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Original Research |
1 All authors: Division of Diagnostic Imaging, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Unit 57, Houston, TX 77030.
Received January 24, 2005;
accepted after revision April 18, 2005.
Address correspondence to W. T. Yang
(wyang{at}di.mdacc.tmc.edu).
Abstract
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MATERIALS AND METHODS. Sixteen phantom breast masses of varying sizes (0.3-1.2 cm), shapes (round and irregular), and densities (high and low) were created from shaved tumor specimens and imaged using both full-field digital and screen-film mammography techniques. We created 408 detection tasks that were captured on 68 films. On each radiograph, six detection tasks were partially obscured by areas of varying breast-pattern complexity, including low (predominantly fatty), mixed (scattered fibroglandular densities and heterogeneously dense), and high (extremely dense) density patterns. Each detection task was scored using a five-point confidence scale by three mammographers. Receiver operating characteristic (ROC) curve analysis was performed to analyze differences in detection of masses between the two imaging systems, and sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were computed.
RESULTS. Full-field digital mammography showed higher area under the ROC curve than screen-film mammography for detecting masses in each breast background and performed significantly better than screen-film mammography in mixed (p = 0.010), dense (p = 0.029), and all breast backgrounds combined (p = 0.004). Full-field digital mammography was superior to screen-film mammography for characterizing round and irregular masses and low- and high-density masses.
CONCLUSION. Full-field digital mammography was significantly superior to screen-film technique for detecting and characterizing small masses in mixed and dense breast backgrounds in a phantom model.
Keywords: breast neoplasms • breast radiography • digital radiography • phantoms
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Because the reduction in the breast cancer mortality rate with screening mammography is partly due to the identification of small nonpalpable masses, it is important to quantitate the detectability of small masses using full-field digital mammography and to compare its performance with that of screen-film mammography, the current standard, thereby validating the role of the newer imaging technique in routine breast cancer screening.
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Specimen Radiography of Phantom Breast Masses
The setup for specimen radiography is illustrated in Figures
1 and
2. Three components were
assembled in three layers. Layer 1 consisted of a supporting base layer of
clear film that was placed on the bucky, with permanent fiduciary ink marks
made in three corners of the film. Two supporting rails made of polymerized
methyl methacrylate (Lucite, Lucite International) (0.6-cm thick and 2.5-cm
wide) were placed at the periphery of the imaging field to fit the
anthropomorphic phantom. Layer 2 consisted of clear film with six designated
locations marked by circles for placement of phantom breast masses. Before
imaging, the simulated masses were attached to layer 2 at the six selected
locations. This second layer was placed on the supportive plate (layer 1), and
the fiduciary marks were aligned. Layer 3, the anthropomorphic breast phantom,
was placed onto the supportive rails to ensure that the masses on layer 2 were
not squashed during imaging. The anthropomorphic breast phantom (RMI 165,
Gammex) had an effective thickness of 5 cm and was used to simulate a spectrum
of complex breast backgrounds
[15]
(Fig. 2).
Imaging was performed with a full-field digital mammography system (Senographe 2000D, GE Healthcare) and with a screen-film mammography system (Senographe DMR, GE Healthcare). Thus, each combination of simulated masses was imaged twice (Figs. 3A and 3B).
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The anthropomorphic breast phantom was imaged with the screen-film technique using automatic exposure control. The contrast option was selected under automatic exposure control, with a prescribed recommendation for using Mo-Mo, at 25 kVp and 240 mAs. Because the manual mode was used to acquire images, several images of the anthropomorphic breast phantom were acquired at several different exposure levels (approximately 225-250 mAs), and a consensus selection was made by the radiologists participating in the reviewer study.
Each radiograph contained six detection tasks randomly distributed in areas of varying breast backgrounds in the anthropomorphic breast phantom, including low (predominantly fatty), mixed (scattered fibroglandular densities and heterogeneously dense), and high (extremely dense) tissue densities (Figs. 3A and 3B). There were 408 detection tasks created and captured on 68 hard-copy films, including 272 true-positives and 136 true-negatives.
