Breast MRIs haven’t been studied in the general population. So, what constitutes a woman at a high-risk for breast cancer, thus warranting the screening breast MRI?

“The only way to get into that [MRI screening] bracket is to have a significant family history of breast cancer,” Russell says.

That means more than just having a relative with breast cancer. A woman would be considered at high risk if at least two women in her family “” her mother, sisters or daughters “” have breast cancer or a combination of breast cancer and ovarian cancer before age 50.

A woman would also be considered high risk if she carries certain mutations in her BRCA1 or BRCA2 genes. Mutations in these genes (or other as yet unidentified genes that confer high risk of early breast cancer) cause less than 10% of breast cancer cases. But 65% of women with BRCA1 mutations, and 45% of women with BRCA2 mutations, have breast cancer by age 70.

Of all U.S. women between the ages of 30 and 70, the number of women the new guidelines affect might be between 1 million and 1.5 million women, Russell says. They should consider having an MRI in conjunction with their yearly mammograms.