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Two years after the first full-field digital mammography (FFDM) system received US Food and Drug Administration (FDA) approval, research inquiring into both the clinical and practical implications of this new technology is beginning to mount.
So far, two systems have been approved by the FDA, both of which are also approved for soft-copy reading. Both of the systems employ amorphous silicon detectors. A third system may soon be added to the list: one vendor is conducting trials in Europe using a selenium detector-based FFDM system. A fourth system utilizing a charge-coupled device detector just received FDA premarket approval.
With an average cost of $400,000 to $550,000 for one digital system (compared to its $80,000 to $150,000 film counterpart), there may be more than a few skeptics as to the sliding scale of advantages vs disadvantages of switching to digital mammography. When Decisions in Axis Imaging News spoke to several professionals working within the digital breast imaging field, however, all stated that besides the cost and possible learning curve factor, digital mammography clearly demonstrates a better technique for recording radiographic images. Such benefits include a lower radiation dose, fewer false-positives, ability to manipulate the information contained in the data set, improved access to prior images, and increased efficiency associated with the use of digital mammography.
DMIST Under Way
Several studies have been published that suggest how digital mammography will improve conventional mammography methods and more are ongoing. The largest study currently under way is the Digital Mammographic Imaging Screening Trial (DMIST), which began shortly after the National Cancer Institute approved a grant for $26.3 million in the latter part of 2001 to the American College of Radiology Imaging Network (ACRIN). The trial involves 49,500 women in the United States and Canada, and it will compare digital mammography to standard film mammography by screening patients with both methods. Suspicious lesions from either study will undergo additional workup and the follow-up of all other patients will be at 1 year.
“The primary aim,” says Edward Staabe, MD, chief of the Diagnostic Imaging Branch of the Biomedical Imaging Program, National Cancer Institute, Rockville, Md, “is to compare the diagnostic performance of digital mammography and screen film mammography as measured by the area under the receiver operating curve (ROC), sensitivity, specificity, and positive and negative predictive values in a prospectively enrolled screening cohort of asymptomatic women across all digital mammography machines.”
Other aims of the trial, according to Staabe, include retrospective reader studies to determine diagnostic accuracy of individual machines; to assess effects of patient characteristics; to study the effects of technical parameters; to model cost-effectiveness; to assess the impact of reduced false-positives; to analyze retrospective reader studies to compare soft copy versus film; to determine differences in image quality and radiation dose among the sites; and to assess temporal variations in a number of parameters.
“This study represents ACRIN’s most ambitious undertaking, and certainly the most important,” Bruce J. Hillman, MD, ACRIN network chair, has said. “The results of this trial will guide women’s breast care into the future.”
Exploring The Benefits
Etta D. Pisano, MD, the study’s principal investigator, says that one possible advantage of digital mammography is that it may be more effective in detecting cancers in women with dense breasts because it has improved contrast resolution. In addition, Pisano says, two smaller previous studies have indicated that digital mammography may result in fewer women called back for work-up of suspicious breast lesions.
One such study1 was conducted by John Lewin, MD, and several associates from the Department of Radiology at the University of Colorado Health Sciences Center, Denver. At two institutions, 4,945 digital and film examinations were performed in women 40 years of age and older. Two views of each breast were acquired with each modality and FFDM images were interpreted independently by soft-copy read. Although the study concluded that there was no difference in cancer detection rate, research did indicate that digital mammography had a significantly lower recall rate.
When asked what attributed to the lower recall rate, Lewin says he believes that it was a combination of the display and the ability to manipulate the image. Not only does digital mammography increase the field of display, it allows radiologists to magnify or zoom in on one particular area and enhance the image by lightening or darkening the view before it is printed.
In a study to determine the rates and causes of disagreements in interpretation between digital and film mammography in a diagnostic setting, Venta et al2 found that the most significant difference between the two was interobserver variability. Three views in both digital and film mammography were obtained, and radiologists at the Lynn Sage Comprehensive Breast Center of Northwestern University Medical School, Chicago, independently assigned a Breast Imaging Reporting and Data System (BI-RADS) category to each of the images. These were then grouped into the general categories of agreement, partial agreement, or disagreement. A third and different radiologist reviewed all cases of disagreement.
Six radiologists reviewed the images from both digital and film, equaling a total of 1,147 breasts in 692 patients. Agreement between digital mammography and film mammography assessment was 82%, partial agreement was 14%, and disagreement 4%. The cause of the 4% disagreement among the radiologists was then assessed. The primary causes of disagreement were differences in management approach/interobserver variability (52%), information derived from sonography or additional film mammograms (34%), and technical differences between the two mammographic techniques (10%).
It was concluded that the most frequent cause of disagreement in interpretation was due to interobserver variability. This source of variability, the study abstract reads, was larger than that due to differences in lesion visibility between film mammography and digital mammography.
On the Horizon
With the clinical implementation of FFDM, many exciting developments within digital mammography lie just beyond the horizon, experts say.
“Expect computer-aided detection (CAD) systems, tomography and tomosynthesis, 3D reconstruction of the breast, contrast subtraction mammography, and other similar advanced applications that are possible only after mammography is digital,” says Pisano.
The future of digital mammography may be predicted, according to one vendor, by comparing it to digital photographic technology. Just a few years ago, three or four megapixel digital cameras cost more than $10,000; a camera now can be purchased for $800. What was seen with digital photography will most likely happen in mammography.
However, one potential problem associated with digital mammography must be resolved: the large data file associated with FFDM and the bandwidth required to swiftly and effectively move the images without creating network havoc. It is widely held that such traffic would overwhelm the bandwidth of most picture archiving and communications system (PACS) networks in existence today. The problem is expected to be addressed by the National Institutes of Health effort to establish a central facility to store digital mammograms. The National Digital Mammography Archive (NDMA), sponsored by the National Library of Medicine, is a research project that will address potential problems such as patient confidentiality and facility control issues that would arise if images are archived off-site.
“The goal of the project, however, is not to compare diagnostic outcomes of any of the images, it is more of a feasibility study to show that centralized storage and retrieval can be workable,” says Pisano.
Subsequent generations of FFDM technology undoubtedly will improve not only digital mammography images, but how they are viewed and stored, with further advances anticipated in the field of CAD.
Marissa Shira is a contributing writer for Decisions in Axis Imaging News.
References:
- Lewin JM, Hendrick RE, D’Orsi CJ, et al. Comparison of full-field digital mammography with screen-film mammography for cancer detection: results of 4,945 paired examinations. Radiology. 2001;218:873-80.
- Venta LA, Hendrick RE, Adler YT, et al. Rates and causes of disagreement in interpretation of full-field digital mammography and film-screen mammography in a diagnostic setting. AJR Am J Roentgenol. 2001;176:1241-8.
