Here's a new guideline for women at high risk of breast cancer. It comes from the American Cancer Society, which recommends yearly screening using MRI, magnetic resonance imaging, in addition to mammography. A study published in the New England Journal of Medicine finds the MRI highly effective in finding cancer.

NPR's Patricia Neighmond reports.

PATRICIA NEIGHMOND: The new guidelines are directed only toward women at high risk of breast cancer. Robert Smith is an epidemiologist with the American Cancer Society.

Dr. ROBERT SMITH (Epidemiologist, American Cancer Society): These are going to be women with two or more close relatives. And by close, I mean first or second-degree relatives who've been diagnosed with breast cancer or breast and ovarian cancer. So, for example, this might be a mother and a maternal aunt or a paternal aunt and a paternal grandmother. This would also include women who had one or more first-degree relatives diagnosed pre-menopausally.

NEIGHMOND: Women can also undergo genetic testing to see if they have a defective gene that puts them at very high risk for breast cancer. It's recommended these women also get yearly MRI screening. So too for women who were exposed to radiation between the ages of 12 and 18; that's mostly women who were treated for Hodgkin's disease.

Epidemiologist Smith says the recommendations come after a series of studies showed MRI more effective than mammograms at finding cancer in these high-risk women. Radiologist Connie Lehman is a breast-imaging specialist at the University of Washington Medical Center. Lehman says mammography measures breast density and tissue distortion, but the MRI, she says, is more precise.

Dr. CONNIE LEHMAN (Radiologist, University of Washington Medical Center): MRI uses sophisticated computers, very strong magnets, radio waves to produce images of what is occurring within the breast tissue and most importantly how blood flow is behaving.

NEIGHMOND: Areas with increased blood flow can mean cancer cells are flourishing. Lehman recently headed a study with 969 women.

Dr. LEHMAN: They had had a recent breast cancer diagnosis. Our question was whether or not MRI would detect cancer in opposite breasts. We knew they had cancer in one breast. The question was did they have cancer in the other breast that mammography had missed?

NEIGHMOND: So all the women had a mammogram on the breast that was not diagnosed with cancer.

Dr. LEHMAN: We then performed MRI to see if we could find the cancer even though there was basically a clean bill of health from the mammogram and the clinical breast exam.

NEIGHMOND: After the MRI, Lehman found cancer in 30 women. That's three percent of the women in the study. That represents 10 times more cancer found than was found by mammogram. For these women, Lehman says, knowing about cancer in their second breast was important.

Dr. LEHMAN: Rather than to have surgery, possibly radiation and chemotherapy for the known cancer, only then to be told a year or three years or five years later another cancer has been found in the opposite breast. This enabled these women to have a full diagnosis of the cancers in both breasts at the same time, consolidating their treatment into one round of therapy.

NEIGHMOND: And for women who were considering double mastectomies as a preventive measure, Lehman says if the MRI found no cancer in the second breast, that can be reassuring information that could influence their decision. Researchers emphasize that for most women a yearly mammogram will detect cancer.

MRIs can make mistakes causing women who don't have cancer to undergo biopsy. In Lehman's study that was rare. MRIs are expensive, about a thousand dollars, most insurers review scientific evidence when deciding what to pay for. Epidemiologist Robert Smith expects most insurers to cover the cost of MRI screening for women at high risk of breast cancer.

Patty Neighmond, NPR News.

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