Crikey, no one can be positive on what causes cancer... loads of environmental issues and nutitional etc....
Myself and another person I know got breast cancer after using a pump for a period of 5 years... could it have been pump novorapid constantly being fed in? Who knows? There is some questions on the net about this angle too.
Hen you have the oestrogen/progesterone to think about etc.... anal cancer is found more in gay men apparently?
There is no definitive cause of cancer... even injecting mice does not really link to humans.. you only have to look at those en that got so ill doing research tests a few years back.