Park BH et al, 2012: Analyzing the effect of distance from skin to stone by computed tomography scan on the extracorporeal shock wave lithotripsy stone-free rate of renal stones
Park BH, Choi H, Kim JB, Chang YS
Department of Urology, Konyang University College of Medicine, Daejeon, Korea
PURPOSE: To determine whether the distance from skin to stone, as measured by computed tomography (CT) scans, could affect the stone-free rate achieved via extracorporeal shock wave lithotripsy (ESWL) in renal stone patients.
MATERIALS AND METHODS: We retrospectively reviewed the records 573 patients who had undergone ESWL at our institution between January 2006 and January 2010 for urinary stones sized from about 5 mm to 20 mm and who had no evidence of stone movement. We excluded patients with ureteral catheters and percutaneous nephrostomy patients; ultimately, only 43 patients fulfilled our inclusion criteria. We classified the success group as those patients whose stones had disappeared on a CT scan or simple X-ray within 6 weeks after ESWL and the failure group as those patients in whom residual stone fragments remained on a CT scan or simple X-ray after 6 weeks. We analyzed the differences between the two groups in age, sex, size of stone, skin-to-stone distance (SSD), stone location, density (Hounsfield unit: HU), voltage (kV), and the number of shocks delivered.
RESULTS: The success group included 33 patients and the failure group included 10. In the univariate and multivariate analysis, age, sex, size of stone, stone location, HU, kV and the number of shocks delivered did not differ significantly between the two groups. Only SSD was a factor influencing success: the success group clearly had a shorter SSD (78.25±12.15 mm) than did the failure group (92.03±14.51 mm). The results of the multivariate logistic regression analysis showed SSD to be the only significant independent predictor of the ESWL stone-free rate.
CONCLUSIONS: SSD can be readily measured by CT scan; the ESWL stone-free rate was inversely proportional to SSD in renal stone patients. SSD may therefore be a useful clinical predictive factor of the success of ESWL on renal stones.
Korean J Urol. 2012 Jan;53(1):40-3. doi: 10.4111/kju.2012.53.1.40. Epub 2012 Jan 25
PMID: 22323973 [PubMed] PMCID: PMC3272555. FREE ARTICLE
It is all statistics. My old boss Hohenfellner used to say: “% is a part of 100”.
The selection criteria that led to an exclusion of more than 90% of the stone patients treated with ESWL are somewhat difficult to understand. The authors do not define what a stone is that does not move. The sample size is very small. The failure group ( n=10) had 6/10 lower pole stones, the success group (n=33) had 13/33 lower pole stones. But this difference was not statistically evaluated. Shock wave frequency was 2/sec which impedes lithotripsy. Neither Hounsfield unit nor stone sizes were significantly correlated to the success rate.
It is difficult to imagine that a difference of 14 mm in skin-to-stone distance (SSD) was really the cause for the observed difference in stone free rates. “The success group clearly had a shorter SSD (78.25±12.15 mm) than did the failure group (92.03±14.51 mm)”.
In the paper by Pareek et al. (Reference 6: Shock wave lithotripsy success determined by skin-to-stone distance on computed tomography, Urology 66: 941–944, 2005) a SSD >10 cm signalized ESWL failure of lower pole kidney stones, but the skin-to-stone distance was measured in a different way.