Retrograde Intrarenal Surgery (RIRS) is a highly effective, minimally invasive procedure for the treatment of renal stones, particularly for those less than 2 cm in size. Current clinical practice relies primarily on two-dimensional measurements of stone size to predict surgical outcomes. However, the role of three-dimensional stone volume as a more accurate predictor of key outcomes, such as stone-free rates (SFR) and complications, remains unclear. This study aimed to compare the predictive capabilities of stone size, volume, and density  on postoperative outcomes, specifically 1.) SFR, 2.) complication rate using the Clavien-Dindo Grading System, and 3.) length of hospital stay, in patients who underwent RIRS for solitary renal stones <2cm.

A retrospective cohort study was conducted on 54 patients who underwent RIRS between August 2022 and August 2024 in a tertiary hospital in the Philippines. Data on patient demographics, stone characteristics (size, volume, density), and operative details were collected. Stone size was measured in centimeters, stone density in Hounsfield units, while stone volume was calculated using the ellipsoid formula. SFR one month after surgery was the primary outcome, while the complication rate (Clavien-Dindo classification) and length of hospital stay were the secondary outcomes. Predictive performance was evaluated using the receiver operating characteristic (ROC) curve analysis.

The stone-free rate of all participants was 94.44%, with 5.56% of patients having residual stones. Stone size showed poor predictive capability for residual stones (AUC: 0.6569) and complications (AUC: 0.6726), however, it demonstrated fair accuracy for predicting prolonged hospital stay (AUC: 0.7624). Stone volume exhibited slightly better but still poor predictability for residual stones (AUC: 0.6993), complications (AUC: 0.5747), and hospital stay (AUC: 0.6547). Stone density performed the worst for predicting residual stones (AUC: 0.5163) but showed moderate accuracy for predicting complications (AUC: 0.7364).

Both stone size and volume demonstrated limited predictive capabilities for residual stones and complications following RIRS, while stone density was moderately useful for predicting complications. These findings suggest that stone characteristics alone may not be sufficient for preoperative decision-making. Future research should focus on developing comprehensive predictive models that incorporate multiple clinical factors to improve surgical outcomes.