A Prospective Evaluation of High-Resolution CT Parameters in Predicting Extracorporeal Shockwave Lithotripsy Success for Upper Urinary Tract Calculi

Abstract

Objective:

To evaluate the ability of noncontrast CT parameters (stone size, stone attenuation, and skin-to-stone distance [SSD]) to predict the outcome of extracorporeal shockwave lithotripsy (SWL) in a prospective cohort of patients with renal and upper ureteric stones.

Patients and Methods:

Patients with stones 5 to 20 mm were prospectively enrolled from 2011 to 2014. Patients had NCCT with recording of stone size, stone mean attenuation, and SSD, as well as various stone and patient parameters. The numbers of needed sessions as well as the final outcome were determined, with SWL failure defined as residual fragments >3 mm. Predictors of SWL failure were assessed by multiple regression analysis.

Results:

Two hundred twenty patients (mean ± standard deviation [SD] age 41.5 ± 12.4 years) underwent SWL. Mean ± SD stone size was 11.3 ± 4.1 mm, while mean ± SD stone attenuation was 795.1 ± 340.4 HU. Mean ± SD SSD was 9.4 ± 2.1 cm. The average number of sessions was 1.64. SWL was effective in 186 (84.5%) patients (group A), while 34 (15.5%) patients had significant residual fragments (>3 mm). On univariate analysis, predictors of SWL failure included stone attenuation >1000 HU, older age, higher body mass index, higher attenuation value, larger stone size, and longer SSD. Increased SSD and higher stone attenuation retained their significance as independent predictors of SWL failure (p < 0.05) on multiple regression analysis both after first session and as final SWL outcome. A positive correlation was found between number of SWL sessions and mean stone attenuation (r = 0.6, p < 0.001) and SSD (r = 4, p < 0.001).

Conclusions:

Stone mean attenuation and SSD on noncontrast CT are significant independent predictors of SWL outcome in patients with renal and ureteric stones. These parameters should be included in clinical decision algorithms for patients with urolithiasis. For patients with stones having mean attenuation of >1000 HU and/or large SSDs, alternatives to SWL should be considered.

Introduction

Since the clinical introduction of extracorporeal shockwave lithotripsy (SWL) during the early 1980s, this treatment modality has occupied a prominent role in the management of patients with urolithiasis. It is generally considered that SWL can be a valid treatment option for nearly 90% of stones seen in adults.¹ Success of SWL depends on various factors, including the efficacy of the lithotripter, stone factors (size, location, and composition), patient factors, and the performance of SWL.² CT is currently the modality of choice for imaging urinary stones, and various CT parameters (such as stone attenuation, skin-to-stone distance [SSD], and abdominal fat measurements) have been investigated to assess their ability to predict SWL success; however, despite a large number of studies during the past 10 years, there is still no consensus regarding the use of these parameters to guide management decisions. The aim of the current study is to evaluate in a prospective cohort of patients the ability of a number of parameters, including stone size, stone attenuation, and SSD, to predict the outcome of SWL.

Patients and Methods

Patients presenting with renal or upper ureteric stones 5 to 20 mm who were eligible for SWL treatment were recruited in the study from January 2011 to February 2014 at our institution. Excluded from the study were patients with high-grade obstruction, patients needing a nephrostomy tube or a ureteric stent, and patients with solitary kidneys. Patients' evaluation included a complete history and physical examination, measurement of body mass index (BMI), routine preprocedural laboratory studies, and imaging with a kidney, ureter, and bladder radiograph (KUB), renal ultrasound, and a high-resolution noncontrast CT.

