In one study, an experimental diagnostic test called molecular breast imaging (MBI) revealed more tumors and gave fewer false positives than mammograms, researchers reported at the American Society of Clinical Oncology conference in Washington, D.C. The test shows promise for detecting cancer in dense breast tissue, which makes tumors harder to find on mammogram. An estimated 25% of women over age 40 have dense breasts.

With MBI, women receive an I.V. dose of a short- acting radioactive tracer that abnormal cells absorb more readily than healthy cells. Gamma cameras detect the telltale glow.

Researchers compared MBI and mammography on 940 women with dense breasts and a high risk of cancer. They found 13 tumors in 12 women. MBI found 10 out of the 13 tumors; mammography only 3. The rate of false positives was 9% for mammography versus 7% for MBI.

One drawback of MBI is that it exposes the patient to 8 to 10 times more radiation than mammography, although researchers are working on ways to reduce the radiation dose. The Mayo Clinic (Rochester, Minn.) is leading a study to evaluate how MBI stacks up against magnetic resonance imaging.