Rationale and Objectives
Mammographic breast density, a strong risk factor for breast cancer, may be measured as either a relative percentage of dense (ie, radiopaque) breast tissue or as an absolute area from either raw (ie, “for processing”) or vendor postprocessed (ie, “for presentation”) digital mammograms. Given the increasing interest in the incorporation of mammographic density in breast cancer risk assessment, the purpose of this study is to determine the inherent reader variability in breast density assessment from raw and vendor-processed digital mammograms, because inconsistent estimates could to lead to misclassification of an individual woman’s risk for breast cancer.
Materials and Methods
Bilateral, mediolateral-oblique view, raw, and processed digital mammograms of 81 women were retrospectively collected for this study (N = 324 images). Mammographic percent density and absolute dense tissue area estimates for each image were obtained from two radiologists using a validated, interactive software tool.
Results
The variability of interreader agreement was not found to be affected by the image presentation style (ie, raw or processed, F-test: P > .5). Interreader estimates of relative and absolute breast density are strongly correlated (Pearson r > 0.84, P < .001) but systematically different ( t -test, P < .001) between the two readers.
Conclusion
Our results show that mammographic density may be assessed with equal reliability from either raw or vendor postprocessed images. Furthermore, our results suggest that the primary source of density variability comes from the subjectivity of the individual reader in assessing the absolute amount of dense tissue present in the breast, indicating the need to use standardized tools to mitigate this effect.
Breast cancer is currently the most commonly diagnosed cancer in women and is projected to account for 29% of all new cancer cases in women in the United States this year . Although it is expected that one in eight women will develop breast cancer over the course of their life , previous studies have identified multiple demographic and lifestyle risk factors that are associated with an increased risk for developing breast cancer, such as age, weight, ethnicity, parity, and family history . Comprehensive assessment of an individual woman’s risk for breast cancer could lead to personalized screening regimens using complementary or alternative imaging modalities to mammography such as ultrasound or magnetic resonance imaging .
In addition to demographic risk factors, several studies have also identified that mammographic breast density, commonly measured as the relative amount of radiopaque fibroglandular breast tissue, is a strong, independent risk factor for breast cancer . Clinically, breast density is most commonly estimated by radiologists via visual assessment as the amount of mammographically dense tissue, or “white areas,” and then categorized using the American College of Radiology four-class breast-imaging reporting and data system (BIRADS) or the Boyd six-category scale . In addition, continuous measures of breast percent density (PD%), acquired using interactive image thresholding software , have also been widely used, primarily in the research setting, as a more precise, quantitative measures in the effort to better estimate the risk for breast cancer associated with increasing amounts of fibroglandular tissue.
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Figure 1
Examples of raw (a) and processed (b) mediolateral-oblique view mammograms of a breast-imaging reporting and data system category II breast with scattered densities from a 53-year-old woman. In general, improved tissue contrast and a more pronounced skin line can be seen in the processed image when compared to the raw digital mammogram.
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Materials and methods
Study Population and DM Image Acquisition
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Radiologist Estimation of PD%
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PD%=DABA PD%
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D
A
B
A
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Statistical Analysis
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Per-woman, Inter-breast Repeatability of Density Estimates
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Reader Agreement in Density Estimates from Raw and Processed DM Images
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Interreader Agreement of Continuous Breast Density Estimates
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Intra- and interreader Agreement of Categorical BIRADS Density Estimates
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Results
Per-woman, Inter-breast Repeatability of Density Estimates
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Table 1
Intrareader Agreement between Individual Women’s Left and Right Breast Density Estimates as Measured by Pearson Correlation and Bland-Altman Statistics
Breast Percent Density (%) Reader 1 Reader 2 Raw Processed Raw Processed Pearson r 0.99 0.99 0.99 0.99 Mean difference 0.03 −0.22 0.02 −0.13 95% limits of agreement (−4.6, +4.7) (−5.6, +5.1) (−1.0, +1.0) (−3.7, +3.4)
Dense Tissue Area (cm 2 ) Reader 1 Reader 2 Raw Processed Raw Processed Pearson r 0.98 0.97 0.99 0.98 Mean difference 0.61 0.25 0.54 0.50 95% limits of agreement (−8.7, +9.9) (−10.1, +10.6) (−5.1, +6.1) (−6.2, +7.2)
Top, breast percent density; bottom, dense tissue area. All correlations are statistically significant ( P < .001).
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Reader Agreement in Density Estimates from Raw and Processed DM Images
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Table 2
Intrareader Agreement between Individual Readers’ Raw and Processed Breast Percent Density and Absolute Dense Tissue Area Estimates as Measured by Pearson Correlation and Bland-Altman Statistics
Reader 1 Reader 2 Breast Percent Density (%) Dense Tissue Area (cm 2 ) Breast Percent Density (%) Dense Tissue Area (cm 2 ) Pearson r 0.97 0.97 0.99 0.99 Mean difference −1.24 −0.81 −0.05 0.96 95% limits of agreement (−9.5, +7.0) (−12.2, +10.6) (−1.8, +1.7) (−2.5, +4.4)
All correlations are statistically significant ( P < .001).
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Interreader Agreement of Continuous Breast Density Estimates
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Table 3
Interreader Agreement between Readers’ Per-woman Density Estimates as Measured by Pearson Correlation and Bland-Altman Statistics
Breast Percent Density (%) Dense Tissue Area (cm 2 ) Raw Processed Raw Processed Pearson r 0.90 0.89 0.87 0.85 Mean difference −5.05 −3.94 −7.05 −5.28 95% limits of agreement (−20.6, +10.5) (−18.8, +11.0) (−29.2, +15.1) (−28.3, +17.7)
All correlations are statistically significant ( P < .001).
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Interreader Agreement of Categorical BIRADS Density Estimates
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Table 4
Categorical BIRADS Density Assignment by Reader Made on Raw (Left) and Processed (Right) DM Images
Reader 1 (D.N.) Reader 1 (D.N.) I II III IV I II III IV Reader 2 (S.G.) I 38 9 0 0 Reader 2 (S.G.) I 39 9 0 0 II 1 19 9 0 II 2 19 7 0 III 0 0 3 2 III 0 0 3 2 IV 0 0 0 0 IV 0 0 0 0
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Discussion
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Acknowledgments
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