Discussion
Significant improvement in identification of asymptomatic cancers occurred for all readers in this study. This is shown by a 63% increase in callbacks of cancer cases with only a 4% decrease in correct identification of the true negative cases. The confidence of the diagnoses of the 102 cases with predictive BIRADS and DMIST likelihood scales was confirmed by using AUC and FOM methodology.
With a short training period experienced radiologists using 2D AWBU significantly improve their ability to diagnose cancer in dense-breasted women. This type of AWBU is similar in appearance to real-time ultrasound images. The slower transducer speed enforced by the AWBU decreases inter-image distance, allowing the reader more time to identify small masses. At a review speed of 10 images per second the observer has 0.5 s to identify a 5-mm mass. A high-resolution computer screen, along with a post-processing technique to expand the grayscale at the black end of the spectrum, results in visually sharper margins and more contrast of masses against the background tissue. These factors are designed to make recognition of invasive cancers easier and more reliable. This automated process for breast ultrasound eliminates operator variability, provides greater consistency, and ensures reproducibility of quality images. Study radiologists increased discovery of T1a and T1b invasive cancers 150% over mammography alone (Table 3).
The average review time per AWBU study was about 11 min shorter than the 19 min for radiologists in the ACRIN 6666 trial of handheld screening ultrasound [8]. As half of our test set subjects had cancers, it would be expected that the average review time we observed for AWBU would be significantly longer than in a typical screening population with mostly normal studies.
Our study had a number of inherent weaknesses. Although the test set was confidential, the readers probably quickly realized that it was enriched. They may have been extraordinarily vigilant resulting in increases in both TPs and FPs.
A false increase in TPs would occur if all the correctly identified cancers were not subsequently confirmed with biopsy. Also, analysis was performed on a case basis in the three patients in whom cancers were present in both breasts; it was assumed if one of the cancers was identified, the cancer in the other breast would be found by the subsequent workup. This assumption might have falsely raised the TPs and reduced the FNs . In addition, we did not have a comparison with hand-held ultrasound.
Any of the following factors could have decreased the readers’ accuracy with AWBU (decreased true positives and negatives, and increased false positives and negatives) compared with a normal screening situation. Fatigue—Readers reviewed an average of 34 AWBUs daily.
Inexperience with ultrasound screening—Some readers do not perform screening ultrasound.
Limited experience with AWBU—This was the first exposure to AWBU for 11 of the readers.
Unfamiliarity with some ultrasound formats—Images from many different manufacturers were used.
In spite of these hindrances our observations clearly show that radiologists improve detection of cancers, especially small invasive ones, by adding AWBU to mammography findings.