3D information necessary for breast tissue density calculation
The utilization of 2D information from x-ray mammography to assess the ratio of dense to fatty tissue in the breast is inaccurate for determining the volume ratios of the tissues, according to Daniel B. Kopans, MD. In an opinion piece published in this month’s issue of Radiology, Kopans argues that a methodological flaw exists in numerous studies that have used 2D information to measure the percentage of tissues by volume.

“Without exposure values, half-value layer information, and knowledge of the compressed thickness of the breast, an accurate volume of tissue cannot be calculated,” he wrote. “The great variability in positioning the breast for a mammogram is also an uncontrollable factor in measuring tissue density.”

The ratio of dense-to-fatty breast tissue has been raised by numerous studies, which suggest that there may be a link between breast tissue patterns in mammography and the risk for breast cancer.

“Although I personally suspect that there is a relationship between density and risk, the degree is far from clear, since I believe the studies that have been published provide analyses that go beyond what is physically possible in terms of accurately measuring the percentage volume of breast tissue that is dense,” Kopans wrote. “Furthermore, ‘dense’ tissues are not homogeneous but are composed of fibrous connective tissue, ducts, and lobules in various percentages. The relative percentages of these components are presently not measurable on two-dimensional (2D) mammographic images. Consequently, it is unclear which tissue compartment, if any, is related to breast cancer risk.”

Simply repositioning the breast can lead to marked variation in estimates of tissue density. (A) Initial CC mammographic projection shows fibroglandular tissue (gray) and fat (black). By using computer segmentation, the percentage of dense tissue was 27.6 percent. (B) The technologist pulled the breast further into the mammography machine on same day, revealing more fat at back of the breast and changing the percentage of dense tissue to 13 percent. Image and caption courtesy of the Radiological Society of North America.

Kopans’ argument with these studies is that the true accuracy of the measurements in terms of a direct relationship to the absolute volumes of these tissues has yet to be shown.

“Basic physics shows that the methods used to quantify the percentage of breast tissue that is dense cannot possibly measure the true volume ratio of dense to fatty tissue with any degree of accuracy, which casts doubt on virtually all published analyses,” he stated.

Kopans noted that computer algorithms have attempted to compensate for the limitations of using 2D data to determine 3D volume ratios and that results with these tools have been shown to be highly reproducible.

“Unfortunately, reproducibility of objective measurements does not mean accuracy,” he noted.

A two-dimensional perspective limits accuracy. Kopans offers the example of a viewer gazing at an eye-level hedge. From that perspective, it is impossible for the viewer to know if the hedge is comprised of one row of plants or multiple rows of plants.

“Investigators who are interested in exploring tissue density and breast cancer risk need to start fresh and think three dimensionally,” Kopans wrote.

He suggested that 3D imaging data obtained via MRI, digital breast tomosynthesis, or CT can provide true volumetric information. However, he noted that investigators will need to decide where the breast actually ends in order to standardize the measurement of the total volume of the breast.
Kopans urged his colleagues to employ 3D imaging tools in their research on breast tissue density and its possible relationship to risk for breast cancer.

“Until truly accurate 3D data are available, I would urge investigators to stop drawing conclusions based on 2D information unless the other important factors are collected at the same time to permit quantitative density and volume determinations,” he wrote. “Without them, quantification is virtually meaningless, in my opinion.”