Is Triple D Score Effective to Predict the Stone-Free Rate After Shockwave Lithotripsy in Pediatric Population?

Abstract

Introduction:

Triple D score was developed using skin-to-stone distance (SSD), stone density (SD), and stone volume (SV) for prediction of extracorporeal shockwave lithotripsy (SWL) outcomes in adults. SWL is the first-line treatment method for kidney stones <2 cm in children, however, it was not validated in the pediatric population. This article aims to validate Triple D score in pediatric patients.

Materials and Methods:

Of the 269 children treated with SWL between 2007 and 2021, a total of 147 children who had adequate follow-up data and evaluated with noncontrast CT before SWL were included in the study. Parameters were calculated for each of the SV, SSD, and SD variables. Receiver operator characteristic analysis was used to set cutoff values. Triple D scores were calculated, and success rates were determined for each score. Stone-free status was determined as complete clearance after 3 months of final SWL.

Results:

The median age of the study group was 7 years (range 1–16). Ninety-three of the 147 (63%) children had stone-free status. Mean SV, SSD, and SD values were significantly higher in the SWL-failed group than in the stone-free group after detecting cutoff values of 155 mm³, 6.5 cm, and 550 HU, respectively. Stone-free rates were detected as 23.8%, 35.1%, 74.0%, and 92.0% with the Triple D scores of 0, 1, 2, and 3 points.

Conclusions:

Our study confirms that Triple D scores support the SWL outcomes in the pediatric population. We believe that our research on Triple D score validation in children is of great clinical importance although various factors may affect to predict the success of SWL. IRB Approval: 2021/514/194/14.

Introduction

Endourologic methods gain popularity against extracorporeal-shockwave lithotripsy (SWL) because of the miniaturization of the instruments used for treatment of stone disease, however, today SWL is still the first-line treatment method in children with kidney stones <2 cm.¹ Urolithiasis in the pediatric population differs from that in adults with its recurrent nature appropriately. SWL stone-free rates were reported between 59.2% and 94.8% for children.² A stone-free status belongs to multiple factors such as genetics, metabolics, and anatomy, and could not be achieved in some circumstances (e.g., oxalosis or nephrocalcinosis).¹

In addition, inappropriate patient selection in SWL is one of the most important factors determining stone-free status in children, and nomograms have been reported for this purpose.^3,4 In recent years, there has been considerable interest in scoring systems, ease to use, and could be involved in reports of noncontrast enhanced computed tomography (NCCT).^5
–7 After calculating the stone density (SD), stone volume (SV), and skin-to-stone distance (SSD).⁸ Tran and colleagues established the Triple D scoring system, which is one of the scoring methods.

Several authors reported “Triple D score” as a predictor of SWL outcomes in adults, to our knowledge no other series has evaluated this system in the pediatric population.^9
–11 The aim of our study was to establish our own cutoff points for the SV, SSD, and SD parameters, then calculate our own Triple D Scoring System to predict success of SWL stone-free rates in children.

Patients and Methods

Data of 269 pediatric patients (<16 age) who underwent SWL in our institution from January 2007 to January 2021 were reviewed. A total of 147 children with previously untreated stones <20 mm that had been evaluated with noncontrast CT before SWL and adequate follow-up imaging data were included in the study. Patients with stone in solitary kidney, decreased renal function (serum creatinine >1.2 mg/dl or >0.031 mmol/L), stones <7 mm, multiple stones in one kidney, history of urinary tract and stone surgeries were excluded from study. Patients who have no stone fragment were accepted as successful SWL (stone-free) session 2–3 weeks after session.

Age, gender, SV, SSD, and SD were recorded. Elipsoid SV was estimated using the formula: SV = π/6 (anteroposterior × transverse × craniocaudal diameters of the stone in millimeters), reported by Finch et al.¹² SSD was calculated as reported previously by Pareek and colleagues, and SD was measured by determining average HU.⁷ Triple D scores were calculated whether the measured parameters of SV, SSD, and SD were less than cutoff values (0 and 1 for values lower and upper than cutoff points) reported by Tran and colleagues.⁸

SWL was performed under sedoanalgesia (ketamine, 0.5–1.0 mg/kg, and midazolam, 0.05–0.1 mg/kg) at outpatient condition. Before SWL, we assessed urine samples and urine culture. The session was postponed and antibiotic treatment was started if there was urinary tract infection. SWL sessions were performed using an electromagnetic shock wave generating ultrasonographic guidance (Dornier CompactSigma; MedTech, Munich, Germany). The SWL procedure was usually started at frequency of 60–90 shock waves (SW)/min, and up to a total of 2500 SWs per session with the same energy level (level 2, 0.029 J/SW) during all treatments. Patients were assessed at 1 week after session with renal unit.

Structural and compositional stone analysis was performed using X-ray diffraction in patients that can collect a large stone fragment from the urine post-SWL. Maximum three sessions were performed. Patients who have stone fragment (>4 mm) after three sessions were considered SWL failure and these patients were evaluated after 4 weeks for spontaneous passage.

