Comparative performance of five nephrolithometry scoring systems and a novel nomogram for predicting stone-free rate following pediatric percutaneous nephrolithotomy

Introduction

In recent years, the global incidence of pediatric urolithiasis has shown a significant upward trend, emerging as a public health issue of increasing importance (1). For children with large (typically defined as >20 mm in diameter) or anatomically complex renal calculi, percutaneous nephrolithotomy (PCNL) is recommended as the first-line treatment by both the European Association of Urology (EAU) and the American Urological Association (AUA) guidelines (2). The reported stone-free rate (SFR) after a single PCNL procedure in pediatric patients ranges from 73% to 98.5%. This efficacy is not only markedly superior to traditional open surgery but is also generally more efficient for large stone burdens compared to other minimally invasive techniques such as extracorporeal shock wave lithotripsy (ESWL) or retrograde intrarenal surgery (RIRS) (3). However, as an invasive procedure, the success of PCNL is influenced by multiple factors, including stone characteristics (e.g., size, location, hardness) and renal anatomy, while also carrying risks of postoperative complications such as hemorrhage, infection, and adjacent organ injury (4).

To objectively quantify stone complexity, thereby enabling preoperative prediction of surgical outcomes and facilitating patient counseling, several nephrolithometry scoring systems have been developed. Among these, the S.T.O.N.E. (size, tract length, obstruction, number of involved calices, and essence) Nephrolithometry Score, the Guy’s Stone Score (GSS), the Clinical Research Office of the Endourological Society (CROES) scoring system, and the Seoul National University Renal Stone Complexity (S-ReSC) score have been widely applied and validated in adult populations (5-8). By integrating multiple dimensions such as stone size, location, degree of hydronephrosis, and calyceal involvement, these systems provide standardized tools for assessing surgical difficulty and predicting postoperative SFR.

However, the vast majority of these scoring systems were established based on adult anatomical and clinical data. Given the significant differences between children and adults in renal volume, collecting system anatomy, and physiological compensatory capacity, the accuracy and reliability of directly applying these adult-based models to pediatric patients remain questionable. Recognizing this limitation, the Stone Kidney Size (SKS) score was developed as the first system designed primarily for the pediatric population (9). Although some studies have explored the application of these scoring systems in pediatric PCNL, there is currently no consensus on which system offers the best predictive performance for children, and findings from different studies vary (10-13). Furthermore, research on the value of these systems in predicting postoperative complications, particularly within the pediatric cohort, is even more limited, representing a pressing issue in clinical practice (10-13).

Therefore, the objectives of this study were: (I) to retrospectively compare the accuracy of five established systems—the S.T.O.N.E. score, GSS, S-ReSC score, CROES nomogram, and SKS score—in predicting postoperative SFR after pediatric PCNL; (II) to evaluate the ability of these systems to predict postoperative complications; and (III) to identify key independent risk factors influencing surgical outcomes based on our institutional data, and subsequently develop and internally validate a more concise and practical predictive tool, named the “Friendship nomogram” (after our institution, Beijing Friendship Hospital). We aim for this work to provide more reliable evidence-based support for clinical decision-making in pediatric PCNL. We present this article in accordance with the TRIPOD reporting checklist (available at https://tau.amegroups.com/article/view/10.21037/tau-2025-1-965/rc).

Methods

Patients

This retrospective study analyzed the clinical records of pediatric patients who underwent mini-PCNL at the Department of Urology, Beijing Friendship Hospital, Capital Medical University, from July 1, 2020, to July 1, 2025. The study was conducted in accordance with the ethical principles of the Declaration of Helsinki and its subsequent amendments, and was approved by the Ethics Committee of Beijing Friendship Hospital (approval No. 2024-P2-543). Due to the observational nature of the data collection, the requirement for informed consent was waived.

A total of 140 patients who underwent mini-PCNL at our center during the specified period were identified. After applying the exclusion criteria, 96 patients under the age of 18 were included in the final analysis. The exclusion criteria were as follows: (I) absence of preoperative computed tomography (CT) imaging data (n=21); (II) loss to follow-up postoperatively (n=6); (III) prior placement of an ipsilateral ureteral stent or nephrostomy tube (n=9); (IV) concurrent bilateral PCNL surgery (n=8); or (V) patients with a bleeding diathesis or uncorrected coagulopathy.

Interventions

To minimize bias associated with the learning curve and technical variations, all surgeries were performed by the same senior endourologist who had extensive experience in pediatric PCNL (>200 procedures) prior to the study period. The surgical protocol remained strictly standardized throughout the five years.