Patients were scanned with a multidetector row helical CT scanner (HiSpeed Dual; GE Medical Systems, Milwaukee, WI). The images were obtained using high quality mode at 200 mA, 120 kV, and 5 mm collimation reconstructed at 3 mm. The postscanning bone window protocol was used to measure stone attenuation value (HU). For the measurement of stone density, the axial plane representing the stone in its largest dimension was defined and mean stone attenuation was calculated from three nonoverlapping regions of interest (area 0.026 cm² or 25 pixels) chosen for stones >1 cm. For smaller stones ≤1 cm, consistent areas of interest (25 pixels) were centrally chosen to minimize volume averaging. Stone site was evaluated, and stone size was measured (defined as the greatest length in any dimension). SSD was also measured by averaging three measured distances from the center of the stone to the skin at 0°, 45°, and 90° using radiographic calipers as described by Pareek and colleagues.³

SWL was delivered using the lithotripter DoliS (Dornier MedTech, Germering, Germany), which has an electromagnetic flat coil with a focusing lens with a focal distance of 150 mm. Patients were placed in the supine position with the side of the stone toward the device and were given sedation analgesia throughout the procedure. Fluoroscopic guidance was used. During each session, a constant number of shock waves (3000 Shocks) were delivered, with gradual ramping of voltage and increasing frequency (depending on the response and patient's tolerance) to a maximal power of 22 to 24 kV and a maximal rate of 120 shock waves/minute. A change in stone size, outline, or separation indicated fragmentation. Repeat treatment was carried out after 3 weeks if inadequate fragmentation of the stone or significant fragments were observed on follow-up plain KUB, up to a maximum of three sessions. The final outcome was assessed 1 month after the last session using a KUB and an abdominal ultrasound. Patients scheduled for retreatment who elected to switch to an endourologic intervention were excluded from the analysis.

The outcomes were evaluated, and patients were classified according to stone clearance into two groups: group A (success group) with complete clearance of the stone (stone free) or residual fragments less than or equal to 3 mm in size (clinically insignificant stone fragments) and group B (failure group) with clinically significant fragments more than 3 mm in size after three SWL sessions (residual stone).

Data management and analysis were performed using SigmaStat program; version 3.5 (Systat Software, Inc.). The numerical data were statistically presented in terms of range, mean, standard deviation (SD), median, and interquartile range (IQR). Categorical data were summarized as percentages. Comparisons between numerical variables of two groups were done by the Student's unpaired t-test for parametric data or the Mann–Whitney Rank Sum test for nonparametric data. Comparing categorical variables was done by chi-square test or Fisher's exact test for small sample size. Multivariate regression analysis was performed to test for independent predictors of SWL failure. All p-values were two tailed and considered significant when p-values were less than 0.05.

Results

A total of 220 patients with renal and upper ureteric calculi were admitted for stone disintegration by SWL. The cases included 157 (71.4%) males and 63 (28.6%) females. Their age ranged from 18 to 70 years with a mean ± SD of 41.5 ± 12.4 years (median [IQR]: 40 [32–51]). The BMI of the recruited subjects ranged from 18 to 36.2 kg/m² with a mean ± SD of 24.9 ± 5.1.

Preoperative imaging with KUB revealed that the majority of cases, 210 (95.5%), had a single stone. Left side stones were seen in 130 (59%) subjects, while 90 (41%) had right side stones. Renal stones were found in 181 (82.7%) patients, while 39 (17.3%) had upper ureteric stones. Twenty-eight (12.7%) patients had stones in the lower pole.

Stone parameters on noncontrast CT are detailed in Table 1. Mean ± SD stone size was 11.3 ± 4.1 mm, while mean ± SD stone attenuation was 795.1 ± 340.4 HU. Mean ± SD SSD was 9.4 ± 2.1 cm.

Table 1.

Noncontrast CT Stone Parameters

Variables	Mean ± SD	Range	Median (IQR)
Stone size, mm	11.3 ± 4.1	5–20.8	11 (8–13)
Stone density, HU	795.1 ± 340.4	193–1741	787.0 (500–1024.3)
SSD, cm	9.4 ± 2.1	5.8–16.52	9 (7.5–11)

IQR = interquartile range; SD = standard deviation; SSD = skin-to-stone distance.

One hundred twenty-six (57.3%) patients received one SWL session, while 46 (20.9%) needed two sessions, and 48 (21.8%) patients underwent three sessions. The average number of sessions was 1.64, and the average number of shock waves per stone was 4922.7. A strong positive correlation was found between mean stone attenuation and the number of SWL sessions needed for complete stone disintegration (r = 0.6, p < 0.001). A positive correlation was also noted between stone-to-skin distance (r = 4, p < 0.001) and patient's age (r = 0.3, p < 0.001) and the number of SWL sessions needed for complete stone disintegration (Table 2).

