Good question...I know there are a lot of factors they consider, including family history and known BRCA gene mutation. I stumbled across a cool website part of Johns Hopkins University, ask the expert. You can look at the scores of questions by topic or post your own. Mine got answered within a couple hours.
more specifically, try http://www.hopkinsmedicine.org/breastcenter/services/ask_expert/
I am impressed with the quality of answers....