After successful puncture with either a visual or a conventional puncture needle, the inner sheath was removed, and a percutaneous renal dilation guidewire was placed. The tract was serially dilated to 12–18 French (F) using fascial dilators. Lithotripsy was performed using a standardized set of equipment: an F8/9.8 rigid ureteroscope in combination with a 500 µm Holmium laser fiber. The laser parameters were consistently set to a high-power mode (40–50 W), with a frequency of 20 Hz and an energy of 2.0–2.5 J. Large fragments were retrieved via fluid irrigation or grasping forceps, while remaining fragments were ensured to be less than 2 mm in diameter. Upon completion of the procedure, a nephrostomy tube, a ureteral catheter or stent, and a urinary catheter were left in place. At 24 hours postoperatively, the ureteral and urinary catheters were removed if the patient had no complications such as fever or hematuria. If a ureteral stent was placed, it was removed 4 weeks postoperatively.

Data extraction and outcome measures

Baseline characteristics, including age, gender, height, weight, body mass index (BMI), anatomical abnormalities (e.g., horseshoe kidney, solitary kidney), and history of previous treatment, were collected through the electronic medical record system. For non-ambulatory patients or those unable to stand, recumbent length was measured and recorded as height. Preoperative evaluation was primarily based on non-contrast CT (NCCT) to obtain data on stone burden, laterality, location, number of involved calyces, stone density (Hounsfield Units), and degree of hydronephrosis. Intraoperative data included access tract length and operative time. Multiple stones were defined as the presence of ≥2 calculi visible on preoperative imaging. All radiological parameters were independently reviewed and measured by two experienced urologists blinded to the surgical outcomes. Discrepancies were resolved by consensus or consultation with a senior radiologist.

Based on preoperative CT images, the SKS score, S.T.O.N.E. Nephrolithometry Score, S-ReSC score, CROES nomogram, and GSS were calculated for all patients as previously described. The scoring systems were characterized as follows: the SKS score ranges from 2 to 4; the S.T.O.N.E. score ranges from 5 to 13; the S-ReSC score is on a 9-point scale; the CROES nomogram generates scores from 0 to 350; and the GSS classifies patients into four grades (I–IV). Stone burden was calculated using the formula Σ (0.25 × π × length × width). The Stone-to-Kidney Index (SKI) was calculated as the ratio of the maximum stone length to the maximum ipsilateral kidney length, measured on coronal preoperative NCCT images.

Stone-free status was defined as the absence of residual fragments >4 mm (clinically insignificant residual fragments). Evaluation was performed at the 1-month postoperative follow-up using NCCT or abdominal ultrasound, adhering to the As Low As Reasonably Achievable (ALARA) principle to minimize radiation exposure in pediatric patients. This <4 mm threshold was adopted because a significant portion of the cohort was evaluated via ultrasound; while ultrasound effectively detects clinically significant >4 mm fragments, it lacks the spatial resolution to reliably confirm an absolute 0 mm status. Patients confirmed as stone-free at this time point were classified into the stone-free group, regardless of any subsequent recurrence during long-term follow-up. Postoperative complications were recorded and graded according to the modified Clavien-Dindo classification system.

Statistical analysis

All statistical analyses were performed using SPSS (version 26.0) and R software (version 4.2.1). The Shapiro-Wilk test was used to assess the normality of continuous variables. Normally distributed data are presented as mean ± standard deviation (SD), while non-normally distributed data are presented as median (interquartile range) [M (Q₁, Q₃)]. Categorical variables are described as counts and percentages (n, %).

In the univariate analysis, differences between groups (stone-free vs. residual stone; complication vs. no complication) were compared. The Mann-Whitney U test was used for non-normally distributed continuous variables, and the independent samples t-test was used for normally distributed continuous variables. The Chi-squared (χ²) test or Fisher’s exact test was used for categorical variables, as appropriate.

To identify independent predictors of SFR, a multivariate logistic regression model was constructed using the backward elimination method. Variable selection for the multivariate model was based on a hybrid approach of statistical significance (P<0.05 in univariate analysis) and clinical relevance. This selection process was strictly constrained by the “events per variable” (EPV) principle to ensure model parsimony and prevent overfitting given the limited sample size. Prior to modeling, multicollinearity among candidate variables was assessed using the Variance Inflation Factor (VIF), and variables with high collinearity (e.g., number of stones and involved calyces versus overall stone burden) were excluded. Additionally, candidate variables with extreme data sparsity or zero-cell counts (e.g., prior treatment history) were excluded a priori to prevent complete separation and model non-convergence. Results are presented as odds ratios (ORs) with their 95% confidence intervals (CIs).

