Genetic and Shared Environmental Influences on Cancer Risk and Cross-Cancer Associations in Nordic Twins

The study analyzed data from 222,530 same-sex twins from Denmark, Finland, Norway, and Sweden with a total of 43,060 cancer cases during a median follow-up of 41.6 years. Lifetime cancer risk was 36.5%, increasing to 51.4% in monozygotic twins and 45.3% in dizygotic twins with an affected co-twin. Overall cancer risk was explained by heritable (28%) and shared environmental (40%) influences. Heritability was highest for prostate cancer (42%), non-melanoma skin cancer (24%), and breast cancer (18%). Analyses revealed extensive genetic and environmental overlap between different cancer types, indicating shared genetic and environmental risk factors. The findings emphasize the importance of incorporating familial shared environmental exposures into cancer risk prediction and prevention strategies.