Statistical analysis

Statistical analysis was performed with IBM SPSS Statistics for MacOs, Version 25.0. (IBM Corp., Armonk, NY). Descriptive statistics for the parameters were provided for the entire cohort. Continuous variables were reported as mean and standard deviation, and categorical variables were reported as percentages. Student's t-test or Mann–Whitney U test was used for the comparison of the continuous variables, and chi-square test or Fisher's exact test was used for the comparison of the categorical variables in the renal stone patients with respect to SWL success.

Receiver operator characteristic (ROC) curves were generated to determine cutoff values for the SV, SSD, and SD, and area under the curve (AUC) values for each parameter were provided. Patients were scored with respect to these cutoff values.

A further comparison of the Triple D score groups for success rates was performed using the chi-square test.

Results

A total of 147 children consisting of 84 girls and 63 boys with a mean age of 7.5 ± 4.5 years were enrolled to the study. The overall population's mean SV, SSD, and SD values were 175 ± 68 mm³, 6.7 ± 0.7 cm, and 590 ± 110 HU, respectively. Stone-free status was established in 93 of the 147 (63%) patients after as much as three SWL sessions, and success rate after the first SWL session was 53.3%. Table 1 summarizes the characteristics of the patients in terms of treatment success. Patients with stone-free and not stone-free outcomes reported similar age, gender, and number of SWL sessions.

Table 1.

Demographic Characteristics of Study Population

Parameters	Overall (n = 147)	Stone free (n = 93)	Not stone free (n = 54)	p ^a
Age, years (mean ± standard deviation)	7.5 ± 4.5	7.3 ± 5.0	8.2 ± 4.3	0.08
Gender, n (%)				0.12
Male	84 (57)	52 (56)	32 (59)
Female	63 (43)	41 (44)	22 (41)
SV (mm³), mean ± standard deviation	175 ± 68	141 ± 3	240 ± 3	<0.001
SSD (cm), mean ± standard deviation	6.7 ± 0.7	6.2 ± 0.5	7.1 ± 0.4	<0.001
SD (HU), mean ± standard deviation	590 ± 110	522 ± 105	692 ± 123	<0.001
Median number of SWL episodes (range)	3 (1–3)	2 (1–3)	3 (1–3)	0.26

Comparison between the stone-free and residual groups.

SD = stone density; SSD = stone-to-skin distance; SV = stone volume; SWL = extracorporeal shockwave lithotripsy.

The mean SV, SSD, and SD values in patients who were not stone-free were substantially higher than in patients who were stone free (all p 0.001). For the SV, SSD, and SD parameters, ROC curves were created, and cutoff points were selected based on the highest sensitivity and specificity values. For all of the parameters, significant AUC values were obtained. AUC value of the Triple D score was 0.933 respectively. The AUC value of the Triple D score was higher than the AUC values of each of the SV, SSD, and SD parameters. The results of AUC and cutoff levels with sensitivity and specificity values are summarized in Table 2, and ROC curves are given in Figure 1, respectively.

FIG. 1.

ROC curves for the SV, SSD, and SD in patients. (a) ROC curve for SV in patients. (b) ROC curve for SSD in patients. (c) ROC curve for SD in patients. (d) ROC curve for Triple D score in patients. AUC = area under the curve; ROC = receiver operator characteristic; SD = stone density; SSD = skin-to-stone distance; SV = stone volume. Color images available online.

Table 2.

Cutoff Values of Triple-D Scores and Parameters After Determining Receiver Operator Characteristic Analysis with Sensitivity and Specificity

Parameters	Sensitivity (%)	Specificity (%)	Cutoff value	AUC (95% CI)
SV (mm³)	89	84	155	0.892 (0.836–0.948)
SSD (cm)	75	68	6.5	0.762 (0.634–0.884)
SD (HU)	81	78	550	0.805 (0.729–0.881)
Triple D score	94	83	1.5	0.933 (0.889–0.976)

AUC = area under the curve; CI, confidence interval.

Triple D scores were calculated after patients were grouped according to the stated cutoff values. The Triple D score was found to be significantly correlated with a 92% success rate, with a Triple D score of 3 resulting in stone-free outcomes. Patients with a Triple D score of 2, 1, and 0 had a stone-free rate of 74%, 35.1%, and 23.8%, respectively.

In this pediatric population, hematuria was seen in 11 (7%) cases after SWL treatment, who were treated conservatively without the need for transfusion or intervention. In 8 (5.4%) of the patients, ureteral stenting was required, and 13 (8.8%) of the patients had urinary tract infections that required antibiotic treatment.