Reviewer Studies of Mass-Detection Tasks
For reviewer studies, all full-field digital mammographic images were
printed on 8 x 10-inch hard-copy films using a laser printer (Scopix
LR5200, Agfa Medical Imaging) and laser film (Scopix LT2 B, Agfa Medical
Imaging). Screen-film images were acquired with Min-R 2 cassettes, Min-R 2000
screens, and Min-R 2000 film (Eastman Kodak Company) and were processed with
an X-OMAT processor (Eastman Kodak Company). A transparent overlay with six
stenciled circles was placed over each hard-copy radiographic image at the
time of interpretation. Each circle indicated the general location of one of
the six detection tasks. The circle sizes were chosen so that each phantom
mass (if present) would be contained within the circle. The circles were
produced on unexposed, developed film with the use of a template (Rapidesign
R-2040, Berol) and a permanent marker. Fiduciary markings on the overlay
ensured proper alignment with the study images.
All hard-copy films were displayed on a motorized mammogram viewer (MS604A, S&S RADX Technology). Screen-film and digital images were randomized for the reviewer study. Two review sessions were conducted with an interval of 2 weeks. Lighting conditions were at viewbox illuminance. All radiologists reviewed images of the anthropomorphic breast phantom without superimposed phantom breast masses to verify that all patterns present in the anthropomorphic breast phantom were rated as true-positive.
The interpretation exercise in this study was separated into two components: the detection of simulated mass phantoms and the classification and characterization of those mass phantoms. For the detection of masses, three experienced mammographers independently reviewed each film and rated the visibility of each detection task using a five-point confidence scale: 1 = definitely not present, 2 = probably not present, 3 = possibly present, 4 = probably present, and 5 = definitely present.
For each detection task with a visibility score of 3 or more, each mammographer also characterized each mass as round or irregular in shape and of high or low density. Therefore, a three-point scale was used for the classification and characterization of mass shape and density: 1 = true-negative, with visibility score of 1 or 2; 2 = visibility score of 3 or greater, with low density or round shape; and 3 = visibility score of 3 or greater, with high density or irregular shape.
Statistical Analysis
A total of 408 scores (six detection tasks per image x 68 images)
were collected and recorded from each reviewer. The scores were sorted
according to mass size, mass shape, mass density, breast background density,
detector system, and reviewer before analysis.
The sensitivity, specificity, PPV, NPV, and accuracy in the detection of masses of different sizes and also in the detection of masses in different breast backgrounds were calculated for full-field digital mammography and screen-film mammography. Confidence level scores of 1 and 2 were considered negative at imaging, and confidence level scores of 3, 4, and 5 were considered positive at imaging in these calculations. The three-point scale previously described was used in the calculation of sensitivity, specificity, PPV, NPV, and accuracy in the detection of masses of different shapes and densities using full-field digital mammography and screen-film mammography. A score of 1 was considered negative at imaging, and score 2 was considered positive at imaging for low-density and round-shaped masses, and score 3 was considered positive at imaging for high-density and irregular-shaped masses.
For visibility of breast masses, ROC analysis (SPSS version 11.5, SPSS) was performed, and the area under the ROC curve was computed for tumor size and confidence level for each imaging system.
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Our findings suggest that full-field digital mammography may partially diminish the problem of dense breast tissue obscuring small, isodense breast cancers and therefore may help to decrease the false-negative rate of mammographic screening.
Mammography is the only method of early detection of nonpalpable breast cancer that has been proven to lead to a significant reduction in breast cancer mortality [16, 17]. Over the past 2 decades, conventional screen-film mammography has become the main tool for detecting and diagnosing early nonpalpable breast cancers. However, some breast cancers cannot be detected by screening with this imaging method. Much of the decreased lesion conspicuity that limits the usefulness of screen-film mammography is due in part to factors inherent to screen-film mammographic technique. Once a screen-film image has been exposed and processed, the film cannot be further manipulated to improve the contrast.
