extremely high. For example in testicular cancer, the lifetime risk was 0.5%.”

The study showed there was an excess familial risk in common cancers such as breast and prostate cancer. However, it also found the same was true for more rare cancers, such as testicular, head and neck, melanoma, ovarian, and stomach cancers.

The researchers found that the heritability of cancer overall was 33%. Significant heritability was found for skin melanoma (58%), prostate cancer (57%), non-melanoma skin cancer (43%), ovarian cancer (39%), kidney cancer (38%), breast cancer (31%), and uterine cancer (27%).

The study was a collaboration among Harvard investigators and investigators at the University of Southern Denmark, and at the University of Helsinki. It was the first study to provide family risk estimates for these and other rarer cancers.

The study also showed that in twin pairs where both developed cancer, each twin often developed a different type of cancer. This suggested that in some families there is a shared increased risk of any type of cancer.

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Mucci said prior studies had provided familial risk and heritability estimates for the common cancers. However, she said for rarer cancers all the previous studies were too small or the follow-up time was too short.

The current study was based on data of both identical and fraternal twins in Denmark, Finland, Norway, and Sweden. All of the individuals participated in the Nordic Twin Study of Cancer and were followed over an average of 32 years between 1943 and 2010.