Based on the final regression model, a nomogram for predicting the probability of postoperative SFR was constructed using the rms package in R. Scores for each variable were assigned by linearly transforming the regression coefficients (β), where the variable with the largest absolute coefficient was scaled to 100 points, and other variables were weighted proportionally. The total score for each patient was calculated by summing the points of all predictors, which was then converted to the predicted probability of SFR. To assess the model’s stability and correct for potential overfitting, internal validation was performed using the bootstrap method with 1000 resamples.

Receiver operating characteristic (ROC) curve analysis was used to evaluate the predictive performance of the S.T.O.N.E. score, GSS, S-ReSC score, CROES nomogram, SKS score, and the newly developed Friendship nomogram for SFR. The area under the curve (AUC) was calculated to quantify the discriminative ability of each model. The optimal cut-off value for each scoring system was determined using the Youden index (J = sensitivity + specificity − 1), and the corresponding sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated. A two-sided P<0.05 was considered statistically significant.

Results

Patient demographics and baseline characteristics

This study included 96 pediatric patients who underwent PCNL, with an overall postoperative SFR of 77.08% (74/96). Postoperative stone-free status was evaluated via NCCT in 20 patients and via ultrasound in 76 patients. Based on postoperative imaging, patients were divided into a stone-free group (n=74) and a residual stone group (n=22). The cohort included patients with anatomical abnormalities, specifically horseshoe kidney (n=3) and solitary kidney (n=2). The baseline demographic and stone-related characteristics of both groups are summarized in Table 1. Univariate analysis revealed significant differences between the groups. Compared to the stone-free group, patients in the residual stone group were significantly older, taller, and heavier (all P<0.001). Regarding stone characteristics, the two groups differed significantly in terms of stone location distribution, number of involved calyces, previous treatment history, stone number, stone CT value, Stone Kidney Index (SKI), and stone burden. Correspondingly, the median scores of all evaluated scoring systems were also significantly different between the two groups (all P<0.001). However, no significant differences were observed in gender, BMI, degree of hydronephrosis, stone laterality, or postoperative length of stay (Table 1).

Independent predictors of SFR and development of the Friendship nomogram

Multivariable logistic regression analysis identified age, stone burden, and SKI as independent predictors for SFR (Table 2). These factors were used to develop a predictive nomogram, termed the “Friendship nomogram” (Figure 1). By summing the scores for each factor, clinicians can estimate the individual probability of achieving a stone-free status after PCNL. The performance evaluation of the nomogram demonstrated its excellent predictive efficacy. The model achieved an AUC of 0.878 (95% CI: 0.792–0.964), exhibiting outstanding discrimination. The calibration curve (Figure S1) indicated a high degree of concordance between the predicted probabilities and the actual observed outcomes. To assess the model’s stability and correct for the risk of overfitting, internal validation was conducted. After 1,000 bootstrap resamples, the optimism-corrected C-index was 0.8681. This finding was highly consistent with the mean AUC of 0.8649 obtained from 10-fold cross-validation, confirming the model’s excellent stability and potential for generalization.

Figure 1 The Friendship nomogram for predicting postoperative SFR. SFR, stone-free rate; SKI, Stone-to-Kidney Index.

Predictive performance of scoring systems for SFR

The predictive performance of each scoring system for SFR was evaluated using ROC curve analysis (Figure 2). The AUC values, ranked from highest to lowest, were as follows: CROES nomogram (0.882), Friendship nomogram (0.878), SKS score (0.843), S-ReSC score (0.841), GSS (0.822), and S.T.O.N.E. score (0.775). Among all models, the CROES nomogram demonstrated the highest predictive accuracy. Detailed performance metrics for each system, including optimal cut-off values, sensitivity, and specificity, are provided in Tables S1,S2.

Figure 2 ROC curve analysis of the S.T.O.N.E. score, GSS, S-ReSC score, CROES score, SKS score, and Friendship nomogram for predicting SFR. CROES, Clinical Research Office of the Endourological Society; GSS, Guy’s stone score; ROC, receiver operating characteristic; S.T.O.N.E., size, tract length, obstruction, number of involved calices, and essence; SFR, stone-free rate; SKS, stone kidney size; S-ReSC, Seoul National University Renal Stone Complexity.

