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1 University of Colorado Health Sciences Center, 4200 E. 9th Ave., Mail Stop
F724, Denver, CO 80262.
2 University of Massachusetts Medical Center, 55 Lake Ave. N., Worcester, MA
01655.
3 Northwestern University Medical School, 357 E. Chicago Ave., Chicago, IL
60611.
4 AMC Cancer Research Center, 1600 Pierce St., Lakewood, CO 80232.
Received July 6, 2001;
accepted after revision February 19, 2002.
Supported by contract DAMD17-96-C-6104 from the United States Army Medical
Research and Materiel Command Breast Cancer Research Program.
Abstract
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SUBJECTS AND METHODS. Full-field digital mammography was performed in addition to screen-film mammography in 6736 examinations of women 40 years old and older presenting for screening mammography at either of two institutions. Two views of each breast were acquired with each technique. The digital and screen-film mammograms were each interpreted independently. In addition to a clinical assessment, each finding was assigned a probability of malignancy for use in receiver operating characteristic analysis. In cases in which the digital and screen-film interpretations differed, a side-by-side analysis was performed to determine the reasons for the discrepancy. With few exceptions, findings detected on either technique were evaluated with additional imaging and, if warranted, biopsy.
RESULTS. Additional evaluation was recommended on at least one technique in 1467 cases. These additional evaluations led to 181 biopsies and the detection of 42 cancers. Nine cancers were detected only on digital mammography, 15 were detected only on screen-film mammography, and 18 were detected on both. The difference in cancer detection is not statistically significant (p > 0.1). Digital mammography resulted in fewer recalls than did screenfilm mammography (799 vs 1007, p < 0.001). The difference between the receiver operating characteristic curve area for digital (0.74) and screen-film (0.80) mammography was not significant (p > 0.1). Reasons for discrepant interpretations of cancer were approximately equally distributed among those relating to lesion conspicuity, lesion appearance, and interpretation.
CONCLUSION. No significant difference in cancer detection was observed between digital mammography and screen-film mammography. Digital mammography resulted in fewer recalls than did screen-film mammography.
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Thirty-two symptomatic patients presenting for bilateral diagnostic mammography were originally imaged but are excluded from this analysis. The data presented here are thus from asymptomatic subjects presenting for screening mammography. Each subject signed a consent form approved by the institutional review board of the enrolling institution and the United States Army Medical Research and Materiel Command. Screen-film mammography and full-field digital mammography were performed on each subject, usually by the same technologist at the same visit. Screen-film mammography was performed on a commercial mammography unit (DMR; General Electric Medical Systems, Milwaukee, WI). Full-field digital mammography was performed on a prototype unit using an amorphous silicon solid-state detector with CsI crystal. The technical details of this digital prototype have been reported previously [7]. A slightly modified version of this device, using the same detector technology, is now available commercially (Senographe 2000D; General Electric Medical Systems). The major technical improvements in this commercial unit are the addition of a system for automatically selecting the technique factors, analogous to the Automatic Optimization of Parameters mode available on the DMR screen-film unit, and an improved postprocessing algorithm for equalizing regional signal values as the breast thickness decreases near the skin.
Technique factors for each screen-film examination were selected by the screen-film unit in Automatic Optimization of Parameters mode. Because no method automatically determines technique factors on the digital prototype, technique factors for each digital image were matched for X-ray beam target material, filtration material, peak kilo-voltage, and (as closely as possible) beam current to that used for the corresponding screen-film image.
The screen-film and digital images were each interpreted independently by a board-certified radiologist with experience in mammography. Years of interpretation experience for each reviewer ranged from 3 to more than 20 years. Digital mammography was interpreted on a prototype two-monitor workstation supplied with the unit. Details of this workstation have been given previously [6]. This workstation was notably less powerful than the workstation used with the FDA-approved commercial unit. For example, the monitor resolution of the prototype workstation was only 1800 x 2300 pixels versus 2000 x 2500 pixels for the commercial system. This difference is significant in that the entire image could not be displayed in full resolution on the prototype system, whereas it can be on the commercial system. Additionally, the software on the commercial system enables far easier manipulation of the images than the software on the prototype system. On the prototype system, image interpretation typically included viewing the image at a magnification of 2x. For the first approximately 200-400 cases, magnification was accomplished with a moving square ("mag glass") showing a portion of the image magnified. Subsequently, each quadrant of each image was magnified in its entirety and examined. The digital workstation was located in a darkened room away from film alternators. Minimizing ambient light was important because the monitors used on the prototype system operated at a maximum luminance of about 40 foot-lamberts versus about 70 foot-lamberts for those on the prototype system. Comparison mammograms were viewed on film using a standard viewbox placed next to the digital workstation. The viewbox was turned off during the detailed evaluation of digital images to avoid glare. The reviewers were trained on the use of the workstation with a small number of sample images before the study.
