Radiology: Comparison mammos helpful, but beware false positives

Having comparison mammograms on hand has generally been thought to increase the accuracy of cancer detection during breast imaging, and a study published in the Oct. 26 issue of Radiology confirmed some of these benefits, but it also cautioned against a high false-positive rate when changes are noted between current and prior screening mammograms.

Bonnie C. Yankaskas, PhD, of the department of radiology at the University of North Carolina School of Medicine in Chapel Hill, and colleagues showed that the use of comparison mammograms, available more than 90 percent of the time, can be helpful, but should be broken down into two groups: comparisons where changes are noted and comparisons where no changes are noted.

“When change is noted, the false-positive rate is high,” wrote the authors. “Attention needs to be paid to those recommended for further work-up to reduce the high rate of false-positive findings without lowering the high sensitivity.” They added that the reverse is true when no change is noted as there are fewer false positives, but a large proportion of cancers are missed.

The prospective study looked at a total of 1,157,980 screening mammograms from 435,183 women at least 40 years old obtained between 1994 and 2008. Presence of comparison mammograms was recorded, as was the presence of changes noted between comparison images, and the women were followed for one year to monitor cancer occurrence.

Comparison mammograms were available in 93 percent of exams, and screenings with comparisons resulted in higher positive predictive rates (5.4 percent with comparisons vs. 4.8 percent without), higher specificity (93.5 percent vs. 85.7 percent) and a lower recall rate (6.9 percent vs. 14.9 percent).

When separating out the comparisons where change was noted, which was 12.3 percent of the comparison exams, the differences in performance are starker. In the comparisons that showed a change, recall rate was 41.4 percent compared with 2 percent when no change was noted. Sensitivity when changes were noted (96.6 percent) was higher than when no change was noted (43.5 percent), but there were many more false positives resulting in a lower specificity (60.4 percent when changes are noted versus 98.1 percent when there is no change).

The positive predictive value for screening with comparison mammograms where a change was noted was 6 percent compared with 3.9 percent in comparison screenings without change.

One of the weaknesses of the study noted by the authors was that no information on the type of change was recorded.

“We did not ask what the change was,” wrote Yankaskas et al. “Another study will explore in more detail whether changes are new findings, a change in density, or something else entirely, along with the actual findings associated with accuracy and change.”

They said that further work is needed to understand what types of changes might indicate cancer and which do not.