Mammograms: More Volume Means Fewer False Positives

Radiologists who interpret a higher volume of mammograms don’t detect more cancers than their lower-volume colleagues - but they do have significantly lower false positive rates, according to a new study just released online in Radiology.

"Contrary to our expectations, we observed no clear association between volume and sensitivity,” said the study’s lead author, Diana S.M. Buist, PhD, MPH, senior investigator at the Group Health Research Institute in Seattle.

For the retrospective study, authors looked at all the mammograms interpreted by
120 radiologists from six Breast Cancer Surveillance Consortium mammography registries in different states, from 2002 to 2006. This included 783,965 screening and diagnostic mammograms. They measured sensitivity, false positives, and cancer detection rates for screening performance.

Although the number of imaging studies interpreted yearly was important, there was also a difference found in the ratio of screening mammograms to total mammograms, which included diagnostic imaging in the mix.

“The data suggest that radiologists who interpret screening mammograms should spend at least a portion of their time interpreting diagnostic mammograms, because radiologists who interpreted very few diagnostic mammograms had worse performance, even if they read a high volume of screening mammograms,” Buist said.

Stamatia Destounis, MD, a radiolgoist at Elizabeth Wende Breast Care, LLC in Rochester, NY, said that the study’s finding that a higher screening volume with a greater screening (versus diagnostic) focus led to a lower false positive rate was something practicing radiologists already suspected.

These results could be used by the healthcare profession, including insurance companies, she said, to find better ways to benchmark physicians. Destounis noted that an interesting point the authors made was that there they did not find a single best performance to set a policy. It’s not just the volume of interpretations that matter. “It’s the experience of the radiologist that comes into play, the workups they do, the diagnostics they do, how many years they’ve been out,” she said.

There’s a lot of interest in whether physicians who read many more mammograms are performing better, finding more cancers, Destounis said. However the study didn’t find that association between volume and sensitivity. “We want a high sensitivity and low false positive,” she said. “From this article it doesn’t seem that doctors who have a higher volume are getting both of those.” She said that additional studies could look at the other factors, including experience and training.

FDA regulations require physicians who interpret mammograms to read at least 960 every two years to meet current standards. Using the data, the authors projected that by increasing that number to 1,000 mammograms a year, there could be 43,000 fewer false positive findings and a decrease in costs by at least $21.8 million. They note, however, that increasing the interpretation standards can cause workforce and access to healthcare issues.

Destounis agreee. “I think if we were to recommend that the volume requirement goes up in the United States, you have to look at false positive exams, workforce issues and cancer detection rates,” she said, adding that if a facility performs only 20 mammograms a day, and the radiologist is told he has to do more, the center may not have enough capacity or demand to continue offering them. “Women’s access to care can change.”