Contrast variations prove main factor for breast cancer visibility By Kate Madden Yee AuntMinnie.com staff writer December 2, 2008
CHICAGO - The Digital Mammographic Imaging Screening Trial (DMIST) of 2005 has provided clinicians with valuable data for comparing digital to analog mammography. Sponsored by the American College of Radiology Imaging Network (ACRIN), the study involved nearly 50,000 asymptomatic women who presented for screening mammography at 33 sites in the U.S. and Canada.
Principal investigator Dr. Etta Pisano, from the University of North Carolina School of Medicine in Chapel Hill, and colleagues used DMIST data to evaluate which factors contribute to lesion visibility in both digital and analog mammography. Pisano presented the findings at this year's RSNA meeting.
"We brought readers in to look at cancer cases, film and digital, side by side," Pisano said. "We asked them to assess whether lesions were more visible in digital or film -- or equally visible -- and to explain why."
The study included seven radiologist readers who reviewed film and digital mammograms of 307 DMIST cancer cases. Positioning, compression, and image contrast were considered.
Readers attributed the majority of differences in DMIST cancer visibility to variations in image contrast between the two modalities rather than to differences in positioning or compression.
"Seventy-three percent of the time our readers reported that the differences in cancer visibility were due to contrast," Pisano said.
The odds of a lesion being more visible on digital mammography (relative to being equally visible on digital and film) were higher for women with dense breasts, compared to women without dense breasts, even when the team adjusted the data for age, lesion type, and mammography system.
"The odds of our radiologists rating cancers as more visible on digital, rather than equally visible on digital and film, were 2.3 times higher for those women with dense breasts rather than those with fatty breasts," Pisano said.
Pisano chalked this finding up to system performance improvements available in each of the units.
"We hypothesize that DMIST results are attributable to differences in display and acquisition characteristics of the devices themselves rather than reader variability," she said.
According to Pisano, one of the study's clinical applications is that digital mammography should replace film mammography for breast cancer screening in women with dense breasts. But the reverse isn't necessarily true, Pisano said, responding to a question from the audience of whether film mammography should be used for fatty breast imaging.
"The DMIST results overall were that digital and film were equivalent," she said. "We don't have evidence that film is better for imaging fatty breast tissue -- we have a trend. The fact that image processing was involved in the results that we got from this study implies that manufacturers should work hard on improving image processing technology for fatty breasts."