Home

Cover Story

Table of Contents

E-Newsletter

Article Archive

Editorial Calendar

Datebook

Writers' Guidelines

Orgs/Links

Opinion Polls

Reprints

Forum

For other articles and previous issues click here.

April 18, 2005

DMIST-ifying Digital Mammography’s Role?
By Dan Harvey
Radiology Today
Vol. 6 No. 8 P. 12

The initial results of the largest digital mammography study will be published soon. The women’s care community is looking forward to a better sense of what the digital revolution means to breast care.

The mammography field eagerly awaits the conclusions of researchers involved in the large-scale Digital Mammographic Imaging Screening Trial or, as it is more commonly known, the DMIST study.

At least part of their wait is nearly over. Later this spring, the initial results of the ambitious, three-year project are scheduled to be released, according to the study’s principal investigator Etta Pisano, MD, professor of radiology and biomedical engineering and director of the Biomedical Research Imaging Center at the University of North Carolina School of Medicine in Chapel Hill.

The initial results were originally scheduled to be published in late 2004 or early 2005, but their release has been delayed until spring so researchers can finish the follow-up portion of the study. The primary goal of the trial, which is funded by the National Cancer Institute and coordinated by the American College of Radiology Network, is to compare the diagnostic accuracy of digital mammography with standard film/screen mammography for the screening population of women without breast cancer symptoms. Further, it will compare the cost-effectiveness of both technologies and look at the quality-of-life issues associated with false positives.

A Necessary Study
The DMIST study enrolled nearly 50,000 women at more than 30 participating facilities in the United States and Canada. Essentially, the study was prompted by the development of digital mammography and intended to assess its potential advantages in the area of breast screening, which include improved diagnostic accuracy, especially in women with dense breast tissue.

Standard film/screen mammography has proven to be an effective method of detecting early breast cancers, and it has had a notable impact on breast cancer treatment outcomes. However, it’s not infallible; some early cancers do get missed. The introduction and expansion of digital imaging prompted healthcare professionals to begin comparing digital mammography’s potential with traditional techniques.

Digital mammography has demonstrated promise as far as earlier detection of breast cancer, but a large study was needed to see whether it was really better than, or at least as good as, standard mammography, according to Lawrence Bassett, MD, principal investigator at the participating center at the University of California, Los Angeles.

Digital mammography has concrete advantages with the storage and electronic retrieval of images and the ability to manipulate images. But heading into the study, there was no solid evidence that the digital images themselves were better for detecting cancer. However, if DMIST does show that digital mammography is as good as or better than standard mammography, it could lead to increased usage that could in turn result in the anticipated improved cancer detection in dense-breasted women. It could also result in fewer call-backs for suspicious lesions. Quality digital images could lead to improved mammography services in rural areas by facilitating remote consultations.

Comparing Images
DMIST was launched in October 2001 as the first large, multicenter study to compare digital and standard mammography. Most researchers expected that the differences between the two technologies wouldn’t be great, but believed a well-controlled comparison was needed.

Women without symptoms or signs of breast cancer enrolled in the prospective study at the time of their regular screening mammogram. Each participant underwent both digital and film/screen mammography. Both exams included two views of each breast. Two radiologists then viewed each woman’s images.

“We had two readers look at both scans independently,” Pisano explains. “That is, the women had two studies, one read by one radiologist and one read by another. We are comparing the diagnostic accuracy of digital vs. film.”

The status of breast health was determined by breast biopsy results, if necessary, or a one-year follow-up.

“We finished enrolling patients in November 2003, and so we are in the follow-up period right now,” Pisano says. “The database will be closed very soon. We thought it was going to close on February 18, but we still have 8,000 patients who will have to come back for follow-up. So we are trying to reduce those numbers. After that, we will be writing the paper.”

Pisano says the study was designed to examine diagnostic accuracy, sensitivity, specificity, positive and negative predictive value, and receiver operating characteristic performance. Specifically, each participant’s images were evaluated for the diagnostic accuracy of digital images displayed on a monitor compared with that of the printed film, the effect of breast density on the diagnostic accuracy of digital mammography compared with film, and the diagnostic accuracy of both digital and standard mammography units.

