Both American and European guidelines on management of urolithiasis depend on the stone burden to determine the modality used for management. ^1,2 There are different methods of reporting stone burden, however. While these guidelines use the cumulative stone diameter (CSD) to assess stone burden, two of four recently published nephrolithometry scoring systems used different methods of assessing stone burden to predict percutaneous nephrolithotomy (PCNL) outcomes. ^3,4 Whereas the nephrolithometric nomogram from the Clinical Research Office of the Endourological Society (CROES) group used the stone surface area formula ∑ (length × width × length × width × π × 0.25) (mm²), ³ the S.T.O.N.E. (size [S], tract length [T], obstruction [O], number of involved calices [N], and essence or stone density [E]) nephrolithometry scoring system used the formula (length × width) mm² as a surrogate for measuring stone burden. ⁴ Two other nephrolithometry scoring systems did not use stone burden as a variable. ^5,6

Recent studies, however, showed that these scoring systems had comparable accuracies in predicting stone-free rate post-PCNL. In the study from Labadie and associates, ⁷ the receiver operating characteristic (ROC) curve revealed that the area under the curve (AUC) for the nephrolithometry nomogram was 0.67 (confidence interval [CI] 95% = 0.60–0.74), for the Guy's scoring system AUC was 0.634 (CI 95% = 0.566–0.702), and for the S.T.O.N.E. scoring system AUC was 0.670 (CI 95% = 0.602–0.738).

This was congruent with the study from Noureldin and colleagues ⁸ in which the AUC for the Guy's scoring system was 0.74 (95% CI 0.66–0.82) and AUC for the S.T.O.N.E. nephrolithometry scoring system was 0.63 (95% CI 0.54–0.72). In addition, their accuracies were not superior when compared with the stone burden as a predictor of PCNL outcome. ⁸ The AUC for the stone burden in the study from Labadie and coworkers ⁷ was 0.67 (95% CI 0.60-0.74) and in the study from Noureldin and associates ^8,9 was 0.64 (95% CI = 0.54–74). Furthermore, stone burden was found to be the only predictor of stone-free rate post-PCNL in a prospective study. ¹⁰

Review of the literature, however, revealed that there are different methods of calculating stone burden:

1. CSD: Calculated as the maximal diameter of a single stone (mm) or sum of the maximal diameters (mm) for multiple stones. ^1,2

Because stone burden is a major predictor of PCNL outcomes with comparable accuracies to the different nephrolithometry scoring systems, there should be a single method of calculating stone burden, and this method should be feasible, easily obtainable without any extra cost or software, and should best predict PCNL outcomes. Therefore, we propose a CROES study in which different methods of stone burden calculation could be measured, compared, and correlated with PCNL outcomes (stone-free rate, post-PCNL complications, operative time, estimated blood loss, and length of hospital stay) to determine the best method of stone burden calculation.

This would be of great benefit for multiple reasons:

1. Better comparison of PCNL outcomes among different centers

Therefore, we aim to search once again the global PCNL database aiming to provide comparative data on stone burden including stone volume. It is expected that these data will answer the main question put forward: Does stone volume provide a better predictor of outcome than largest size of the stone? In follow-up, we will be able to thus provide better care to our patients through another CROES project.