Risk of Sex-Specific Cancers in Opposite-Sex and Same-Sex Twins in Denmark and Sweden.
BACKGROUND
Increasing evidence shows that some cancers originate in utero. It is hypothesized that elevated exposure to some steroid hormones might increase cancer risk and that hormone transfer between twin fetuses could result in different prenatal exposure to testosterone.


METHODS
This large-scale prospective twin study compared opposite-sex (OS) and same-sex (SS) twins to test the impact of intrauterine exposures on cancer risk. On the basis of the Danish and Swedish twin and cancer registries, we calculated incidence rate ratios for OS and SS twins, whereas standardized incidence ratios (SIR) with 95% confidence intervals (CI) were calculated for OS/SS twins compared with the general population.


RESULTS
A total of 18,001 cancers were identified during 1943-2009. No significant differences were observed between OS and SS twins, neither for the sex-specific cancers nor for cancer at all sites. All-cause cancer was slightly reduced for OS and SS twins compared with the general population, significant for OS males (SIR, 0.95; 95% CI, 0.92-0.98) and for SS males and females (SIR, 0.97; 95% CI, 0.94-0.99).


CONCLUSIONS
Our data suggest that having a male co-twin-which may entail higher exposure to prenatal testosterone-does not increase the risk of sex-specific cancers in OS females. Furthermore, the study supports that twinning per se is not a risk factor of cancer.


CONCLUSIONS
Findings are reassuring, as they fail to provide evidence for the hypothesis that endocrine or other difference in the in utero milieu affects the risk of sex-specific cancers.
The Danish Twin Registry, Department of Public Health, University of Southern Denmark, Odense C, Denmark. lahrenfeldt@health.sdu.dk.
Linda J
LJ
Ahrenfeldt
The Danish Twin Registry, Department of Public Health, University of Southern Denmark, Odense C, Denmark.
Axel
A
Skytthe
The Danish Twin Registry, Department of Public Health, University of Southern Denmark, Odense C, Denmark.
Sören
S
Möller
Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden.
Kamila
K
Czene
Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden. Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, Massachusetts.
Hans-Olov
HO
Adami
Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, Massachusetts.
Lorelei A
LA
Mucci
Department of Public Health and Institute for Molecular Medicine, University of Helsinki, Helsinki, Finland. Department of Health, National Institute for Health and Welfare, Helsinki, Finland.
Jaakko
J
Kaprio
The Danish Twin Registry, Department of Public Health, University of Southern Denmark, Odense C, Denmark.
Inge
I
Petersen
The Danish Twin Registry, Department of Public Health, University of Southern Denmark, Odense C, Denmark. Department of Clinical Biochemistry and Pharmacology and Department of Clinical Genetics, Odense University Hospital, Odense, Denmark.
Kaare
K
Christensen
The Danish Twin Registry, Department of Public Health, University of Southern Denmark, Odense C, Denmark. Max-Planck Odense Center on the Biodemography of Aging, University of Southern Denmark, Odense C, Denmark.
Rune
R
Lindahl-Jacobsen
D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D003718:Denmark / Q000453:epidemiology; D004200:Diseases in Twins / Q000453:epidemiology*; D005260:Female; D005544:Forecasting; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008875:Middle Aged; D009369:Neoplasms / Q000453:epidemiology*; D011159:Population Surveillance / Q000379:methods*; D011446:Prospective Studies; D012307:Risk Factors; D012737:Sex Factors; D013548:Sweden / Q000453:epidemiology; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D055815:Young Adult