Cancer: Equal Opportunity for All

Knowing the origins of cancer help to understand how it can be treated, and in some cases, how it can be prevented.  We know cancer can be caused by exposure to harmful environments or by genes gone haywire.  However, only an estimated 5-10% of cancers have been found to have genetic components.  This would leave the other 90% of cancer risk to be attributed to the outside world.  But, cancer seems to develop in places that we do not expect, where there is less exposure to the environment, or where there is not a direct genetic link to a greater risk of developing cancer.  Why is this?