Yamashita S, Kohjimoto Y, Iwahashi Y, Iguchi T, Iba A, Nishizawa S, Hara I.

Department of Urology, Wakayama Medical University, Wakayama, Japan.

OBJECTIVES: The objective of the present study was to investigate the usefulness of three-dimensional images of stones to measure mean stone density for predicting the outcome of shock wave lithotripsy. METHODS: We retrospectively identified 239 patients who underwent shock wave lithotripsy with pretreatment non-contrast computed tomography. We automatically measured the mean stone density of three-dimensional images of stones using a high-functional viewer. For comparison, mean stone density was also measured by two previously reported techniques using both the abdominal windows and the bone windows on the axial slice at the level of the largest diameter of the stone. We compared the outcome predictive power after the first treatment with outcomes according to measurement by four other methods. We also carried out logistic regression analysis, including mean stone density measured by three-dimensional images. RESULTS: The single treatment success rate was 48.5%. The effect size (14.148) of the mean stone density measured by three-dimensional images was higher than those of the other four manual methods. In addition, the area under the curve (0.6330) of the mean stone density measured by three-dimensional images was significantly higher than those of the other methods. Increasing stone volume (P = 0.002) and increasing mean stone density measured by three-dimensional images (P = 0.023) were significant independent predictors of the treatment outcome on multivariate analysis. CONCLUSIONS: This is the first study to compare the predictive powers for shock wave lithotripsy outcome of various mean stone density measuring methods. There is an indication that mean stone density automatically measured by three-dimensional images of stones is more useful than other measuring methods for predicting outcomes of shock wave lithotripsy.

Int J Urol. 2018 Oct 17. doi: 10.1111/iju.13827. [Epub ahead of print]

Peter Alken
on Friday, May 17 2019 08:15

On first glance it makes more sense to measure the mean density (MSD) of a whole stone to determine the stone’s susceptibility to ESWL than to measure the density at arbitrarily defined areas of the stone. But on the other hand one would expect that not the brittle parts of a stone but the total volume of the densest parts determine the susceptibility to ESWL. In the present paper CT images of stones with different densities are shown similar to the image (Fig.1) designed by the reviewer:

Fig.1 Schematic image of a plane through a stone with the different HU measured from the outer surface area (red) through all layers to the core (white)

In the present paper the images of a patient evaluated were generated in a single session with the CT scanner. All measurements were carried out by three radiologists and an average of these measurements was calculated.

Special software was used to measure the density of the 3D total stone volume. Values were compared to those generated by measuring the density in an axial slice at the level of the largest diameter of the stone in manually selected ROI areas: an elliptical plane or at three different points. These latter values were measured using both using both the abdominal windows and the bone windows. Thus 5 values were measured for each stone. Table 1 is adopted after table 2 in the publication

Table 2 MSD (HU) values of three different measurement methods in successful and unsuccessful cases (adapted from: Yamashita S et al. Three-dimensional mean stone density measurement is superior for predicting extracorporeal shock wave lithotripsy success. Int J Urol. 2018 Oct 17)

access paper https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4817509/ which references several other PMC papers on that subject.

Mean values ± SD.

In the table the reviewer has rounded the original MSD values for easier understanding and has added the bold, italic figures. Dividing the mean (measured) ROI HU values in the respective line by these factors will roughly give the mean (measured) 3 D image HU (in first line) (e.g. 730 ÷ 1,7 = 420). This suggests that not the measured values themselves have an impact on the effect size but the fact that the 3-D values have a smaller standard deviation.

The authors of this publication briefly quote an own paper (Reference 20) on the same subject and probably the same patient group (Yamashita S et al. Variation Coefficient of Stone Density: A Novel Predictor of the Outcome of Extracorporeal Shockwave Lithotripsy. J Endourol. 2017 Apr;31(4):384-390. doi:10.1089/end.2016.0719 (https://www.ncbi.nlm.nih.gov/pubmed/28052698). Curiously enough in that paper, the area under curve of variation coefficient of stone density (VCSD) (0.7181) was larger than that of mean stone density (MSD) (0.6384, p = 0.09) and that of the standard deviation of stone density (SDSD) (0.5412, p 0.01). In the present paper the area under curve values for the measured parameters are not as good as in the previous publication.

There are different ways to measure and handle CT data on stone density and different impacts on ESWL success has been demonstrated. Stone heterogeneity is one of these. To the interested reader I recommend this free.