Analysis of postoperative complications

Postoperative complications occurred in 42 of the 96 patients (43.75%). According to the modified Clavien-Dindo classification system, all of these were minor complications. Specifically, 35 cases were Grade I (e.g., transient postoperative fever) and 7 cases were Grade II (e.g., infections requiring intravenous antibiotics). There were no severe complications (Grade III or higher) observed in this cohort. Univariate analysis showed that among all clinical variables assessed, only the preoperative degree of hydronephrosis (P=0.02) and the S.T.O.N.E. score (P=0.03) were significantly different between the complication and no-complication groups (Table S3). None of the other scoring systems showed a significant association with the occurrence of postoperative complications. To further assess the predictive value of hydronephrosis and the S.T.O.N.E. score for complications, an additional ROC analysis was conducted. The analysis yielded an AUC of 0.672 for the degree of hydronephrosis and an AUC of 0.623 for the S.T.O.N.E. score, indicating that both have limited ability to predict postoperative complications (Figure S2 and Table S4).

Discussion

PCNL represents a first-line therapy for complex renal calculi in children, with the postoperative SFR serving as a key metric of its efficacy. Achieving a high SFR is crucial for reducing the risk of stone recurrence and improving the long-term quality of life for pediatric patients. However, the accurate preoperative prediction of SFR remains a significant clinical challenge in pediatric urology. In our pediatric cohort, existing scoring systems demonstrated varied predictive efficacy. The CROES Nephrolithometric Nomogram (AUC =0.882) and our newly developed Friendship nomogram (AUC =0.878) showed the best predictive performance, followed by the SKS score (0.843), the S-ReSC score (0.841), and the GSS (0.822). The S.T.O.N.E. Nephrolithometry Score was comparatively the weakest predictor (AUC =0.775). Notably, none of the scoring systems performed well in predicting postoperative complications, and our analysis did not identify any strong predictors for this outcome.

These findings are largely consistent with several previous studies. For instance, research by Akdogan et al. (12) similarly found that the CROES nomogram (AUC =0.839) was significantly superior to the GSS (AUC =0.670) for predicting SFR in children, and also reported that both had limited utility for predicting complications, which aligns with our observations. A report by Altunkol et al. (10) showed the GSS (AUC =0.817) to be the best predictor, followed by the SKS and S.T.O.N.E. scores, an order that is generally comparable to our results. However, our findings contrast sharply with those of Aldaqadossi et al. (14), whose study reported exceptionally high predictive power for the S.T.O.N.E. score (AUC =0.92), far surpassing the CROES and GSSs. Furthermore, a meta-analysis (15) in adult populations indicated that the Guy’s, CROES, and S.T.O.N.E. scores had similar efficacy for predicting SFR, which again highlights the discrepant performance of the S.T.O.N.E. score across different age groups. These discrepancies among studies can likely be attributed to several factors. First, patient heterogeneity is a critical element; children are not merely small adults, and differences in renal anatomy, physiology, and surgical tolerance can influence outcomes. Furthermore, pediatric urolithiasis is often associated with distinct underlying conditions (e.g., metabolic abnormalities like cystinuria, or anatomical malformations) that differ from the adult population. These etiologies influence stone composition and hardness, which in turn affect lithotripsy efficiency and the probability of stone clearance. The failure of current nephrolithometry scoring systems to account for these biological and compositional variables may explain some of the discrepancies in their predictive accuracy when applied to pediatric cohorts. The complexity of the stone burden (e.g., the proportion of staghorn calculi) in different cohorts also directly impacts surgical difficulty and SFR. Second, variations in surgical technique, such as the use of mini-PCNL versus standard PCNL, affect procedural trauma and stone clearance efficiency. Surgical patient positioning and surgeon experience are also significant variables. Finally, methodological differences, such as the limitations of a single-center retrospective design and varied definitions of “stone-free” (e.g., <2 vs. <4 mm residual fragments), can alter the calculation of SFR and thus the apparent performance of each scoring system.

