Variation of Thickness Across the Breast Effect on Density Analysis

Despite the success of the quantitative techniques, thus far described, in predicting breast cancer risk, neither the automated methods nor the PD measure has provided as strong a risk measure as does the subjective six-category classification system. The reason for this may be explained by Eqs. (1) and (2). Among many other factors, the optical density at a given point on the image depends on both the composition of the breast and the total thickness of tissue through which the X-rays must pass. Techniques relying solely upon the image brightness are unable to separate these factors.

A breast under compression can be considered to consist of two regions: a central area of approximately uniform thickness; and, a margin where the thickness is reduced. These are illustrated schematically in Fig. 7. Depending on the size and compressed thickness of the breast, the margin can represent a substantial portion ofthe projected area. In the central area, the image will vary in brightness according only to the relative composition of fat and glandular tissue. When a threshold, ┬┐DY, is set using the PD method, while considering the uniformly

FIGURE 7 Schematic illustration of the compressed breast. Under compression the breast is considered to consist of two regions, one of approximately uniform thickness referred to as the central region, and a margin where thickness varies. In the margin, variation in transmitted X-ray fluence occurs because of changes in both breast thickness and composition.

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