Evaluation of possible predictive variables for the outcome of shock wave lithotripsy of renal stones

Abstract

Purpose: The aim of this study was to evaluate possible predictive variables for the outcome of shock wave lithotripsy (SWL) of renal stones in a single center.

Materials and methods: Between March 2008 and March 2010, a retrospective review was performed of 115 patients who underwent SWL for solitary renal stones. The patients' characteristics and stone size, location, skin-to-stone distance (SSD), and Hounsfield units (HU) of stone were reviewed. The impact of the possible predictors on the disintegration of the stones was evaluated by logistic regression analysis. Receiver operator characteristic (ROC) curves were generated to compare the predictive powers of the variables.

Results: Seventy-nine patients (68.7%) had successful outcomes, whereas 36 patients (31.3%) had residual stones. Significant differences were found in the mean size and mean HU of the stones (size: 8.34±3.58 mm vs. 13.57±5.41 mm, p<0.001; HU: 675.29±254.34 vs. 1,075.00±290.41, p<0.001). In the unadjusted model, age, stone size, and stone density were significant predictors. In the reduced model, stone density and size were significant predictors for the outcome of SWL. The area under the ROC curve (AUC) was significantly higher for stone density and size than for the other parameters, but the AUC between stone density and size did not differ significantly (stone density: 0.874, stone size: 0.827, p=0.388).

Conclusions: Stone density and size were significant predictors of the outcome of SWL for renal stones less than 2.0 cm in diameter. We should consider HU and stone size when making decisions on the treatment of renal stones.

Keywords: Kidney calculi; Lithotripsy; X-ray computed tomography.

FIG. 1

Three regions of interest with a diameter of 2 mm were drawn on the stone in the axial plane of NCCT where the stone length was the longest. The mean number of HU calculated from the 3 regions represents the density of the stone. NCCT: non-contrast computed tomography, HU: Hounsfield unit.

FIG. 2

Measurement of the skin-to-stone distance at 0°, 45°, and 90° on an axial scan of NCCT. NCCT: non-contrast computed tomography.

FIG. 3

Comparison of ROC curves to test the statistical significance of the difference between the area under different ROC curves. In pairwise comparison of all predictors for the outcome of shock wave lithotripsy, stone density and stone size was not different (AUC difference: 0.0465, p=0.388). ROC: receiver operator characteristic, AUC: area under the ROC curve.