The primary contribution of this study is the development and internal validation of a novel predictive tool, the Friendship nomogram. Its principal advantage lies in its ability to offer high predictive accuracy while maintaining remarkable clinical simplicity, positioning it as a potentially valuable tool for clinical decision-making. The Friendship nomogram is composed of three readily available preoperative variables: patient age, stone burden, and the SKI. First, age was identified as an independent protective factor for achieving a stone-free status (OR =0.83, P=0.02), with younger children having a higher SFR. Although the association between age and SFR is debated in adult or mixed populations (16), our results are highly consistent with a key pediatric study by Dogan et al. (17). Second, stone burden, a fundamental factor influencing PCNL outcomes, is a core component of several established scores (e.g., the S.T.O.N.E. score, CROES nomogram). It has been robustly demonstrated in both adult (18) and pediatric (14) studies that a smaller stone burden is significantly associated with a higher SFR. Third, the SKI, the core of the SKS score, effectively normalizes for the physiological increase in kidney size with age by using a ratio of stone-to-kidney length, thereby providing a more accurate reflection of the relative stone size in children (9). Previous research (19,20) has already shown that a lower SKI is significantly correlated with a higher clearance rate. All three variables are easily obtained from standard preoperative imaging and medical records. In terms of predictive performance, the AUC of the Friendship nomogram (0.878) was nearly identical to that of the more complex, six-variable CROES nomogram (0.882) in our cohort. At its optimal cutoff value of 0.75, the Friendship nomogram demonstrated a high specificity of 90.91% and a PPV of 96.30%, indicating that it can reliably identify children who are highly likely to become stone-free postoperatively.

Several limitations of this study must be acknowledged to ensure a balanced interpretation of our findings. First, the retrospective, single-center design inherently limits the generalizability of our findings to broader patient populations. Second, although our sample size (n=96) is comparable to other pediatric PCNL studies, the number of events in the residual stone group (n=22) is relatively small for multivariable modeling. We acknowledge that developing a predictive model with this sample size raises concerns regarding potential overfitting. Although we utilized bootstrap internal validation (1,000 resampling iterations) to demonstrate the model’s stability, the Friendship nomogram should currently be considered a preliminary, exploratory model. Third, the assessment of SFR relied on a combination of NCCT and ultrasound, rather than a uniform postoperative NCCT. While this heterogeneity—stemming from the ethical imperative to minimize radiation exposure (ALARA principle) in children—reflects real-world clinical practice, it may impact the sensitivity of detecting small residual fragments compared to purely CT-based studies. Consequently, our definition of stone-free status (<4 mm) is clinically practical but relatively generous; adopting a stricter definition (e.g., absolute 0 mm stone-free) might alter the calculated SFR and potentially yield different predictive distinctiveness and performance rankings among the evaluated scoring systems. Fourth, while we standardized the measurement of variables, the calculation of nephrolithometry scores relies on manual interpretation of imaging, which inevitably introduces a degree of subjective observer bias. Additionally, due to the retrospective nature, comprehensive metabolic evaluation was not available for all patients, limiting our ability to analyze the impact of metabolic etiologies on SFR. Finally, the Friendship nomogram was developed and validated within the same internal dataset. Therefore, its clinical utility and robustness must be confirmed in large-scale, prospective, multicenter studies before widespread application, and it is important to emphasize that this preliminary nomogram cannot yet replace established scoring systems until it is successfully validated in external cohorts.

Conclusions

This study confirms that all evaluated nephrolithometry scoring systems—including the S.T.O.N.E. score, GSS, S-ReSC score, CROES nomogram, and SKS score—can effectively predict the postoperative SFR in pediatric PCNL, with the CROES nomogram demonstrating the highest predictive accuracy. Notably, our novel Friendship nomogram, based on just three variables (age, stone burden, and the SKI), achieved predictive performance nearly identical to that of the more complex CROES nomogram.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the TRIPOD reporting checklist. Available at https://tau.amegroups.com/article/view/10.21037/tau-2025-1-965/rc

Data Sharing Statement: Available at https://tau.amegroups.com/article/view/10.21037/tau-2025-1-965/dss

Peer Review File: Available at https://tau.amegroups.com/article/view/10.21037/tau-2025-1-965/prf

Funding: This research was supported by Capital’s Funds for Health Improvement and Research (No. 2022-2-1102), the Beijing Natural Science Foundation (No. 7232027), and the Beijing Tongzhou District Science and Technology Plan (No. WS2025028).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tau.amegroups.com/article/view/10.21037/tau-2025-1-965/coif). J.L. reports that this research was supported by Capital’s Funds for Health Improvement and Research (No. 2022-2-1102), the Beijing Natural Science Foundation (No. 7232027), and the Beijing Tongzhou District Science and Technology Plan (No. WS2025028). The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the ethical principles of the Declaration of Helsinki and its subsequent amendments, and was approved by the Ethics Committee of Beijing Friendship Hospital (approval No. 2024-P2-543). The requirement for informed consent was waived due to the retrospective and observational nature of the study.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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