Harvard University researchers have developed an imaging contrast agent that targets a compound found in cancerous breast tissue. The researchers hope that the agent will help doctors identify breast cancer at an earlier stage, when treatment is easier, and that it will provide a better alternative for patients for whom mammography is not very effective. A test that uses the agent could also cut down on false positives.

The imaging agent emits a particular wavelength of near-infrared light when it binds to a compound called hydroxyapatite, a calcium salt that builds up in breast tissue as milk-producing cells die. These so-called microcalcifications are one of the things radiologists look for in mammograms to diagnose breast cancer.

Among women in the United States, breast cancer is the most common cancer and the second leading cause of cancer death. The U.S. Preventive Services Task Force recommends that women between the ages of 40 and 75 have mammograms every one or two years. Mammography is not recommended for younger women, whose breast tissue is denser and therefore yields images that are more difficult to interpret. Mammograms in younger women find fewer cancers and save fewer lives. Doctors also suspect that they are missing tumors in older women with denser breasts.

Screening with mammography has been very effective: breast-cancer death rates have been steadily declining since the 1990s. But John Frangioni, a physician and associate professor at Harvard Medical School, who led the research, thinks that targeted imaging agents could do better.

With agents like Frangioni's, "You know when you see it binding it's because of an accumulation of calcium," says A. Dean Sherry, a professor in the Advanced Imaging Research Center at the University of Texas Southwest Medical Center. "It tells you something specific about the biology."

The wavelength of light needed to activate the agent and the wavelength of light the agent emits fall within what researchers call the near-infrared window, an area of the spectrum where light is very good at passing through biological tissues.

The hydroxyapatite-targeting molecule is one of many near-infrared imaging agents being developed by Frangioni for breast-cancer screening. The advantage of near-infrared imaging is that "you can use different flavors for different processes," says Bruce Tromberg, professor of biomedical engineering and director of the Beckman Laser Institute at the University of California, Irvine. The idea is to use imaging to provide a molecular profile of a patient's cancer. Tromberg and Frangioni are developing imaging molecules that fluoresce in different areas of the spectrum, and each targets different cancer-associated compounds.