For testicular cancer the risk is 75 times that of the general population for men under 35 . The risk for breast cancer is 35 times higher than average for women under 35 .
The study by researchers led by Professor Tony Swerdlow at the London School of Hygiene and Tropical Medicine confirmed a significant genetic element in both diseases. It also showed that high levels of oestrogen present in the womb during twinning probably increases the risk of both cancers.
This was supported by the fact that more non-identical than identical twins get the diseases, pointing to a non-genetic risk factor. Non-identical twins have a placenta each, resulting in higher levels of oestrogen whereas identical twins share a placenta, so oestrogen levels are lower.
Twins occur in about one in every 84 births in Britain and about one in 300 pregnancies results in identical twins. Multiple births may occur as a result of the almost simultaneous release and fertilisation of more than one ovum, which results in non-identical twins, or from the fertilisation of a single ovum which then separates into two parts, creating two identical embryos.
The increased risk for identical twins is only seen in families where one twin has already had either breast or testicular cancer. In that case the other twin has a 29 per cent chance of developing breast cancer before she is 40, and, for men, a 14 per cent chance of developing testicular cancer before the age of 40. These young twins constitute an "extremely high-risk group, identifiable without genetic testing", said the researchers in a paper published in the Lancet.
Theoretically, young women at high risk of breast cancer could easily be identified by asking if they are one of a pair of identical twins, with a twin sister who has already had the disease.
Data for the research was obtained by scouring national cancer registration records from 1971 to 1989. The researchers identified 500 individual twins with breast cancer and 194 with testicular cancer.Reuse content