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Breast Cancer Risk in Monozygotic and Dizygotic Female Twins: A 20-Year Population-based Cohort Study in Finland from 1976 to 1995¹

Department of Public Health, University of Helsinki, FIN-00014 Helsinki [P. K. V., J. K.]; Department of Environmental Sciences, National Public Health Institute, FIN-70701 Kuopio [P. K. V.]; Department of Mental Health and Alcohol Research, National Public Health Institute, 00300 Helsinki [J. K.]; Finnish Cancer Registry, FIN-00170 Helsinki [E. P.]; and Department of Public Health, University of Turku, FIN-20520 Turku [M. K.], Finland

This population-based study investigated the occurrence of breast cancer over a 20-year period in a cohort of monozygotic (MZ) and dizygotic (DZ) twins in Finland. Altogether, 13,176 female twins of known zygosity who were living in Finland at the end of 1975 were identified from the Finnish Twin Cohort Study and followed-up for cancer through the Finnish Cancer Registry for the years 1976–1995. Standardized incidence ratios (SIRs) were calculated, based on national cancer incidence rates. The relative risk of breast cancer for MZ twins compared to DZ twins was decreased [SIR_MZ/SIR_DZ ratio = 0.78; 95% confidence interval (CI), 0.58–1.0]; the decreased risk for MZ twins (SIR = 0.76; 95% CI, 0.58–1.0) accounted for this result, whereas the risk for DZ twins did not differ from the general population risk (SIR = 0.98; 95% CI, 0.84–1.1). There was no risk decrease among MZ twins in other cancers related to reproductive behavior; i.e., number of children and age at first birth seem not to explain the decreased risk of breast cancer. Our results, which are in line with earlier studies on the same topic, suggest that prenatal influences or postnatal behavioral factors may protect MZ female twins from breast cancer.

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