For each finding on the mammogram, the interpreting radiologist was instructed to provide its location, an assessment and recommendation using the American College of Radiology Breast Imaging Reporting and Data System (BI-RADS) nomenclature [8], and a probability of malignancy on a scale of 0-100, for use in receiver operating characteristic (ROC) curve analysis. Each reviewer in the study interpreted an approximately equal number of screen-film and digital examinations. Prior mammograms and patient history were equally available for the screen-film and digital interpretations.
Findings detected on either screen-film or digital mammography were further evaluated, in a manner consistent with routine clinical practice, using any combination of additional mammography, sonography, and biopsy. The additional mammographic images were generally obtained using the technique on which the finding was better seen, with the exception that all magnification images were obtained on screen-film mammography because the grid on the prototype was not easily removed. Each subject was followed up for 1 year after her enrollment to determine whether she developed breast cancer. Cancer was considered to be present at the time of imaging if it was proven by biopsy within 1 year.
Discrepancy Analysis
For a given finding, if the interpretation of the screen-film examination
differed from that of the digital examination, the case was set aside for
discrepancy evaluation. The interpretations were considered different if one
resulted in a recommendation of immediate additional imaging or biopsy and the
other resulted in a recommendation of short-interval or routine follow-up.
These recommendations generally corresponded to BI-RADS assessments of 0, 4,
or 5 in the former instance, and to BI-RADS assessments of 1, 2, or 3 in the
latter case.
Discrepancy evaluation consisted of a side-by-side comparison of the screen-film and digital images by two radiologists using soft-copy display for digital and hard-copy for screen-film. Ninety percent of discrepancy evaluations were performed by the two radiologists who had originally interpreted the case; in 10% of cases, one of the two radiologists was different. The purpose of the evaluation was to determine the causes of different interpretations for each finding. A discrepancy evaluation form was used that consisted of an 11-point scale for measuring the relative visibility and conspicuity of a finding on screen-film versus digital mammography; the form gave a choice of 39 reasons for the discrepancy. The reviewers were instructed to choose, by consensus, a single major reason and, optionally, a single minor reason for the discrepant interpretations between the two techniques.
At discrepancy evaluation, reviewers had the option of deciding not to work up a finding, but they were instructed to use this option only if the finding detected on one technique could be dismissed with a definite explanation based on the other technique or if the discrepancy was due to an error in interpretation. This option was rarely used: only 68 (13%) of 510 digital-only findings and 14 (2.0%) of 751 screen-film-only findings were dismissed. Two invasive lobular carcinomas in the regions of digital-only findings that were dismissed at discrepancy evaluation became palpable within 1 and 5 months of the screening examination. Because additional imaging was not performed after the cancers became palpable, it was not possible to exactly correlate the imaging finding and the palpable abnormality, but in both cases the cancers involved the quadrant of the finding. In one case the finding was a density that, on the basis of evaluation of the postlumpectomy mammogram, was removed at lumpectomy. The other finding was calcifications, and evaluation of the mastectomy specimen showed calcifications in the tumor. On the basis of the available information, both of these cases were considered true-positive for digital mammography. An additional digital-only finding that was dismissed became palpable 10 months after screening and was diagnosed as a fibroadenoma at surgical biopsy.
Analysis of ROC Curve
Analysis of the ROC curve was performed using the alternative free-response
ROC method [9]. The area under
each curve was calculated using the trapezoidal method. Areas were compared
using the Wilcoxon's statistic, which accounts for the correlation gained by
the use of the same cases for each curve
[10]. Statistical analysis on
agreement tables was performed using the McNemar chi-square test for paired
data [11].
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Tables 1 and 2 list the most common major and minor reasons given at discrepancy analysis for the findings seen only on screen-film and those seen only on digital mammography, respectively. In both cases, the two most common reasons, by far, were "fortuitous positioning" and "minor difference in opinion." Figure 1A,1B gives an example of fortuitous positioning resulting in a discrepant finding detected only on screen-film mammography. "Minor difference of opinion" was defined as an interpretation difference between the reviewers resulting in a recall by one reviewer even though both reviewers had a low suspicion of malignancy. A minor reason was given for only about half the discrepant interpretations. The most common minor reason given for both screen-film-only and digital-only findings was "compression difference."
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One hundred seventy-nine biopsies were performed on findings detected in the study. Eighty-seven biopsies were of findings originally detected only on screen-film mammography, 38 biopsies were of findings originally detected only on digital mammography, and 56 biopsies were on findings originally detected on both techniques. The difference between the number of biopsies resulting from screen-film findings and the number resulting from digital findings is statistically significant (p < 0.001). Forty-two cancers were diagnosed. Screen-film mammography detected 33 of the cancers and digital mammography detected 25. These numbers include the 18 cancers detected on both techniques. Eight additional cancers were detected by palpation and were biopsied within 1 year of negative interpretations on both techniques. The difference in cancer detection was not statistically significant (p>0.1).
