Two other commonly used imaging techniques—magnetic resonance imaging (MRI) and ultrasound—often supplement mammography to detect breast cancer but are not yet reliable enough to be used by themselves for screening. MRI uses magnetism and radio waves to measure differences in the water content of tissue, which provides more detail about differences in the composition of breast tissue than do x-rays. But because a benign cyst often looks like a tumor on an MRI, screening with MRI also increases the rate of false positives. For that reason, the American Cancer Society recommends annual MRI screenings only in women with a strong family history of breast or ovarian cancer. In addition, breast MRIs are too expensive for routine use in the general population, running $2,000 to $6,000 a test compared with a few hundred dollars for a mammogram.
Ultrasound relies on high-frequency sound waves to characterize the internal structures of the breast. Unlike MRI, it can determine if a lump is a harmless, fluid-filled cyst. But its images cannot distinguish malignant tumors from benign growths filled with harmless breast tissue. It thus gives more false negatives than mammography does, which is why it is less than ideal as a stand-alone method for initial screening.
Researchers, in their quest for better solutions, are experimenting with new twists on an age-old method: feeling for lumps. Tumors are stiffer than healthy breast tissue—which is why they feel different. The problem with waiting until a tumor has grown large enough to be felt, however, is that the delay increases the odds that the cancer will have spread by the time it is diagnosed. The goal is to create ways of detecting that stiffness while a tumor is still too small to be felt by human hands.
One method for measuring such stiffness depends on a combination of low-frequency sound waves and MRI. Dubbed magnetic resonance elastography, the technique was first developed more than a decade ago by Richard Ehman, a radiologist at the Mayo Clinic. Currently applied to the diagnosis of nonmalignant liver diseases, the approach is now being tested on breast tumors. Patients are scanned in an MRI while sound waves with a frequency of 60 hertz pass through plastic tubes to the breasts. The MRI picks up tiny variations in how tissue is moved by the sound waves. Ehman says his team has gotten pretty good at distinguishing between normal and malignant breast tissue based on the stiffness revealed by the MRI. But for elastography to work as a population-wide screening tool, the cost of using an MRI would have to be much lower.
Another technique for measuring tissue stiffness dispenses with the expense and claustrophobia of an MRI. Bruce Tromberg, a biomedical engineering professor at the University of California, Irvine, has built a handheld scanner that sends near-infrared light through the skin and into the breasts and then measures how the light energy scatters on its path through the body before it gets reflected back to the scanner. The light photons travel differently through tumors than through normal tissue. The experimental device is being tested in cancer patients to see whether it can be tuned to accurately measure tumor shrinkage in response to therapy. Tromberg hopes, though, that the technology can eventually be refined enough to spot malignancies while they are still microscopic.
A simpler screen, like a blood test, would be ideal. Physicians do have blood tests that detect the recurrence of breast and ovarian cancer, such as by measuring a molecule called CA125, but they are not accurate enough for large-scale screening. Several researchers, however, are tracking chemical markers in the blood and in the breath with the goal of creating screening tools that could someday not just find cancer but also indicate how dangerous it is likely to be.
Emory’s Gabram-Mendola and chemists at the Georgia Institute of Technology have found markers in the breath of 20 breast cancer patients that were not present in the breath of 20 control subjects. The team is not focusing on fully replacing mammography but on saving it to use as a follow-up tool in places where resources are scarce. “There is a huge need to come up with something that can be used in countries where mammography is less available,” says Charlene Bayer, who is leading the effort at Georgia Tech. A breath test performed in a doctor’s office could also have great appeal for the many women in developed countries who shun screening mammograms because of the discomfort and inconvenience.