Our recommendation is that everybody screen at 40 because your biggest risk is being female. Then the second biggest risk is getting older. It's very difficult to predict.
There are some people you have a better chance of predicting, such as if they have a very strong family history or if they've had radiation to the chest before 30. There are some people for whom you know you're going to have to work harder and get them into MRI. You're going to have to dig a little deeper and make sure that you really screen them hard.
However, the average woman walking around without a family history is still at risk for breast cancer, and because that is the vast majority of women, that is also the vast majority of breast cancers.