Also, researchers were comparing cost-effectiveness. “If digital mammography were found to be better, I think it would replace film. If it is found to be the same, then you’d have to make a cost argument,” says Pisano. “You’d have to figure out a way, given the increased cost [of the digital technology], that it would still be worth it. So, it would become a cost-effectiveness argument—[Are] the storage, retrieval, and transmission capabilities that represent an improvement over film still worth the additional costs?—because you won’t be able to make the argument that you’re saving more women’s lives. But if digital is found to be better, then that argument is easier to make. Digital should replace film because you could then, presumably, save more women’s lives. That’s easy.”

Pros and Cons
Researchers are also looking at the quality-of-life question: Would digital mammography reduce the number of false positives, and thus lessen the emotional turmoil a patient experiences when told that her mammogram revealed a suspicious lesion that requires more testing?

Digital mammography is approved by the FDA and has been used for several years at both academic centers and community facilities. The technique uses computers and special detectors to produce a digital image of the breast that can be displayed on high-resolution monitors.

Standard mammography and digital mammography are similar in two ways: both utilize low-energy x-ray beams that pass through breast tissue to produce images, and both require breast compression (no one has figured out how to eliminate that step from breast imaging).

That’s where the similarities end. With digital mammography, the x-ray beam is picked up by a sensor and directly converted into digital information. That digital information is then converted into a photograph.

Specifically, the electronic x-ray sensor converts the x-ray photons to light. That light passes through a fiber-optic cable and is then converted to a digitized signal and displayed on a computer monitor. The displayed image can be manipulated on screen. A technologist can enlarge or invert the image, magnify specific areas of the breast, or heighten or decrease the brightness or contrast. The digital image can also be printed out onto film. In that way, the technology is similar to using a digital camera, where the user can display and manipulate an image before making a hard-copy photograph.

Digital advocates believe the manipulation capabilities can make digital mammography more effective when it comes to detecting cancers, especially in women with dense breast tissue. With standard mammography, the technologist can’t manipulate the analog images when the exposure isn’t optimum.

The known advantages of digital mammography include the separation of the acquisition, storage, and display functions. Also, the speed of the technology benefits both the technologist and patient. The technologist knows almost instantly whether images are acceptable.

A potential advantage is that digital mammography could lead to the development and usage of new imaging techniques, such as 3-D imaging, dual-energy subtraction mammography, contrast-enhanced mammography, or low-dose mammography tomosynthesis, where slices can be made through the breast, which could lead to the earlier detection of cancer. These techniques can’t be done with film because of the high doses. But, once again, these are potential—not proven—advantages.

Digital mammography is not without its disadvantages. Even though it can provide better contrast resolution, its spatial resolution reportedly is no better than standard mammography. In fact, the spatial resolution can even be inferior. Because of this, standard film images may be better at detecting microcalcifications and other subtle tissue changes that may indicate early cancer.

The biggest, most obvious disadvantage of the digital mammography for most facilities is that a digital mammography system typically costs three to five times more than a film/screen mammography unit. Maintenance and image storage can add to the cost. Unless a facility performs a substantial volume of mammographic studies, it would be difficult to justify the expense. Installing a new, more expensive technology to replace an older technology that is just as good, if not better, will be a tough sell. DMIST could provide a definitive conclusion in this area.

Earlier Studies
DMIST’s size fuels hope that it will yield much more information than previous studies. Earlier studies indicate no significant difference in cancer detection between standard and digital mammography. Conclusions had indicated that both approaches pick up and miss nearly the same numbers of biopsy-confirmed cancers. The largest previous study—conducted at the University of Colorado Health Sciences Center and the University of Massachusetts—involved approximately 7,000 women who underwent both film/screen mammography and digital mammography. It reached the same conclusion about missed and detected cancers, but the study also determined that digital mammography showed potential for rapid advancement and that it provides a significant speed advantage. Also, it indicated that there were fewer negative biopsies after digital mammography but the difference was not significant.

Digital mammography has generated excitement about its potential capabilities for breast cancer screening. The technical advantages of digital technology have been well-documented, but researchers involved in DMIST want to go beyond the so-called bells and whistles and get down to the basic question: When it comes to film/screen mammography vs. digital mammography, which is more accurate?

— Dan Harvey is a freelance writer based in Wilmington, Del. He is a frequent contributor to Radiology Today.

Subscribe to Radiology Today Magazine!