Stone volume versus stone size as predictors of stone-free status after percutaneous nephrolithotomy

Introduction. At present, the principal parameter used to determine stone burden is the maximum linear diameter, as assessed by non-contrast computed tomography. From a conceptual standpoint, however, linear size reflects the calculus in only one plane and should be less accurate than stone volume in representing the true burden, because volume captures the three-dimensional structure of the stone. 
Objective. To compare the prognostic value of stone volume and maximum stone diameter in predicting achievement of a stone-free status after percutaneous nephrolithotomy (PCNL).
Materials & method. This retrospective study included data from 388 patients with nephrolithiasis who underwent standard PCNL. Stone volume was calculated using automated lithometry based on CT data with Vitrea software v. 4.1.52, employing the segmented region volume measurement function. Stone size was determined by the maximum diameter. Postoperatively, stone-free rate (SFR) was assessed. Stone-free status was evaluated by non-contrast CT performed 3 days after surgery. SFR was defined as the absence of residual stone fragments larger than 3 mm on CT. To compare the predictive value of stone volume and stone size, ROC analysis was performed with calculation of the AUC, and statistical significance was confirmed using the DeLong test. Multivariable logistic regression was used to verify the independent predictive role of stone volume and size. In addition, the Mann–Whitney U test was applied for quantitative variables and the chi-square test for categorical variables. Differences were considered statistically significant at p < 0.05.
Results. Multivariable logistic regression demonstrated that, after adjustment for all relevant factors, both stone volume and stone diameter were independent statistically significant predictors of complete stone clearance. The AUC for the volume-based model was higher than that for the size-based model (0.80 vs 0.75). Subgroup analysis showed that volume had greater predictive ability for solitary non-staghorn stones, whereas no statistically significant difference was observed for multiple and staghorn calculi.
Conclusion. Preoperative measurement of renal stone volume in patients with a single large non-staghorn calculus more accurately reflects the true stone burden, since, unlike maximum linear diameter, it accounts for the threedimensional configuration of the stone. For multiple and staghorn calculi, however, no statistically significant difference was found in the ability to predict the outcome of PCNL.

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