Discussion

Despite the advances in endourology, SWL is still the first-line treatment protocol for the majority of the renal and proximal ureteral stones in children because of its noninvasive nature, its suitability for use with outpatients, and its high patient compliance.¹³ In daily practice, pediatric nomograms provide valuable information and are mostly needed to assess treatment outcomes.^3,4 Unfortunately, those nomograms fail to support the stone composition, SD, SSD, or infundibular angle obtained by modern radiologic imaging techniques. Moreover, physicians do not have time to fill up these nomograms with their patients' information because of complicated, unpractical, and confusing nature of nomograms.³

In recent years, there has been growing interest in the Triple D scoring system, that is easy to calculate and may be included in routine radiologic reports to guide physicians to make treatment decisions for kidney stones. However, those articles focused on adults only.^8

–11 The aim of this study was to validate the Triple D score in a pediatric population, and our results showed that it is useful in predicting effective SWL outcomes in children with kidney stones.

Tran and colleagues were among the first to employ the Triple D score, and they used ROC curves to highlight the relevance of the SV, SSD, and SD, with each parameter having a significant correlation with success rates.⁸ Similarly, Gökce et al reported the significance of the SV, SSD, and SD with ROC curves separately both in renal and ureteral stones of adults, and the corresponding AUC values were 0.712, 0.687, and 0.725 in renal stones and 0.858, 0.775, and 0.844 in ureteral stones, respectively. In our study, pediatric population was also found to have a significant correlation with success rates after determining the ROC curves for each parameter in the kidney stones.

Previous studies regarding Triple D score have shown an overall 57.1%–73% stone-free rate in adults. The stone-free rate of SWL therapy has been reported to be >70% in the literature, and it can reach 100% with numerous sessions in pediatrics.¹⁴ However, in our study, the success rate of stone-free status was found to be 63%. This difference may be related to the definition of success in our study that residual fragments after SWL was accepted as nonstone free. Asymptomatic and noninfected fragments <4 mm were accepted as clinically insignificant residual fragments (CIRFs).

However, this term may not be suitable in the pediatric population because CIRFs might cause recurrent stone formation and persistent urinary tract infection. In a previous report, 34 of 85 children (40%) with CIRFs were symptomatic, and 18 of the 85 (21%) showed stone regrowth over the next 6 months.¹⁵ Another study by El-Assmy and colleagues found that 31.2% of children with residual fragments after post-SWL and post-percutaneous nephrolithotomy will have clinically significant outcomes at a median follow-up of 18 months.¹⁶

One of the major components in determining SWL success is stone size. Ellipsoid SV was used in our study to determine the stone size by a formula. SV was found to be a significant predictor of stone-free status after SWL with a cutoff value of 155 mm³. This value is in line with Tran et al with 150 mm³ of SV. Gökce et al stated cutoff values of 187.5 and 185 mm³ for the renal and ureteral stones, respectively, as well. The disparity might be because of the amount of SWL sessions and the definition of success.

From the results of all the studies mentioned earlier, SSD was found to be one of the predictors of effective SWL with cutoff values of 9–11.5 cm in adults. As expected, we found a lower cutoff value of 6.5 cm. However, differences in cutoff values can be related to short SSD and lower body mass index of pediatrics, variability in SWL device and technique, the definition of success, and the frequency of SWL sessions used.⁷

Another key parameter is SD evaluation in NCCT studies by HU measurement, which has been the issue of several articles proposing cutoff values. In our study, a cutoff value of 550 HU with a sensitivity of 81% and a specificity of 78% was detected. Similarly, Tran and colleagues defined the cutoff value as 600 HU in adults. In contrast, some reports proposed the cutoff level of 900–970 HU.^6,17,18

Triple D scores of 0, 1, 2, and 3 in our pediatric population were found as 23.8%, 35.1%, 74.0%, and 92.0%, respectively, that were quite similar to the results of Gökce et al with the success rates of 20%, 36.4%, 85.4%, and 95.5% for the renal stone cases.⁹ Although no time recordings were made throughout this study to create the Triple D score, it was realized that making the appropriate measurements on the NCCT pictures takes ∼5 minutes longer. The Triple D score is made up of three criteria that are easily collected from NCCT tests. As a result, it has the potential to be utilized in daily practice and to be included in routine radiography reports.

Multiple factors may have an impact on pediatric SWL results. We recommend that each institution establishes its own cutoff values for SV, SSD, and SD characteristics, and then generates the Triple D score for its patients based on these cutoff levels to predict SWL success.

We are aware that our research may have some limitations. First of all, retrospective nature of study causes excluding some data without CT scan, because of which stone attenuation and SSD could not be measured. Second, small sample size is another limitation of the study. Third, although metabolic evaluation is an important parameter data in pediatric stone disease outcome, these data were not included in this study.

After determining our own cutoff levels for SV, SSD, and SD parameters, the Triple D score was calculated. Our study confirms that Triple D scores support the SWL outcomes in the pediatric population. To our knowledge, this study is the first to evaluate Triple D score in children with renal stones, which may serve as a practical tool for the outcome of SWL. We believe that our research will serve as a base for future studies on Triple D score validation in children in whom various factors may affect to predict the success of SWL.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

Abbreviations Used

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