Debate intensifies over clinical superiority of digital systems

Final results from the a trial conducted under the auspices of the Norwegian Breast Cancer Screening Program have shown no statistical differences in cancer detection between digital and screen-film mammography. The study was a randomized trial involving nearly 25,000 women.

The team, led by Prof. Per Skaane at Ullevaal University Hospital in Oslo, evaluated recall rates, positive predictive values, and cancer detection rates in women from two age groups-45 to 49, and 50 to 69-who were followed for two years. Digital mammograms were read in soft-copy format. Preliminary findings were reported in July (Radiology 2004;232: 197-204), and updated findings presented at December's RSNA meeting.

Full-field digital mammography did detect more cancers than screen-film, but the difference was not statistically significant, in part due to higher recall rates tied to the technique. Positive predictive values for both technologies were comparable.

In the 50-to-69 age group, FFDM demonstrated a higher cancer detection rate of 0.93% versus screen-film's 0.63%. In the 45-to-49 age group, the higher cancer detection rate of 0.33% for FFDM versus 0.24% for SFM was not statistically significant.

Skaane notes, however, that overall cancer detection rates for FFDM were significantly higher than SFM (p = 0.040), and in the 50-to-69 age group, the p value was close to statistical significance (p = 0.058.)

Scans were evaluated on a five-point scale, and any finding that ranked between two and five was discussed in a consensus meeting, Skaane said. That policy may be the reason the Oslo study, unlike smaller studies, reported a higher recall rate with digital mammography than with film.

"I thought in our first study that the higher recall rate for digital was due to unfamiliarity with equipment and soft-copy reading," Skaane said. "But this higher rate is also confirmed in our final assessment."

As a result, the higher cancer detection rate of digital is at least partially explained by the higher recall rate, Skaane said.

"Ultimately, our goal was to determine whether full-field digital mammography with soft-copy reading is suitable for breast cancer screening," he said. "This study confirms the findings of our initial study that it is."

The recall rates in the age group 50 to 69 were 2.5% (SFM) and 3.8% (FFDM), and these values are much lower than for other countries, especially the U.S.

In a study conducted as part of the Oslo trial, Skaane compared the cancer detection performance of CAD with independent double reading of screen-film mammography and FFDM soft-copy exams. The prospective, paired study involved 3683 women who underwent both screen-film mammography and FFDM. CAD correctly marked 90% of cancers on the screen-film exams and 93.3% on the FFDM scans. Double readers correctly identified 93.3% of cancers on screen-film and 73.3% on FFDM.

CAD sensitivity for calcifications on both modalities was 100%. CAD marked one of two cancers missed by double readers on screen-film mammography and all seven cancers missed by double readers on FFDM. Of the 26 cancers that were subsequently diagnosed, the initial double reading was positive in four cases on FFDM and in three other cases on screen-film mammography.

In terms of overall performance, CAD picked up 9% of the cancers that were overlooked by double reading on screen-film mammography and 25% of the cancers that were overlooked by double readers on FFDM, Skaane said.

"One problem with the study is that the learning curve for digital may overestimate the additional benefit of CAD," Skaane said. "The additional benefit of CAD in digital would be at the beginning of the curve."