Foda K et al, 2013: Calculating the number of shock waves, expulsion time, and optimum stone parameters based on noncontrast computerized tomography characteristics
Foda K, Abdeldaeim H, Youssif M, Assem A
Department of Urology, College of Medicine, University of Alexandria, Alexandria, Egypt
OBJECTIVE: To define the parameters that accompanied a successful extracorporeal shock wave lithotripsy (ESWL), namely the number of shock waves (SWs), expulsion time (ET), mean stone density (MSD), and the skin-to-stone distance (SSD).
METHODS: A total of 368 patients diagnosed with renal calculi using noncontrast computerized tomography had their MSD, diameter, and SSD recorded. All patients were treated using a Siemens lithotripter. ESWL success meant a stone-free status or presence of residual fragments
RESULTS: Three hundred one patients were ESWL successes. A significant positive correlation was elicited between number of SWs and stone diameter, density and SSD; between ET and stone diameter and density. Multiple regressions concluded 2 equations: Number of SWs = 265.108 + 5.103 x1 + 22.39 x2 + 10.931 x3 ET (days) = -10.85 + 0.031 x1 + 2.11 x2 x1 = stone density (Hounsfield unit [HUs]), x2 = stone diameter (mm), and x3 = SSD (mm). Receiver operating characteristic curves demonstrated a cutoff value of ≤934 HUs with 94.4% sensitivity and 66.7% specificity and P = .0211. The SSD curve showed that a distance ≤99 mm was 85.7% sensitive, 87.5% specific, P <.0001.
CONCLUSION: Stone disintegration is not recommended if MSD is >934 HUs and SSD >99 mm. The required number of SWs and the expected ET can be anticipated.
Urology. 2013 Nov;82(5):1026-31. doi: 10.1016/j.urology.2013.06.061. Epub 2013 Sep 14
PMID:24044913 [PubMed - in process]
In this report from Egypt the authors have presented formulas for "predicting" the number of shockwaves and the time to fragment expulsion in a study referred to as "the largest prospective study to date of renal calculi managed by ESWL". The purpose of the investigation was to use formulas based on observations on CT examinations to determine the requirements for successful stone disintegration. It is stated by the authors that the "dilemma" encountered with ESWL is its "uncertainty". In this regard it might be appropriate to note that there in fact are very few surgical procedures that can be considered as 100% predictable; this is the case also for PNL or URS.
For this analysis the authors made a selection of their patients and excluded those who were considered poor candidates for SWL because of large stones or unfavourable lower calyx geometry. The outcome of the analysis is in line with previously reported results. It is surprising that the authors have used the longest diameter rather than the surface area of the stones, which they nevertheless measured. The problem is that the formulas were derived from data obtained in those 301 patients who were successfully treated. If a patient is selected with a stone diameter of 12 mm, MSD of 800 and SSD of 95, the predicted number of shockwaves would be 5571. Such a patient might, however, be representative also for one in the failure group. When limiting variables are put into the shockwave formula (HU 935, stone diameter 21 mm and SSD 99 mm) the predicted number of shockwaves will be 6588, which according to the routines at the authors´ department means two sessions. But the real expectation is a failure.
It is thus unlikely that the formulas can contribute to any better "certainty" in terms of outcome than by just examining the CT image. For stone elimination also other important factors have to be included, for instance BMI, age and mobility.