There’s a well-established medical technology that many physicians believe could help catch undiagnosed breast tumors in one-third to one-half of women – yet it’s not being used routinely. It’s ultrasound imaging, a longtime fixture of obstetricians’ offices. Some doctors use ultrasound along with traditional x-ray mammography in breast exams, but the majority have been waiting for clearer evidence of its benefits. And that’s exactly what a key clinical trial now under way in the United States could provide.

At least half of women under 50 and about a third of older women have naturally dense breast tissue, making it harder to distinguish between healthy tissue and questionable masses in mammograms. Ultrasound can get around that problem because the different types of tissue reflect sound waves differently, says Wendie Berg, the leader of the trial and an independent breast-imaging consultant in Lutherville, MD. “Ultrasound is widely available and relatively inexpensive,” she says. “It’s reasonable to consider it for routine screening.”

In past studies involving thousands of women, ultrasound did detect dozens of dangerous cancers missed by mammograms. But the studies didn’t adequately measure the reverse: the number of cancers detected by mammograms but missed by ultrasound.

The new trial will assess 2,808 women at 20 different U.S. and Canadian locations over three years and is designed to count “false negatives” accurately. All participants will get ultrasound exams in addition to their annual mammograms. A final verdict on the two technologies’ effectiveness isn’t expected until 2008.

Oncologists say ultrasound would only be a supplement, not a replacement, for x-ray mammography, since both methods sometimes miss tumors and misidentify healthy tissues as cancerous. But the two together, says Berg, could catch more cancers than mammography alone.