A new system nimbly finds tumors masked by dense breast tissue that mammograms ordinarily can't spot - and at half the cost and with a vastly lower rate of false positives.

The technique, known as Molecular Breast Imaging (MBI), is especially targeted at the nearly one-fourth of women above 40 who have dense breast tissue, which allows malignancies to lie hidden until it's too late.

What's done with MBI is simply to inject a radioactive tracer into the breast. It's absorbed by the cancer cells, and the resulting "glow" is picked up by special cameras. The body excretes the tracer within a day.

Recent research done at the Mayo Clinic on 940 women at high risk for breast cancer discovered 13 tumors in 12 women. Of these, MBI detected 10 of them, while mammograms detected only three. Two of the tumors were spotted by both MBI and mammography, leaving two undetected. The latter two came to light later, with the passage of time.

"We all know that mammography is, in and of itself, an imperfect tool, and we clearly need to do better in the future," said Dr. Eric Winer of the Dana-Farber Cancer Center in Boston, a spokesman for the study.

In addition to mammography's poor track record in dense-breast-tissue women, the procedure is plagued with false positives, leading to many fruitless biopsies.

Breast cancer is the second-most common cancer in the world. (The most widespread is lung cancer.) Cancer of the breast is the world's fifth-most common cause of cancer mortality.

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