Since the first description of the technique in 1976 (Fernstro¨m and Johansson, 1976), percutaneous nephrolithotomy has been widespread for the treatment of renal calculi. It is the golden standard for the treatment of 2-cm kidney stones (Miernik et al., 2014). PCNL’s success rate is between 56% and 96% in various series (Matlaga et al., 2005; Akman et al., 2011; Rosette et al., 2011; Labadie et al., 2015). Many factors contribute to the success of stone clearance including the stone size, location, number, and grade of hydronephrosis, as well as surgeon’s experience. To predict the outcomes after PCNL, several scoring systems have been devised including Guy’s stone score, S.T.O.N.E nephrolithometry system, CROES nephrolithometry nomogram, and S-ReSC score (Al Adl et al., 2020). Guy’s stone score is easy to apply and has been validated in multiple studies. The S.T.O.N.E. score is based on factors determined through CT imaging, which is the currently preferred imaging modality for patients with nephrolithiasis (Noureldin et al., 2015a). The CROES nomogram was developed from data in a large multicenter database and has high statistical power. Determination of the S-ReSC score relies on stone location only, providing a simple approach to grading disease complexity (Noureldin et al., 2015b). Each system has advantages and disadvantages, but several studies suggest that their ability to predict the stone-free rate is comparable (Wu and Okeke, 2017).

Machine learning techniques have been used extensively in the field of clinical medicine, especially when used for the construction of prediction models. The outperformance of ML over conventional data analysis models has been shown in the urology-oncology literature (Hung et al., 2018; Andras et al., 2020; Rodrigo. et al., 2020; Ström et al., 2020).

In predicting post-lithotripsy outcomes with machine learning, there are only three studies published until now (De Perrot et al., 2019; Tayyebe. et al., 2019; Aminsharifi et al., 2020). Aminsharifi et al. (2020) first used the machine learning method for predicting post-PCNL outcomes compared to current scoring systems. They found machine learning-based software was superior in predicting SFS after PCNL, with an AUC of 0.915 compared to 0.615 (GSS) and 0.621 (CROES nomograms) (p < 0.01). More than 20 variables of 146 patients were inputted for the training of machine learning in their study. Alireza used a support vector machine (SVM) as the machine learning technique. We know that the machine learning algorithm includes some other methods, such as decision trees, random forests, artificial neural networks, Bayesian learning, Deep Learning, and so on. In this study, we used four machine learning methods (Lasso logistic, random forests, SVM, and Naive Bayes) to predict the SFS of PCNL with the information of 222 patients. We compared the outperformance of ML to Guy’s score and the S.T.O.N.E score system at the same time.

A total of 222 patients (132 males, 59.5%) were enrolled. The mean age was 54.8 ± 13.3 years, and the mean stone burden was 563.4 ± 517.6 mm². The mean Guy’s score was 3.2 ± 0.9, and the mean S.T.O.N.E. score was 8.9 ± 1.8. Table 1 shows the preoperative factors including individual variables and renal and stone factors. Table 2 shows the actual postoperative data for these patients. The overall SFS was 50% (111/222). Figure 2 shows the stone-free rate in each subgroup of GSS grades and the S.T.O.N.E score systems. The number of fever and infections during hospitalization was 18.9% (42) and 8.6% (19). Postoperative blood transfusion due to significant blood loss happened in nine patients (4.1%). With the follow-ups for at least 1 year, there were 12 patients (5.4%) who accepted ancillary procedures to manage residual renal stones.

We have used four machine learning methods to analyze the outcomes to predict the stone-free status. Table 3 shows the AUC, sensitivity, specificity, and accuracy of each prediction method to the results of the stone-free status. When using AUC as a measure of the predictive model performance, as shown in Table 3, the AUC of Lasso logistic was 0.879. It was superior to those of RF, SVM, and Naive Bayes (0.803, 0.818, and 0.803, respectively). The AUCs of the GSS and S.T.O.N.E were 0.800 and 0.844, respectively, which were lower than the Lasso logistic. Figure 3 shows the ROC curves of the four MLMs, as well as the GSS and S.T.O.N.E score system.

