The article entitled “Anisotropy of Solid Breast Lesions in 2D Shear Wave Elastography Is an Indicator of Malignancy,” 2016, 23:53–61 , presented an interesting and novel idea of using two-dimensional shear wave elastography anisotropy in breast masses as a measure of malignancy. If this were true, it could have a major impact on breast tumor diagnosis. However, as researchers of ultrasound shear wave imaging, we think that there are statements and results in this article that need further clarification before these results can be applied in a clinical setting:
1. In the “Introduction,” equation 1 has assumptions—the tissue is homogenous (isotropic) and incompressible, and has boundaries at infinity. Given the authors’ hypothesis of anisotropy of breast cancer and breast lesion size on the order of several centimeters, this equation cannot be used for this study. Instead, shear wave speeds should be employed.
2. In the “Materials and Methods,” the following need further clarifications:
a. Excitation frequency of the shear wave, one of the key factors of shear wave elastography, is missing, which is usually in a range of 100–1000 Hz.
b. The authors used radial and anti-radial planes for imaging group A breast masses. It is unclear how these orthogonal planes were selected. In fact, there are infinite numbers of these radial planes containing the radial axis through the mass. An illustration is needed to clarify the imaging approach.
c. Where are the definitions of the anisotropy difference and anisotropy factor from? Please explain the underlying physical or physiological theory. To our knowledge, equations 2 and 3 are not measures of anisotropy. They actually reflect the variance of two independent measurements of the same breast mass. To support the argument that they relate to anisotropy, comparison should be made to the measure of random samples taken in other orientations, which would show statistically significant differences. Furthermore, the difference between the anisotropy factors of benign and malignant masses may be explained by the abovementioned variation in measurements, that is, the malignant masses are more heterogeneous with high variance in tissue stiffness due to necrosis, etc, as compared to the benign masses.
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References
1. Skerl K., Vinnicombe S., Thomson K., et. al.: Anisotropy of solid breast lesions in 2D shear wave elastography is an indicator of malignancy. Acad Radiol 2016; 23: pp. 53-61.
2. Zhou J., Yang Z., Zhan W., et. al.: Anisotropic properties of breast tissue measured by acoustic radiation force impulse quantification. Ultrasound Med Biol 2016; 42: pp. 2372-2382.
3. Wang Z.: Mechanical and optical methods for breast cancer imaging. PhD thesis, University of Iowa2010.
4. Weaver J., Doyley M., Van Houten E., et. al.: Evidence of the anisotropic nature of the mechanical properties of breast tissue. AAPM/COMP/CCPM Meeting, Montréal, Quebec, July 152002.