Researchers have scrutinized the performance of digital mammography since the first DR-based full-field digital mammography (FFDM) system was approved by the U.S. Food and Drug Administration in 2000. Most major studies have found DR-based mammography to be comparable to film-screen mammography, although some variations have been documented between the technologies.
CR-based FFDM units were introduced in 2006 as a less-expensive alternative to DR that enables breast centers to go digital without having to purchase an entirely new system. CR users can acquire studies with their existing mammography equipment and a special cassette-based detector; images are then converted to digital with a CR reader.
DR versus CR
While numerous studies have been published comparing DR-based mammography with film-screen mammography, less is known about the comparative effectiveness of DR and CR, according to Anna Chiarelli, PhD, from Cancer Care Ontario in Toronto, and colleagues.
Anna Chiarelli, PhD, from Cancer Care Ontario.
Chiarelli and colleagues used data from the Ontario Breast Screening Program, identifying three groups of women 50 to 74 years old who were screened between January 2008 and December 2009. A total of 403,688 women were screened with film-screen mammography, while 220,520 individuals had DR and 64,210 underwent CR-based exams. The women were followed for 12 months after screening (Radiology, May 14, 2013).
The data included 60,515 abnormal mammograms, of which 59.5% were acquired with film-screen mammography, 32.4% with DR, and 8.1% with CR. Of these, 3,895 cancers were identified: 61% with film-screen mammography, 32.4% with DR, and 6.6% with CR.
Film-screen mammography and DR had similar cancer detection rates, although DR was better at identifying ductal carcinoma in situ by 30%, according to the authors. CR was less likely to help detect invasive cancers, cancers in women between the ages of 50 and 59, and cancer at subsequent screening exams.
CR's recall rates were lower than those of film-screen mammography, although the adjusted odds ratios did not differ significantly. As for positive predictive values, DR and film-screen mammography were similar (6.4 and 6.6, respectively), while CR's performance was lower (5.2).
Fewer detected cancers
In the key area of number of detected cancers, as mentioned, CR lagged both DR and film-screen mammography. DR detected 4.9 cancers per 1,000 mammograms, comparable to film-screen mammography's rate of 4.8 cancers per 1,000 mammograms. CR's detection rate was significantly lower, at 3.4 cancers per 1,000 mammograms, making CR 21% less effective than DR, according to Chiarelli.
"This could result in about 10 fewer cancers detected per 10,000 women screened in this age group," she told AuntMinnie.com.
The study results differed from previous work, which found no difference or significantly higher cancer detection rates and positive predictive values for CR compared to film-screen mammography, the authors wrote.
"For CR systems, we found substantially lower recall rates and positive predictive values, although the adjusted odds ratio was not significantly different and cancer detection rates were significantly lower compared with values seen with [film-screen mammography], even after adjustment for potential confounders," the group wrote.
Dr. Derek Muradali.
Although CR's cancer detection rate is lower than that of DR or film-screen mammography, the risk of missed cancers overall is slight, co-author Dr. Derek Muradali said.
"The miss rate is there, but it's low clinically," he told AuntMinnie.com. "Some of these missed cancers would be interval cancers or mammographically occult anyway. So although we acknowledge that the cancer detection rate is lower with CR, the risk of missed cancers overall is very, very low."
Why is there a discrepancy between cancer detection rates for DR and CR? Technical factors, according to Chiarelli's group.
"Several technical reasons related to the physics of CR systems may influence [its performance]," the team wrote. "For example, scattering of the laser light in CR phosphor material as the detector plate is scanned to read out and create the image results in a loss of spatial resolution. In addition, inefficiency of charge tapping in the phosphor and of collection of stimulated light results in increased image noise."
Where to go from here?
Screening programs should monitor the performance of CR separately and may consider informing women of the potentially lower cancer detection rates, according to Muradali.
"We acknowledge that there's decreased cancer detection with CR," he said. "Women should have the best imaging possible: DR or [film-screen]. In our practice, we inform our family physicians, radiologists, and all of our support network about our results, and translate these results for them so they can use them effectively in clinical practice."
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