In brief

MRI a valuable add-on to mammography for some women

(This article was first printed in the October, 2004 issue of the Harvard Women’s Health Watch. For more information or to order, please go to http://health.harvard.edu/women.)

Women at high risk for breast cancer may benefit from adding magnetic resonance imaging (MRI) to their regular screening mammograms, according to a study in the July 29, 2004, New England Journal of Medicine. The research suggests that in women with an increased genetic or hereditary risk for the disease, MRI scans find cancers that mammograms miss. However, experts caution that the technique’s drawbacks make it an unacceptable screening approach for most women.

The average American woman has about a 14% lifetime chance of developing breast cancer. The study, conducted at several cancer centers in the Netherlands, focused on women at higher risk, such as those with a family history of breast cancer or a gene mutation that predisposed them to it. For example, women with BRCA1 or BRCA2 gene mutations have a 50%–85% lifetime chance of getting breast cancer. Researchers followed 1,909 women for three years. Out of 45 breast cancers, MRI found 32 — of these, 22 were not seen on a mammogram — and missed 13. Mammography caught 18 breast cancers (8 were not visible with MRI) and missed 27.

It’s not entirely clear why MRI picks up more breast cancers than mammography in high-risk women, but there are several possibilities. Breast cancer often occurs in these women at a younger age, when their breasts are denser. (In the study, 75% of the women were premenopausal.) Certain abnormalities in dense breast tissue are harder to spot on a mammogram than with MRI. But breast density is only part of the story, says Dr. Darrell Smith, a radiologist at Harvard-affiliated Brigham and Women’s Hospital in Boston. Hereditary breast cancers may have other features, such as a less conspicuous appearance or a rapid growth rate, that hamper mammography’s effectiveness.

MRI scanning has its own drawbacks. Compared to mammography, it produces a greater number of unclear or false-positive results, leading to needless examinations and biopsies. This is one reason MRI is not recommended for routine screening of women at average risk for breast cancer. In high-risk women, such as those with BRCA mutations, the chance of finding breast cancer is much greater, so it may be worth the risk of a false positive occurring.

But MRI alone is not the best option for screening, says Dr. Smith, even in high-risk women. It typically misses microcalcifications, tiny specks of calcium that are often the only sign of ductal carcinoma in situ, or pre-invasive breast cancer. Mammography, on the other hand, can often find microcalcifications, even in dense breasts. Dr. Smith recommends that women with BRCA gene mutations have annual MRI scans in addition to their mammograms. Women with such mutations should start yearly mammograms at age 25–30.

Women at average risk for breast cancer should continue to schedule regular mammograms every year, usually starting at age 40.