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

OBJECTIVE: 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. 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. RESULTS: 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. CONCLUSION: 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.