New guidelines from the American Cancer Society (ACS) recommend that women at very high risk for breast cancer receive magnetic resonance imaging (MRI) along with annual mammography, reports the October 2007 issue of Harvard Women’s Health Watch.

In one recent study comparing MRI, mammography, and ultrasound in 171 women whose lifetime risk of breast cancer was very high (20% or more), MRI proved best at finding cancer. Of the six cancers diagnosed, MRI detected all six, mammography only two, and ultrasound only one. Only MRI detected the four cancers found in women with dense breast tissue.

Even so, the ACS and other groups don’t recommend routine MRI screening for women at average risk, for reasons such as these:

It leads to too many unnecessary biopsies. MRI picks up any abnormal tissue, whether cancerous or not.

Mammography is getting better all the time. Digital mammography works better than traditional mammography in women with dense breast tissue and is becoming increasingly available.

MRI has limitations. It can’t be performed on women with pacemakers or implantable cardioverter-defibrillators, and its ability to detect some early cancers is limited. It also isn’t widely available.

It’s expensive. And insurance won’t pay for it for women at average risk.

It’s unclear whether MRI helps women live longer. The most important outcome of any screening test is improved survival. We don’t know yet if using MRI helps women live longer.

For most women over 40, having an annual mammogram and clinical breast exam is still the best way to catch early-stage, highly treatable cancers, says Harvard Women’s Health Watch.