As shown in Table 3, the accuracies of the four MLMs were also superior to those of the S.T.O.N.E score system. The sensitivities of the MLMs were 75.8–83.3%, which were higher than the S.T.O.N.E. score system. The machine learning system of LL recognized stone burden and stone location as the most highly weighted preoperative factors affecting the post-PCNL-SFR.

The incidence and prevalence of kidney stones have increased by three times over the past 4 decades (Thongprayoon et al., 2020). The prevalence of kidney stones is estimated at about 5–10% in Europe, 4% in South America, and 1–19% in Asia currently (Sorokin et al., 2017; Liu et al., 2018). Without a doubt, kidney stones represent a considerable burden for public healthcare systems.

Thomas et al. (2011) were the first to introduce Guy’s stone score (GSS) to predict the success of the post-PCNL stone-free status (SFS). The model is reproducible, provides quick and easy office-based categorization of renal stones in four grades based on stone shape and configuration, and correlates well with the SFS; however, it fails to take into account the size and density of the stone. The S.T.O.N.E. nephrolithometry scoring system of Okhunov et al. (Zhamshid. et al., 2013) is based on non-contrast CT (NCCT) having five variables; a score of 5–6 (low complexity) has an overall SFS of 94–100%, and a score 9–13 (high complexity) has an overall SFS of 27–64%. Also, greater S.T.O.N.E. scores are associated with a greater estimated blood loss (EBL), longer operative times (LOTs), and increased length of stay (LOS) in hospital. Smith et al. (2013) developed the CROES (Clinical Research Office of the Endourological Society) nomogram to predict the SFS after PCNL based on a global database study of 5,830 patients. Six characteristics (stone burden, number, location, multiple, staghorn, and institute-level case volume) are included in this nomogram. It achieved a remarkable prediction accuracy of 76%, but it is laborsome and time-consuming.

Many studies have compared the predictive performance of these score systems in post-PCNL SFR. Most studies have examined the performance of these scoring systems to predict SFR equally but not equally to predict complications. The AUC ranges from 0.63 to 0.853 (Wu and Okeke, 2017), and the different scoring system has its drawbacks or limitations. For example, in Guy’s score system, partial staghorn stone was not clearly defined. The S.T.O.N.E. nephrolithometry scoring system relies solely on preoperative CT. The CROES nomogram requires information that might not be readily available (case volume and treatment history). So one simpler and easier application stone score system is needed nowadays. Alireza and his colleagues (Aminsharifi et al., 2017) were the first to use machine learning methods to evaluate the stone-free rate and complications after PCNL. They used ANN to predict the stone-free rate. The accuracy was 81.0–98.2%. The AUC was 0.861. In 2019, his team (Aminsharifi et al., 2020) reported they used software to predict the SFR after PCNL with the AUC of 0.915. In our study, we used four machine learning methods to predict the SFR of PCNL compared with Guy’s system and the S.T.O.N.E. nephrolithometry system. The machine learning methods (MLMs) include Lasso logistic, random forests, SVM, and Naive Bayes. The AUC of the MLMs was superior than that of Guy’s stone score system. The sensitivity and accuracy of MLMs were superior to that of the S.T.O.N.E. nephrolithometry system.

Machine learning is built on the statistical framework. Different approaches are designed to make the most accurate prediction possible. It has been proved to have a good performance to predict the SFR post-PCNL. Although we did not have an advantageous performance of AUC of 0.915 (Aminsharifi et al., 2020), in this study, we found the MLMs could predict the stone-free rate with the AUC not inferior to that of Guy’s stone score or the S.T.O.N.E score system. The machine learning algorithm mainly includes random forests, decision trees, artificial neural networks, Bayesian learning, and Deep learning. Each approach has its advantage and disadvantage. We have tried four methods to predict the stone-free rate in this study, and all of them got a fairly superior performance, as well as the clinical scoring systems being currently available. Machine learning methods are a good tool to predict the stone-free rate with AUCs after PCNL.

So far, in the field of urinary stones, there have been few studies using machine learning methods to predict operative outcomes or help make operative decisions. As one author commented (Peng et al., 2021), to improve the application of MLMs in uritholiasis, two categories should be considered: first, more people including urologists, statisticians, and computer experts need to be involved in this project; second, more data from different regions or population should be collected for future event prediction. We need to establish, manage, and share a cross-country or nationwide database, through which machine learning or AI would contribute to the field of calculi or other issues in the near future.