INTRODUCTION AND OBJECTIVES
In the clinical trials on urolithiasis, stone size is mostly assessed by maximum diameter; however, most stones are not spherical. This could possibly result in bias and misleading results, because stone fragmentation time, the number of fragments, extraction time, and other outcomes are dependent on stone size. The current use of the diameter stands for an implicit correlation between the real stone volume and the diameter.
Software solutions can provide an automated assessment of a stone’s volume by clicking on the stone of interest on a computed tomography (CT) scan. We validated this software (syngo.via/ Siemens, Forchheim, Germany) at our institution and evaluated the variation between diameter, expected volume, and volumetry results.
MATERIAL AND METHOD
Over 100 stones were measured by three urologists independently, who assessed the maximum diameter using a digital sliding caliper and the volume using a water displacement/overflow method. The interrater correlation was calculated and the mean values were used as a reference. The same stones were then positioned in a radiologic phantom and CT scans were acquired at low-dose settings. Three radiologists measured the maximum diameter and volume using the software for automated measurement with 0.75 mm reconstructions.
In order to assess the value of the automated stone volumetry, we calculated the expected volume using the radiological maximum diameter and the formula: V=4/3 πr³ and compared the calculated volume to the volume measured with volumetry.
RESULTS
The interrater correlation for the reference measures was very good, qualifying those measurements as reference values. Some stones with the same diameter differed in volume by a factor of 2. There was a close correlation of the reference volume with the automated radiologic volume assessment, meaning that the displayed values are reliable and useful for size evaluation. The correlation of the expected volume with the reference volume was significantly worse.
CONCLUSION
Automated measurements of stone volume based on CT scans is possible and the accuracy is significantly higher compared with volumetric calculations based on the diameter.
The currently used size parameter ‘max diameter’ causes potential bias in studies. In order to avoid misleading results in clinical trials, size should be measured as volume or a combination of diameter and volume. Therefore, easy-to-use software solutions must be developed and introduced into clinical/scientific practice.
