New technology helps see through breast tissue that can hide cancers.
In recent years, advanced imaging technologies, such as magnetic resonance imaging (MRI), ultrasound, and digital imaging have been used in breast cancer screening. But it hasn't been clear whether any of these techniques offered a better way of spotting breast cancer than standard mammography. While new technologies can help improve image quality or make diagnosis more precise, they haven't replaced traditional mammography.
However, that could change, in light of results from the Digital Mammographic Imaging Screening Trial (DMIST), a large clinical study of digital versus traditional mammography. The DMIST trial was funded by the National Cancer Institute and led by investigators at the University of North Carolina at Chapel Hill. Researchers in the United States and Canada used both technologies to examine the breasts of nearly 50,000 women, ages 47–62. For the group as a whole, digital was neither better nor worse than standard mammography. But in women most likely to have dense breasts, digital did a better job of locating breast cancers. The findings were published in the New England Journal of Medicine (October 27, 2005) and online at www.nejm.org.
Results and implications
The authors reported that digital imaging improved cancer detection by 15% in women under age 50 and in those nearing menopause — as well as by 11% in women of any age with dense breasts. Dense breast tissue has less fat and more glandular and connective tissue. High breast density increases the risk of breast cancer, although the reasons aren't entirely clear. On a mammogram, cancer may be obscured by dense breast tissue. Such tissue may also be more vulnerable to malignancy — a possibility researchers are exploring. Although dense breast tissue is more common in younger women, 30%–40% of women over age 50 also have dense breasts.
One of the chief advantages of digital over standard mammography is that radiologists can fine-tune images so that tiny abnormalities stand out better when breast density is an issue (see "Why digital mammograms help"). Older breasts, which tend to have less glandular tissue, don't pose the same challenge. Indeed, the DMIST study found that digital imaging was no more accurate, overall, than standard mammography in detecting breast cancer in women over age 50 and in those who don't have dense breasts or who are no longer menstruating.
Why digital mammograms help
On mammograms, fat looks dark gray, and breast tissue, which is denser, is white. Abnormalities, such as microcalcifications and lumps, also appear white, making it difficult to distinguish them from the surrounding tissue. In the digital image (left), a cancerous mass can be seen as solid white (boxed in red), just behind the nipple. The tumor is harder to spot on a standard mammogram (right) of the same breast.
These results don't mean that every woman should rush out to get a digital mammogram. For one thing, digital mammography is not yet widely available. Digital systems are far more expensive than standard mammography equipment; they also require special training on the part of radiologists and technologists. Ultrasound, widely available, can be added to standard mammography to help answer questions raised by a suspicious mammogram.
But many institutions are moving toward digital systems, in part because of the technical advantages. Digital images are easier to store, and they can be transmitted quickly — for example, to get second opinions or to transfer records. They may also reduce callbacks for further imaging. And to the extent that digital mammography better reveals abnormalities in dense breast tissue, it will make for another strong incentive to go digital.
More studies of digital imaging in large populations should help clarify the benefits of this technology. For now, there are two important messages: First, regular mammograms for women age 40 and over matter. Whether digital or on film, when read by a well-trained eye, standard mammograms help find breast cancers early. Second, for any woman with dense breast tissue, digital mammography is an option worth investigating.
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