Identifying predictors of failure in pediatric retrograde intrarenal surgery: Why stone volume matters

Pediatric urolithiasis is often associated with metabolic alterations and urinary malformations, leading to a higher risk of recurrence. Effective management requires minimally invasive procedures that achieve high stone-free rate (SFR). Identifying risk factors that affect SFR is essential for selecting the most suitable surgical approach for each patient. This study aims to identify variables associated with reduced SFR in pediatric patients undergoing RIRS to support pediatric urologists' clinical decision-making.

This retrospective study included patients under 18 years of age with renal stones <20 mm treated with RIRS between August 2021 and August 2023. All patients underwent preoperative computed tomography scanning. Stone volume was calculated using the ellipsoid formula. In cases of multiple stones (≥2), diameters and volumes were calculated for each stone individually and then summed to determine the total stone burden. SFR was defined as the absence of stones >3 mm on ultrasound performed at the third postoperative month. Receiver operating characteristic (ROC) curves were generated to determine cut-off points for continuous variables to support clinical decision-making and to evaluate the predictive capacity of stone burden variables.

A total of 62 RIRS were analyzed. The average patient age was 10 years, with 37% having multiple stones and 56.4% with stones located in the lower calyx. Mean stone diameter, volume and density were 11.2 mm, 438 mm³, and 872 HU, respectively. The overall SFR was 82%. Significant predictors of reduced SFR included multiple stones (p = 0.001), lower calyx location (p = 0.017), stone density (p = 0.019), volume (p = 0.001), and diameter (p = 0.001). ROC analyses identified key cut-off values for stone burden. A stone volume ≥1000 mm³ and a stone diameter ≥15 mm were associated with RIRS failure, showing high specificity (98% and 92%, respectively) and markedly increased risks of residual stones (107-fold and 18-fold, respectively). Although both stone burden variables demonstrated high predictive capacity for success on ROC curves, stone volume showed a significantly higher AUC (p = 0.032).

RIRS is a safe and effective treatment for pediatric renal stones <20 mm, achieving high success rates with a low incidence of complications. Several factors were associated with SFR, including stone number, location, density, diameter and volume. However, stone volume demonstrated the highest predictive accuracy and was the only independent predictor of surgical success in multivariable analysis. A stone volume ≤1000 mm³ may therefore represent a practical and clinically meaningful threshold for selecting RIRS as first-line therapy in pediatric renal lithiasis.