Cancer risks in twins: Results from the Swedish family‐cancer database

Abstract

Twin studies on cancer have addressed two general questions, one about the possible carcinogenic effects of twinning and the second about heritable effects of cancer. The first question is answered by comparing the occurrence of cancer in twins to that in singletons; the second is answered in probandwise analysis of monozygotic twins compared to dizygotic twins or siblings. We used the nationwide Swedish Family-Cancer Database on 10.2 million individuals and 62,574 0 -66-year-old twins to calculate standardized incidence ratios (SIRs) and 95% confidence intervals (CIs) for all main cancer compared to cancer in singletons. In probandwise analysis, the SIR was calculated for the co-twin of an affected twin. The overall risk of cancer in same or opposite sex twins was at the level of the risk for singletons. Testicular cancer was increased among same sex twins and all twins to an SIR of 1.43. Melanoma was decreased in these groups of twins to an SIR of 0.84. Some other cancer sites were increased or decreased in some groups of twins, but none in all twins. The SIR of breast cancer was 1.01 and 1.04 in same and opposite sex twins, respectively. Probandwise analysis showed increased risks for Hodgkin's disease in males and breast cancer and childhood acute lymphoid leukemia among females. The data on this unselected population of twins suggest that twinning per se is not a risk factor of cancer. However, because twins are smaller than singletons at birth, some possible effects could be masked by such differences. In utero hormonal exposures may be related to the risk of testicular cancer. The protective effects in melanoma may be due to socioeconomic factors.

Childhood and Adult Cancer in Twins: Evidence from the Utah Genealogy

“…We found, as have several others (4,6,7), that there is no reduction in overall risk of adult cancer among twins, although not all studies are in agreement, with some finding a small deficit in adult cancer incidence (5,30). Given the heterogeneity of adult cancers, however, such a grouping is unlikely to detect real differences in risk, particularly for rare cancers.…”

Models of genetic susceptibility to breast cancer

“…In this study, they estimated the breast cancer incidence to be constant and approximately 1.3% per year, independent of age and duration of follow-up. More recently, Hemminki and Li (2002) have found this incidence rate to decrease with time since the diagnosis of the first twin, but the numbers in this study were small and therefore not necessarily inconsistent. Peto and Mack also note that this incidence rate is about twice the incidence rate for contralateral breast cancer (which are also roughly constant with time), consistent with the hypothesis that the high rate of contralateral breast cancer chiefly reflects genetic susceptibility (but with only one breast being at risk).…”

Birth weight and melanoma risk: a population-based case–control study

“…In two previous studies, we observed a lower incidence of melanoma in twins as compared to singletons (Iversen et al, 2001;Franco-Lie et al, 2005) as have others (Hemminki and Li, 2002;Hemminki and Chen, 2005;Neale et al, 2005), but for which no biological explanation is evident.…”