Aradioactive tracer that "lights up" cancer hiding inside dense breasts showed promise in its first big test against mammograms, revealing more tumors and giving fewer false alarms, doctors reported last week.

The experimental method -- molecular breast imaging, or MBI -- would not replace mammograms for women at average risk of the disease.

But it might become an additional tool for higher-risk women with a lot of dense tissue that makes tumors hard to spot on mammograms, and it could be done at less cost than an MRI, or magnetic resonance imaging. About one-fourth of women 40 and older have dense breasts.

"MBI is a promising technology" that is already in advanced testing, said Carrie Hruska, a biomedical engineer at the Mayo Clinic in Rochester, Minn., which has been working on it for six years.

Mammograms -- a type of X-ray -- are the chief way now to check for breast cancer. MBI uses radiation, too, but in a different way. Women are given an intravenous dose of a short-acting tracer that is absorbed more by abnormal cells than healthy ones. Special cameras collect the "glow" these cells give off, and doctors look at the picture to spot tumors.

Researchers tried both methods on 940 women who had dense breasts and a high risk of cancer because of family history, bad genes or other reasons.

Thirteen tumors were found in 12 women -- eight by MBI alone, one by mammography alone, two by both methods and two by neither.

Looked at another way, MBI found 10 out of 13 tumors, missing three; mammograms detected three out of 13 tumors and missed 10. Using both methods, 11 out of 13 tumors would have been detected.

"These images are quite striking. You can see how the cancers would be hidden on the mammograms," Hruska said.

Mammograms gave false alarms -- led doctors to conclude that cancer was present when it was not -- in about 9 percent of patients, compared to only 7 percent for MBI. The MBI tests led to more biopsies than mammograms did, but they more often revealed cancer.

The Susan G. Komen for the Cure foundation and Bristol-Myers Squibb, which makes the imaging agent used in the study, paid for the work.

The next test will be to see how MBI stacks up against MRI. The federal government is paying for a new study Mayo is leading that compares the two in 120 high-risk women with dense breasts.

MRI is often used now for women with dense breasts, but it gives many false alarms that lead to unnecessary biopsies. Doctors hope MBI will prove more accurate and cost less -- under $500 versus more than $1,000 for an MRI.

One drawback of MBI: It uses about eight to 10 times the radiation of mammograms, a dose that engineers like Hruska are trying to lower with newer technology. Other medical centers are also testing MBI.

"We're just beginning to see what this technology can do," she said.

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