The detection of masses requires a contrast-to-noise ratio that exceeds the threshold value of 3-5 [18]. The contrast-to-noise ratio can be improved by increasing the contrast of the mass to the background contrast level by optimizing the X-ray spectrum, by reducing the noise level by using higher exposures and more efficient detectors, or by using more effective scatter rejection methods. Scattered radiation can greatly decrease the contrast-to-noise ratio, leading to poorer detectability of microcalcifications and soft-tissue masses, particularly in heavily attenuated regions.
A variable in the detection of masses that is less well understood is the overlapping of masses with structures in the background tissue that may obscure the masses. These overlapping tissue structures, although not directly related to the X-ray photon influence at the detector input, can obscure low-contrast objects such as low-density masses in mammographic images and severely limit the diagnostic value of mammography. This is particularly pertinent in mammography of women with dense breasts.
Newer techniques such as full-field digital mammography may theoretically improve the detection and diagnosis of breast cancers because they separate the process of image acquisition from image display and storage, permitting postprocessing. Digital detectors appear to increase lesion conspicuity by improving the efficiency of absorption of X-ray photons and providing a linear response over a wide exposure range and lower system noise. Furthermore, each image can be optimized with postprocessing, thereby improving the visualization of any lesion.
A comparative clinical study involving 4,945 women in whom screening mammography was performed using both full-field digital mammography and screen-film mammography showed equivalent cancer detection rates for the two techniques [6]. This study also showed a 39% increase in the cancer detection rate when full-field digital mammography was used in addition to screen-film mammography and indicated a lower recall rate for screening mammography with the use of digital mammography. The authors speculated that this might be due to the ability of the full-field digital technique to penetrate dense breast parenchyma and thereby better distinguish between normal tissue and occult masses [6]. An extended study including 6,736 women by the same investigators did not confirm the previously reported increase in the cancer detection rate using full-field digital mammography. The authors found that screen-film mammography detected more cancers (presenting as both masses and calcifications) than full-field digital mammography, although the difference was not statistically significant [7].
Digital mammography is likely to be interpreted in a soft-copy format in the near future. A significant limitation of this study is that all digital images were reviewed on hard-copy films rather than on a workstation. This was due to the lack of workstation setup and stability at the time that this study was performed. Although the limitations of printed digital images in this study were minimized by careful windowing and leveling before printing, it is possible that the postprocessing abilities of digital mammography interpretation might affect the diagnostic test performance. The advantages and disadvantages of workstation monitors that affect image presentation are therefore not included in this study.
A further limitation of this study is that presenting reviewers with six tasks on a single film for interpretation, each delineated by a circle, may overestimate both sensitivity and specificity. Reviewers may be directed to look more closely at the designated area when compared with an unmarked film, thereby inflating sensitivity, and they may be less likely to misinterpret an abnormality outside the circle, thereby inflating specificity. This format was, however, selected to increase detection tasks per film and to decrease reviewer burden, while accumulating a large number of detection tasks.
Finally, the PPV, NPV, and accuracy are values that are dependent on the prevalence of disease in the study population. This is especially true in this study, in which the prevalence of disease is 67% (272/408), far higher than would be expected in a general screening population. On the other hand, the sensitivity, specificity, and area under the ROC are parameters that are independent of disease prevalence and are therefore more meaningful values in this study.
If the use of full-field digital mammography is to become widespread, its performance characteristics for the most difficult tasks must be well understood. Thus, its detection capability for small borderline masses in anatomic regions of the breast that are difficult to image, particularly the heterogeneously and extremely dense regions, needs to be studied. If full-field digital technique is to replace screen-film mammography for routine screening or in the diagnostic evaluation of possible abnormalities identified at screening, its performance must be at least equivalent to screen-film mammography. In addition, any advantages of full-field digital mammography should be known and explored. If full-field digital mammography is able to more accurately characterize normal features masquerading as pseudomasses, its use could lead to fewer false-positive screening results and unnecessary recalls. If full-field digital mammography is better able to detect true masses, its use could lead to more true-positive results and fewer false-negative results and to increased detection of small breast cancers, especially in breasts with dense parenchyma. The initial results of this phantom study are encouraging. Determining the potential clinical effect of the apparently improved detectability of small breast masses with the use of full-field digital mammography will require further study in a large clinical population over a long time period.
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