Table 2.

Correlation Between Number of Extracorporeal Shockwave Lithotripsy Sessions and Patients' Variables

	r-coefficient	p
Age, years	0.255	<0.001^a
BMI, kg/m²	0.132	0.051
Stone density, HU	0.554	<0.001^a
SDD, cm	0.356	<0.001^a

Statistically significant.

BMI = body mass index.

Overall, SWL was effective in 186 (84.5%) patients (group A), while 34 (15.5%) patients had significant residual fragments (>3 mm) after the final SWL session (group B). Of the 186 effective cases, 170 (91.2%) required one or two sessions of SWL and 16 (8.8%) needed three. Table 3 details the univariate analysis of variables, predicting failure of SWL (presence of significant residual fragments). According to the univariate analysis, predictors of SWL failure included right side stones, stone attenuation >1000 HU, older age, higher BMI, higher attenuation value (as a continuous variable), larger stone size, and longer stone-to-skin distance (Table 3).

Table 3.

Univariate Analysis for Variables Predicting Failure of Disintegration by Shockwave Lithotripsy

	Success (n = 186)	Failure (n = 34)
Variables	n (%)	n (%)	p
Sex
Male	136 (73)	21 (61.8)	0.254
Female	50 (27)	13 (38.2)
Obesity
Not obese	119 (64)	16 (47)	0.095
Overweight and obese (BMI >25)	67 (36)	18 (53)
Stone site
Renal	154 (73.2)	27 (79.5)	0.817
Ureteric	32 (16.8)	7 (20.5)
Laterality
Right	68 (36.6)	22 (64.7)	0.004^a
Left	118 (63.4)	12 (35.3)
Lower caliceal location
Yes	25 (13.4)	3 (8.8)	0.643
No	161 (86.6)	31 (91.2)
Stone attenuation
≤1000 HU	152 (81.7)	3 (8.8)	<0.001^a
>1000 HU	34 (18.3)	31 (91.2)

Numeric variables	Mean ± SD	Mean ± SD	p
Age, years	40.3 ± 12.5	48.0 ± 9.1	<0.001^a
BMI, kg/m²	24.2 ± 4.9	27.2 ± 5.8	0.001^a
Stone attenuation value, HU	720.9 ± 305.1	1198.7 ± 218.7	<0.001^a
Stone size, mm	10.8 ± 3.9	13.7 ± 4.4	<0.001^a
SSD, cm	9.1 ± 2.1	11.2 ± 0.8	<0.001^a

Statistically significant.

On testing the significant predictors by multiple regression analysis, increased SSD and increased stone attenuation retained their significance as independent predictors of SWL failure (p = 0.001) for all variables (Table 4).

Table 4.

Results of Multiple Logistic Regression Analysis for Predictors of Extracorporeal Shockwave Lithotripsy Failure

	B-coefficients	p	OR	95% CI of OR
Age	0.024	0.252	1.025	0.983, 1.068
BMI	0.019	0.739	1.019	0.913, 1.137
Stone size	−0.025	0.754	0.976	0.837, 1.138
Stone attenuation	0.007	<0.001^a	1.007	1.004, 1.010
SSD	0.449	0.001^a	1.567	1.198, 2.050
Laterality (right vs left)	0.730	0.181	2.075	0.713, 6.041
Constant	−14.849	<0.001

Statistically significant.

CI = confidence interval; OR = odds ratio.

Looking at success after the first SWL session, we found that 126 (57.3%) patients had their stones effectively disintegrated and cleared with one SWL session, while 94 (42.7%) patients did not. Tables 5 and 6 detail the univariate analysis (Table 5) and multiple regression model (Table 6) for patients' variables predicting failure after one SWL session. Larger stone size (p < 0.001), increased stone attenuation (p < 0.001), and increased SSD were the significant independent predictors of failure after one SWL session.

Table 5.

