Yamashita S et al., 2018: Noncontrast Computed Tomography Parameters for Predicting Shock Wave Lithotripsy Outcome in Upper Urinary Tract Stone Cases
Yamashita S, Kohjimoto Y, Iwahashi Y, Iguchi T, Nishizawa S, Kikkawa K, Hara I.
Department of Urology, Wakayama Medical University, 811-1 Kimiidera, Wakayama 641-0012, Japan.
Kidney stones are a major public health concern with continuously increasing worldwide prevalence. Shock wave lithotripsy (SWL) is the first line treatment choice for upper urinary tract calculi with ureteroscopy and has advantages of safety and noninvasiveness, but the treatment success rate of SWL is lower than that of other therapies. It is therefore important to identify predictive factors for SWL outcome and select a suitable treatment choice for patients with upper urinary tract calculi. In recent years, computed tomography (CT) has become the gold standard for diagnosis of upper urinary tract calculi. Several factors based on CT images, including skin-to-stone distance, mean stone density, stone heterogeneity index, and variation coefficient of stone density, have been reported to be useful for predicting SWL outcome. In addition, a new method of analysis, CT texture analysis, is reportedly useful for predicting SWL outcomes. This review aims to summarize CT parameters for predicting the outcome of shock wave lithotripsy in stone cases in the upper urinary tract.
Biomed Res Int. 2018 Dec 2;2018:9253952. doi: 10.1155/2018/9253952. eCollection 2018. Review.
Prediction of the outcome of SWL is difficult. Although different variables obtained from the NCCT examination can be used to get an idea of which efforts that might be required for a successful disintegration and fragment elimination these measures are relative. If repeated SWL sessions can be accepted also stones with negative CT-variables can be successfully treated.
In this article the authors emphasize the influence of stone volume that rather easily can be derived with modern imaging techniques. Stone volume accordingly is a better estimate of the stone burden than both diameters and surface areas. It is also emphasized in the article that determination of stone size from KUB or with ultrasound is inferior to measurements on CT-images, provided a bone window is used!
The lower calyx anatomy is mentioned briefly and infundibulum length of > 25 mm is considered problematic. The calyx angle and width are not mentioned.
One variable that always comes up in articles discussing prediction of SWL is the skin-to-stone distance (STS). An STS exceeding 10 cm is suggested as a limit above which SWL failure should be expected. With a slightly increased energy this obstacle can be overcome. Why STS should be measured in three different angles has never been clear to me. Better is to measure the distance in the line of the shockwave path. With modern technology that distance should be possible to measure directly and easily in the lithotripter.
Hounsfield units as an estimate of the stone hardness are useful predictors of SWL success and so are measurements of stone heterogeneity. In the future texture analysis might be useful.
When results of SWL are reported it is of importance to include the variables discussed in this article, but my personal opinion is that there is no need for an absolute prediction of the outcome of SWL. Most patients can be treated with this non-invasive technique and for those few patients who nevertheless end up with insufficient disintegration the advantage is that SWL does not require general or regional anesthesia.
With the described imaging improvements, it also might prove advantageously, as an alternative, to modify the stone treatment index (STI) that we formulated some years ago , for instance by using the stone volume (SV) instead of stone surface area (SA). Moreover, an adapted expression of stone hardness (density) (HU/1000) possibly can replace hardness index (HI). Factors of mean number of abnormalities for unfavorable lower calyx anatomy, large STS and other anatomical problems of importance can be added.
Previously described formula for STI 
1. Tiselius HG, Ringdén I.
Stone treatment index: a mathematical summary of the procedure for removal of stones from the urinary tract.
J Endourol. 2007 Nov;21(11):1261