Breast cancer risk in male twins: joint analyses of four twin cohorts in Denmark, Finland, Sweden and the United States.
Whiteman DC., Murphy MF., Verkasalo PK., Page WF., Floderus B., Skytthe A., Holm NV.
To test the hypothesis that in utero exposure to high levels of oestrogen increases the risk of male breast cancer, we followed 115 235 male twins for more than 3.5 million person-years at risk. We observed 11 cases of male breast cancer versus 16.16 expected based on national rates (standardized rate ratio 0.68, 95% confidence interval 0.34-1.22) and conclude that any adverse influence of in utero oestrogen exposure is likely to be small.