Table 3 gives the detected cancers by mammographic type. Architectural distortion and calcifications accounted for most of the differences in cancer detection.
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Table 4 gives the major reasons at discrepancy analysis for cancers being found only on one technique. No reason dominated, and the overall categories were fairly evenly divided among visibility, appearance, and interpretation, with no discernible trends between the techniques. Figures 2A,2B,3A,3B,4A,4B are examples of cancers missed on one technique because of appearance, visibility, and interpretation, respectively.
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Five of the 15 cancers detected only on screen-film mammography were judged to be equally conspicuous on both techniques. The other 19 discrepant cancers were determined to be more conspicuous on the technique on which they were found.
The positive predictive value of screening, defined as the percentage of screening examinations with positive findings that led to a diagnosis of cancer, was slightly lower for screen-film mammography (33/1001, 3.3%) than for digital mammography (27/793,3.4%).
Free-response ROC curves for screen-film and digital mammography are shown in Figure 5. The area under the curve was 0.80 for screen-film mammography and 0.74 for digital mammography. This difference was not statistically significant (p = 0.18).
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Given that digital mammography resulted in both fewer false-positive examinations and fewer true-positive examinations (the latter not reaching statistical significance), other measures are needed to compare the overall diagnostic performance of the two techniques. The key question is whether the greater number of cancers detected on screen-film mammography is simply the result of a lower threshold for determining a finding. One measure for comparison is the positive predictive value. The fact that this value was nearly equal for the two techniques suggests that the difference in cancer detection was more than simply a shift in threshold, because such a shift would be expected to result in a lower positive predictive value. In other words, the increase in sensitivity from such a shift would be accompanied by a decrease in specificity. The trade-off between sensitivity (true-positive rate) and specificity (1 — false-positive rate) is directly illustrated by plotting an ROC curve. The area under this curve can then be used to compare two tests independent of the threshold used for positivity. This measurement also favored screen-film mammography (Fig. 5), although the difference was not statistically significant.
Although each technique detected cancers at or above the expected rate for our population [12], each missed a large fraction of those detected by the other. No dominant cause accounted for the misses. In some cases, the appearance of the cancer was significantly different on the two techniques, possibly because of a slight variation in the projection of the lesion during imaging. Figure 2A,2B is an example of one such cancer, which was detected only on digital mammography. In other cases, the relationship of nearby normal tissue affected detection. In some of these cases, fibroglandular tissue superimposed over the cancer on the image of one technique obscured the lesion from view, whereas on the other technique fibroglandular tissue projected next to the cancer, allowing it to be detected. This difference in superposition of structures occurred even though both techniques used identically designed compression devices and the same technologist positioned the patient for both examinations. Generally, the breast appeared to be positioned identically, on the basis of standard landmarks such as the pectoral muscle, inframammary fold, and nipple. In still other cases, interpretation differences resulted in cancers that looked similar on both techniques being detected on only one, as illustrated in Figure 4A,4B.
There are limitations to applying the results of this study to clinical practice. First, digital mammography technology is still evolving. The workstation now being sold for clinical use is technically superior to those used in this study; it incorporates brighter, higher resolution monitors and has more ergonomically designed controls. A breast thickness equalization algorithm is now applied to each image to reduce the amount of image adjustment needed for interpretation. Such adjustments are fatiguing and possibly distracting to the interpreting radiologist in a screening environment when examinations are interpreted in a batch. An improved workstation would possibly have resulted in more cancers being detected on digital mammography; for six of the cancers missed on digital mammography, the major reason was related to interpretation, including one cancer miss thought to be directly caused by the workstation. On the other hand, for three of the cancers missed on screen-film mammography, the misses were determined to be interpretation-related. It might be valuable to use the images acquired in this study in a multireviewer performance study that could use the latest digital mammography workstation and postprocessing to see whether the results would change. Unfortunately, such a study can never exactly reproduce the screening environment in which, on average, approximately 250 examinations must be interpreted to find a single cancer.
Digital mammography is still in its infancy compared with screen-film mammography. The latter has had the benefit of more than three decades of clinical use and technologic improvement. For this reason, perhaps we should not be surprised that a digital prototype cannot outperform screen-film mammography clinically. Because full-field digital mammography is a new technology, technical improvements can be expected to occur at a faster rate than for screen-film mammography, and future studies may show that the technical advantages of full-field digital mammography have translated into a clinical advantage.
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, dotted line) and full-field
digital mammography (
, solid line) based on rating scale of
0-100. Scale for x-axis is probability of false-positive finding
occurring on two screening images of given breast and is analogous to
false-positive rate in standard ROC experiment. Area under screen-film curve
is 0.80; area under digital curve is 0.74. Difference is not statistically
significant (p = 0.18).