Bridging the gap in PCNL preoperative planning: software-free stone metrics that outperform conventional measures

Accurate preoperative stone size assessment is crucial for predicting outcomes after percutaneous nephrolithotomy (PCNL), yet a gap persists between the accurate but impractical gold standard (3D software volume) and the practical but suboptimal conventional metrics (maximum diameter and cross-sectional area). This study aimed to validate two novel, software-free stone size metrics—the triaxial diametric product (X) and stacked volume (V)—designed for routine clinical computed tomography (CT) and compare their efficacy in predicting the stone-free rate (SFR) against conventional methods. We conducted a retrospective analysis of 283 patients who underwent PCNL. Stone size was measured using four metrics on pre-operative non-contrast CT: maximum diameter (R), maximum cross-sectional area (S), triaxial diametric product (X = x·y·z), and stacked volume (V). The predictive power for SFR was evaluated and compared using receiver operating characteristic (ROC) curve analysis and DeLong’s test. All four metrics were significant predictors of SFR (p < 0.001). The predictive accuracy, ranked by the area under the curve (AUC), was as follows: maximum diameter (R, AUC = 0.703) < maximum cross-sectional area (S, AUC = 0.734) < triaxial diametric product (X, AUC = 0.789) < stacked volume (V, AUC = 0.801). The novel metrics X and V both significantly outperformed the conventional metrics R and S (p < 0.01), with no significant difference between X and V. The triaxial diametric product (X) is recommended as a practical and more accurate alternative to conventional metrics for routine clinical practice, especially for stones > 1.5 cm, as it offers an improved balance of predictive accuracy, straightforward calculation, and easy integration into existing workflows without needing specialized software or additional costs. The stacked volume (V) serves as a feasible, software-free tool for practical size estimation in research settings. This study provides urologists with immediately applicable tools to enhance preoperative planning.