Twins carry a significant excess familial risk for cancer overall and for specific types of cancer, including prostate, melanoma, breast, ovary, and uterus, according to a study published in JAMA.1

An international team of researchers sought to estimate the familial risk and heritability of cancer types in a large twin cohort. The prospective study included 80,309 monozygotic and 123,382 same-sex dizygotic twin individuals (N=203,691) within the population-based registers of Denmark, Norway, Sweden, and Finland. The researchers followed the twins for a median of 32 years between 1943 and 2010. Within the study group, 50 990 individual persons died of any cause, and 3804 emigrated and were lost to follow-up.

The researchers used time-to-event analyses to estimate risk of cancer in a person given a twin’s diagnosis of cancer (familial risk) and the proportion of variance in cancer risk due to interindividual genetic differences (heritability). Follow-up was via cancer registries.

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Study findings showed 27 156 incident cancers diagnosed in 23 980 individuals; this translates to a 32% cumulative incidence of cancer. Cancer diagnoses were made in both twins in 1383 monozygotic pairs and 1933 dizygotic pairs, with diagnoses of the same cancer type in 38% of monozygotic and 26% of dizygotic pairs.

Excess cancer risk was seen in twins whose co-twin had a cancer diagnosis, with estimated cumulative risks being an absolute 5% higher in dizygotic twins and an absolute 14% higher in monozygotic twins whose twin also developed cancer compared with the cumulative risk in the overall cohort (32%).

The researchers report monozygotic twins have higher cumulative risks and significant familial risk compared with dizygotic twins. Overall heritability was 33%. Significant heritability was seen for skin melanoma, prostate, nonmelanoma skin, ovarian, kidney, breast, and corpus uteri cancers.


1. Mucci LA, Hjelmborg JB, Harris JR, et al. Familial risk and heritability of cancer among twins in Nordic countries. JAMA. 2016;315(1):68-76. Accessed January 6, 2016.