Univariate Analysis for Patients' Variables Predicting Failure of Disintegration After First Extracorporeal Shockwave Lithotripsy Session

	Success (n = 186)	Failure (n = 34)
Variables	n (%)	n (%)	p
Sex
Male	136 (73)	21 (61.8)	0.254
Female	50 (27)	13 (38.2)
Obesity
Not obese	119 (64)	16 (47)	0.095
Overweight and obese (BMI >25)	67 (36)	18 (53)
Stone site
Renal	154 (73.2)	27 (79.5)	0.817
Ureteric	32 (16.8)	7 (20.5)
Laterality
Right	68 (36.6)	22 (64.7)	0.004^a
Left	118 (63.4)	12 (35.3)
Lower caliceal location
Yes	25 (13.4)	3 (8.8)	0.643
No	161 (86.6)	31 (91.2)
Stone attenuation
≤1000 HU	152 (81.7)	3 (8.8)	<0.001^a
>1000 HU	34 (18.3)	31 (91.2)

Numeric variables	Mean ± SD	Mean ± SD	p
Age, years	40.3 ± 12.5	48.0 ± 9.1	<0.001^a
BMI, kg/m²	24.2 ± 4.9	27.2 ± 5.8	0.001^a
Stone attenuation value, HU	720.9 ± 305.1	1198.7 ± 218.7	<0.001^a
Stone size, mm	10.8 ± 3.9	13.7 ± 4.4	<0.001^a
SSD, cm	9.1 ± 2.1	11.2 ± 0.8	<0.001^a

Statistically significant.

Table 6.

Multiple Logistic Regression Model to Evaluate the Significant Predictors of First Session Failure

	B-coefficients	p	OR	95% CI of OR
Age	0.020	0.224	1.020	0.988, 1.053
Stone size	0.225	<0.001	1.253	1.118, 1.404
Stone attenuation	0.005	<0.001	1.005	1.003, 1.006
SSD	0.332	0.002	1.394	1.134, 1.713
Constant	−11.237	<0.001

The relationship between stone attenuation and stone size is explored further in Table 7. Subjects with stone attenuation >1000 HU had a significantly lower clearance rate, higher failure rate, and most needed three sessions, regardless of stone size (Table 7). SWL final outcome was therefore affected to a much larger extent by stone density than stone size.

Table 7.

Outcome of Extracorporeal Shockwave Lithotripsy in Relation to Stone Density and Size

	Calculus density
	≤1000 HU (n = 95)	>1000 HU (n = 20)
Subjects with calculi ≤1.1 cm in diameter (n = 115)	n (%)	n (%)	p
<3 sessions (n = 100)	94 (98.9)	6 (30)	<0.001^a
≥3 sessions (n = 15)	1 (1.1)	14 (70)
Clearance	95 (100)	6 (30)	<0.001^a
Residual fragments	0	14 (70)

	Calculus density
	≤1000 HU (n = 60)	>1000 HU (n = 45)
Subjects with calculi >1.1 cm in diameter (n = 105)	n (%)	n (%)	p
<3 sessions (n = 71)	48 (80)	23 (51.1)	0.003^a
≥3 sessions (n = 34)	12 (20)	22 (48.9)
Clearance	57 (95)	28 (62.2)	<0.001^a
Residual fragments	3 (5)	17 (37.8)

Statistically significant.

Discussion

The success of stone disintegration by SWL depends on a number of variables, including stone factors (size, location, and composition), patient factors, the efficacy of the lithotripter, and the performance of SWL.² Despite a growing body of literature on the impact of various factors on SWL outcome, clinical decision-making in most cases still depends principally on stone size and location, with little consideration to additional newer predictive factors that can help refine management planning.

Noncontrast CT has become the preferred modality for evaluating patients with urolithiasis, with stone detection and precise estimation of size and location in nearly 96% to 97% of urolithiasis patients.⁴ In addition, different CT parameters have been found to predict with reasonable accuracy SWL outcomes, including stone size (as measured by maximum diameter or stone surface area), stone attenuation (as measured by mean attenuation value [MAV], minimum and maximum HU, and the related mean stone density), and SSD. There are also studies looking at the possible impact of markers of obesity such as abdominal circumference and intra-abdominal fat parameters on SWL outcomes. Most of these studies, however, have been retrospective in nature and have used different CT protocols and outcome measures, rendering comparisons of results difficult.

