Over the past two or three decades, film-screen mammography (film mammography) has greatly aided the detection of early breast cancers. Annual screening of women age 40 and older has lowered breast cancer deaths by as much as 30 percent. Nonetheless, as many as one in five cancers still are overlooked because the mammographic changes may be very subtle. A fundamentally novel approach called full-field digital mammography (digital mammography) may be the logical next step for locating some of these cancers. In contrast to conventional film mammography, which uses radiographic (x-ray) film to acquire, store and display an image, digital mammography conveniently offers a means of separating these functions. Further, digital mammography has a number of attributes that may help a small breast tumor stand out from surrounding normal tissue: the efficient absorption of incident x-ray photons, a linear response over a broad range of radiation intensity, and comparatively little system "noise" (extraneous information).

How Digital Works

Like standard film mammography, digital mammography uses x-rays to produce images of breast tissue. The difference is that with digital mammography, an electronic x-ray detector—a phosphor screen—replaces the film cassette and converts the x-ray photons to light, which in turn passes through a fiberoptic cable to a device that converts the light to a digitized signal for display on a computer monitor. The radiologist can alter the orientation, magnification, brightness and contrast of the images as desired. After the first prototype digital mammography system was exhibited at the Radiological Society of North America's (RSNA) Scientific Assembly and Annual Meeting in 1995, numerous systems were devised and still are undergoing clinical testing in the United States. Several digital mammography systems have since been approved for clinical use by the Food and Drug Administration (FDA).

As with film mammography, optimal positioning and compression are critical in identifying a suspicious lesion. From the patient's point of view, having a digital mammography study is exactly like film mammography except for a shorter wait time to know if the images are satisfactory. Unlike film mammography, which requires the technologist to develop films in a darkroom where the wait time ranges from two to five minutes, with digital mammography the technologist will know within 30 seconds whether the images are satisfactory.

The question of image display is a key practical aspect of these methods. An efficient means of reading screening mammograms is vital for reasons of both cost containment and reader fatigue. Film has proved to be a convenient and time-efficient mode of display. Paradoxically, the valuable capability of digital mammography to separate the detection and display functions also creates a problem. Using a monitor and laser-printed film, it becomes more difficult to display gray levels without manipulating the images, which at this stage is inefficient. The interpretation time required by digital mammography may increase the radiologist's workload and decrease productivity. Some progress has been made in resolving the display problem by a process that compensates for tissue thickness.

How Well Does Digital Mammography Perform?

Preliminary results of the Digital Mammographic Imaging Screening Trial (DMIST), one of the largest cancer screening trials ever conducted by radiologists, were published September 16, 2005 by the New England Journal of Medicine and at a meeting of the American College of Radiology Imaging Network (ACRIN). This large-scale, multi-center clinical trial was designed to measure differences in diagnostic accuracy between digital mammography and film mammography, an x-ray examination used to screen for breast cancer.

DMIST results showed that, for the entire population of women studied, digital and film mammography had very similar screening accuracy. However, the study revealed that digital mammography was significantly better in screening women in three categories, such as those:

• under age 50 (no matter what level of breast tissue density they had).
• of any age with very dense or extremely dense breasts.
• of any age who are pre- or peri-menopausal (defined as women who had a final menstrual period within 12 months of their mammograms).

The study's results suggest that for women who fall into these three subgroups, digital mammography may be better than conventional film mammography at detecting breast cancer.

DMIST showed that there is no apparent benefit of digital mammography over film mammography for women who:

• are over age 50.
• do not have dense or very dense breast tissue.
• are no longer menstruating.

Women who fit these categories should continue to undergo screening mammography per the advice of their physician and the guidelines of national health organizations. (For more detailed information on the DMIST study, see "Results of Large Trial Reveal Certain Women Could Benefit from Digital Mammography.")

Potential Advantages of Digital Mammography

• Compared with the 10-15 minutes needed for conventional film mammography, digital mammography images are acquired in less than a minute. A faster exam may induce more women to comply with screening at recommended intervals, and it should make mammography facilities more efficient.
• Spot-view digital systems now are available to help guide breast biopsy by recording a snapshot of the area of interest. This permits more rapid and accurate needle placement, speeding the biopsy procedure and minimizing discomfort.
• The superior contrast resolution of digital mammography (especially in dense breast tissues) and its ability to manipulate images might in time make for more accurate detection of breast cancers.
• Digital mammography lends itself well to another innovation, computer-aided detection (CAD), which obtains a second, computerized reading in the hope of finding more cancers or more accurately gauging signs of malignancy.
• Computerized mammograms may be archived in various ways, easily retrieved, and copied exactly. Digital mammograms also can be transmitted from one location to another, permitting off-line interpretation and expert consultation (telemammography).
• Digital mammography offers the possibility of three-dimensional breast tomography using relatively low radiation doses.

Drawbacks and Limitations of Digital Mammography

• Until ways are found to increase the spatial resolution of digital mammography, conventional film mammography will remain better able to detect subtle tissue changes such as microcalcifications that may be a clue to early cancer. A dual-energy digital mammography technique may help with this problem.
• Especially in dense breast tissues, digital mammography may overlook some lesions that are masked by overlying structures. Dual-energy subtraction imaging has been proposed as a possible solution.
• The full potential benefit of digital mammography may not be realized until "soft copy" images can be interpreted on a user-friendly computer workstation.
• At present, digital mammography is considerably more costly than film mammography. The needed equipment is three to four times more expensive than that currently in place at more than 10,000 screening sites in the United States, meaning that converting to digital mammography will be a formidable enterprise. Reimbursement rates for film mammography, among the lowest for any imaging procedure, are not likely to provide the needed capital.