Doubts cast on digital detection rates

Digital mammography provides no clear-cut improvements over traditional film X-rays in the ability to detect cases of breast cancer, says a new technology assessment report. The technology does, however, offer other benefits, including slightly lower radiation doses for patients, which may prompt its increasing use despite its much higher costs.

Known as "full-field digital mammography," the high-tech equipment records images of the breast electronically instead of on X-ray film. The pictures are stored directly in a computer system, where they can be enhanced, magnified or shared. The digital devices were first approved for marketing in the United States in 2000.

Three large trials of the technology have been completed, with several thousand women receiving digital mammograms in each. No statistically significant differences in detection rates were observed between the x-ray and the new digital technology. The patients in each group experienced identical positioning and compression of the breast.

"The studies that have been done so far haven't provided a 'slam-dunk' for full-field digital mammography," says Robert Maliff, associate director of the Health Systems Group at ECRI, whose Technology Assessment Group published the report. ECRI is a non-profit health services research agency that produces systematic evidence reviews on medical devices, drugs, biotechnologies, procedures, and behavioural health services.

Maliff says radiation for patients may be "incrementally lower" with digital screening depending on the type of detector used, but the report did not assess the literature on radiation doses -- rather it assessed its efficacy as a screening technology for breast cancer.

A new large clinical trial sponsored by the National Cancer Institute has recently been completed, and published results are expected soon. Nearly 50,000 women in the United States and Canada received both types of mammograms and were asked to return one year later for follow-up exams to determine whether digital mammography detects breast cancer more accurately than X-rays.

At best, the new technology is expected to offer incremental improvements in cancer detection rates, which Maliff says may be enough to spark a large-scale transition to the new technology.

"Cost-effectiveness will ultimately determine whether full-field digital mammography technology is adopted, since hospitals must justify their purchase based on exam volume and patient population," the report says.

Digital mammography systems cost at least three times as much as film systems. In addition to reducing radiation, they can also facilitate the use of computer-aided detection systems and can be easily transmitted for second opinions, according to the report. The technology lends itself to operational improvements. Without films to process, handle, and store, radiology workflow becomes more efficient, says Maliff.

"Many health care professionals predict that screen-film mammography will eventually be replaced by full-field digital mammography once it is proven that the image quality and diagnostic accuracy of both technologies are at least equivalent," says the ECRI report.

According to ECRI, the current body of evidence that suggests that digital mammography may be able to reduce recall rates, and possibly necessary follow-up procedures, which could offset some of its higher initial costs. But more evidence is needed to confirm these potential benefits.

Maliff predicts some facilities may "take the leap" to digital mammography for marketing reasons to position themselves as offering the best women's care services.

Breast cancer is the most common malignancy in women and the third most frequent cancer throughout the world. More than one million new cases were diagnosed worldwide in 2002, and incidence rates are increasing at about five percent a year.