The relationship between stone attenuation on CT and response to disintegration by SWL was first demonstrated in vitro by Saw and colleagues.⁵ Many clinical studies have found a relationship between a higher stone attenuation and poor disintegration and SWL failure.^3,6
–8 Stone attenuation was a predictor of SWL outcome on univariate analysis in the studies by Wiesenthal and colleagues and Celik and colleagues,^9,10 while it was an independent predictor of SWL outcome on multivariate analysis in the studies by Perks and colleagues, El-Nahas and colleagues, and Geng and colleagues.^4,11,12 Our current study lends further support to the value of mean stone attenuation as the most significant independent predictor of SWL success both after the first session and at final evaluation in our cohort (B-coefficient 0.007; p < 0.001 on multiple regression analysis). Higher stone attenuation was also associated with a higher number of SWL sessions needed for disintegration, and this was also seen in the study by Gupta and colleagues,⁸ who found that calculi with attenuation more than 750 HU were 10.5 times more likely to need three or more sessions compared with stones with attenuation 750 HU or less. The cutoff for stone attenuation predicting failure of SWL varied in various studies and was suggested at 750 HU,^8,10 900 HU,^4,9 950 HU,⁶ 970 HU,¹³ and 1000 HU.^11,14 Our reliance on KUB (rather than CT) for determining SWL success may have potentially introduced a systemic bias, with overestimation of treatment success in lower-density stones. We believe the routine use of ultrasound at 1 month after the last session should have minimized the chance of this bias. Novel predictors of SWL success based on HU attenuation are being explored, such as stone heterogeneity index as the SD of HU.¹⁵

SSD is usually measured as described by Pareek as the average of three measurements taken at 0°, 45°, and 90° from the stone center to the skin. Others have measured only the 45° distance between the stone and the skin.⁹ SSD has been suggested by Pareek and colleagues as a significant predictor of SWL outcome, with an SSD greater than 10 cm predicting treatment failure.³ Other investigators were able to confirm the predictive ability of SSD,^4,9,10,16,17 with suggested cutoffs for SSD ranging from 9 cm¹⁸ to 11 cm,⁹ while in other studies SSD was not found to be an independent predictor of SWL outcome.^12,19 In our prospective cohort, of patients, increased SSD was an independent predictor of SWL failure on multivariate analysis (p = 0.001), with an additional positive correlation with increasing number of SWL sessions. It may be noted that some of the studies referenced above used older SWL machines with shorter focal distances; more modern machines with deeper penetration may be more efficient at handling patients with longer SSDs.

Although it is conceivable that the CT parameters (stone attenuation and SSD) evaluated in the current study may impact stone disintegration more than stone clearance (which depends on more factors), we defined SWL success in our study as the absence of significant fragments, as this was the more practical and clinically relevant outcome.

The value of BMI in predicting SWL outcome is debatable. Some researchers^3,11 found BMI to be an independent predictor of outcome, while others^4,20 did not. Higher BMI was a predictor of SWL failure on univariate analysis in our study, but it did not maintain its significance in the multiple regression analysis. Although not evaluated in our study, certain CT markers of obesity such as abdominal circumference and total fat area were found to correlate with SWL outcome. The suggestion is that increased intra-abdominal fat may interfere with stone targeting or may suppress shock waves.¹²

The main strength of the current study is the prospective patient recruitment and data collection, allowing for a standardized evaluation and measurement of outcomes. The study design and number of recruited patients allowed for a multivariate analysis of the predetermined risk factors for SWL failure. Potential limitations in our study include the lack of chemical analysis of the retrieved stone fragments, the use of standard CT 5 mm collimation width, which may falsely decrease the average attenuation of smaller stones, and the fact that we relied on plain radiography and ultrasound to determine the outcome of SWL, which may have underestimated the SWL failure rate.

Conclusion

The current study confirms that stone MAV and SSD on noncontrast CT are significant independent predictors of SWL outcome in patients with renal and ureteric stones. The body of evidence is large enough to warrant inclusion of these parameters (in addition to stone size and location) in clinical decision algorithms for patients with urolithiasis. For patients with stones having MAD of >1000 HU and/or large SSDs, alternatives to SWL should be considered.

Footnotes

Acknowledgment

Mohamed Mostafa helped with the statistical analyses.

Author Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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