Extracorporeal Shockwave Lithotripsy Versus Ureteroscopy: A Comparison of Intraoperative Radiation Exposure During the Management of Nephrolithiasis

Abstract

Background and Purpose:

Both shockwave lithotripsy (SWL) and ureteroscopy (URS) may be used in the treatment of similar stones and both need fluoroscopic imaging to achieve this. Fluoroscopy, however, is a source of ionizing radiation. The purpose of this study is to compare the effective radiation dose (ERD) between patients undergoing SWL vs URS.

Patients and Methods:

The ERD was measured among consecutive patients who were undergoing either SWL or URS between January 2010 and February 2011. For SWL, ERD was calculated using fluoroscopic exposure time, current, voltage, skin-to-source distance, and field size. For URS, it was calculated from the measured dose-area product. We measured several patient and stone factors. Univariate and multivariate analyses were performed.

Results:

A total of 190 patients were included (87 SWL and 103 URS). In the univariate analyses, no differences were found in ERD (7.32 vs 6.00 mSv, P=0.262 and 7.23 vs 6.07 mSv, P=0.198, for renal and ureteral stones, respectively). In the multivariate analyses, among renal stones, SWL was associated with a higher ERD than URS (β=2.06, P=0.026), and body mass index and stone size were also significant predictors (β=0.212, P=0.045 and β=0.452, P=0.004, respectively). Among ureteral stones, no differences were found (β=0.425, P=0.674), and only the presence of a stent was related to ERD (β=2.53, P=0.013).

Conclusions:

Among patients with renal stones, SWL was associated with a modest increase in ERD compared with URS, but for ureteral stones, both modalities were associated with similar levels of radiation. This information may be relevant for frequent stone formers needing treatments for which cumulative exposures may become significant.

Introduction

F or many ureteral and renal stones needing removal, both shockwave lithotripsy (SWL) and ureteroscopy (URS) are acceptable first-line treatments.^1,2 While both approaches offer similar stone-free rates, each technique offers specific advantages and disadvantages. For example, SWL is a minimally invasive procedure that may avoid the need for a general anesthetic while URS is more invasive but may be more effective for harder stones. In many cases, patients may need physician counseling to help them decide which treatment to pursue.

Both approaches rely on fluoroscopic imaging. SWL requires it for stone localization, shockwave targeting, and assessment of stone fragmentation, while with URS, it is used for instrument guidance and as an aid for endoscopic navigation. Fluoroscopy, however, can represent a significant source of ionizing radiation with its attendant risk of subsequent solid organ or hematologic malignancy.³ While many patients may choose either treatment for similar stones, it is not known if either approach confers greater radiation exposure. The objective of this study was to compare the effective radiation dose to which patients are exposed during treatment with either SWL or URS.

Patients and Methods

We prospectively collected data on all consecutive SWL and URS procedures performed at our institution by seven different attending staff urologists between January 2010 and February 2011. For the URS procedures, fluoroscopy was performed by a radiology technician using a digital portable OEC 9800® Elite (GE Healthcare, United Kingdom) C-arm with automatic exposure control and a 12-inch image-intensifier. This device produced a radiation dose report that included a calculated dose-area product (DAP, rad·cm²) as well as the total fluoroscopic time (TFT). The DAP was then converted to effective radiation dose (ERD, mSv) using accepted conversion tables.⁴ Our URS technique has been described in detail.⁵

For the SWL procedures, fluoroscopy was also performed by a radiology technician, and we used the LithoTron lithotripter (Healthtronics, Austin, TX) accompanied by the Philips BV 25 (Andover, MA) C-arm that uses a 6-inch image-intensifier. This device reports average fluoroscopic current and voltage. We used these data with TFT to calculate the MAS (or product of mA and TFT, in seconds). The ERD was then calculated from the MAS, skin-to-source distance, and field size.⁶ SWL was performed using recognized standards. The total number of shocks per kidney did not exceed 3000, regardless of the number of stones treated. A shock rate of either 60 or 120 Hz with starting and ending voltages of 14 to 24 kV, respectively, was used.

We collected both patient- and stone-related data. Patient data included age, sex, and body mass index (BMI, kg/m²). Stone-related data included size, density, location, number, and the presence of hydronephrosis. These data were obtained from preoperative CT scans using Centricity^© Enterprise Web (GE Medical Systems) Picture Archiving and Communication System software. Size was defined as the greatest stone width (mm). Density was recorded as the mean Hounsfield units (HU) from a cross-section of the stone. Location was defined as either ureteral or renal. Among patients with more than one treated stone, the total number of stones treated was recorded, but the dimensions (size, density, and location) of only the largest stone were used.

Both uni- and multivariate analyses were performed. For the multivariate analyses, we chose certain candidate predictors that we believed might influence the relationship between treatment modality (SWL vs URS) and ERD. These included BMI, number, size, and density of stone treated (or largest stone if more than one stone was treated), and stone location. Factors also considered were the presence of a previously placed ureteral stent and the presence of hydronephrosis on preoperative imaging (among those without stents), as well as surgeon volume (<10 cases, 10–30 cases, >30 cases). We used SPSS™ (IBM, Illinois) to perform the analyses. A P value of <0.05 was considered statistically significant.

Results

Patient characteristics

During our study period, a total of 190 procedures were performed among 168 patients. Among these, 87 SWLs and 103 URSs were performed (among 76 and 92 patients, respectively). The mean age and sex and race distribution were not different between the groups. Patients undergoing URS were slightly more obese than those undergoing SWL (30.8 vs 27.6 kg/m², respectively) (Table 1).

Table 1.

Patient Characteristics

	SWL	URS	P value
Procedures (%)	87 (45.8)	103 (54.2)	NA
Patients (%)	76 (45.2)	92 (54.8)	NA
Patients >1 procedure (%)	10 (13.2)	8 (8.7%)	0.199
Age (mean, range, years)	49.9 (19–75)	51.9 (17–81)	0.145
Male (%)	42 (55.3)	55 (59.8)	0.488
BMI (mean, range, kg/m²)	27.6 (16–44)	30.8 (16–55)	0.004
Race (%) Caucasian	(75.9)	(79.6)	0.118
African American	(10.3)	(8.7)
Asian	(2.3)	(0)
Hispanic	(5.7)	(10.7)
Other	(5.7)	(1.0)

SWL=shockwave lithotripsy; URS=ureteroscopy; BMI=body mass index.

Stone characteristics

There were more stones treated per URS than per SWL (2.2 vs 1.1, P<0.001). URS patients were more likely to have been treated for a distal ureteral stone and less likely to have been treated for a renal stone compared with SWL patients (38.8% vs 11.5% and 27.2% vs 54%, respectively, P<0.001). URS patients were also more likely to have arrived in the operating room with a ureteral stent already in place (36.9% vs 12.6%, P<0.001) (Table 2).

Table 2.

Stone Characteristics

	SWL	URS	P value
Right side (%)	33 (37.9)	50 (48.5)	0.142
Stones treated (mean, range)	1.1 (1–3)	2.2 (1–8)	<0.001
Single stone treated (%)	72 (82.8)	56 (54.1)	<0.001
Size^a (mm, mean, range)	8.2 (3–25)	7.1 (2–22)	0.061
Density^b (HU, mean, range)	490.9 (100–1190)	438.9 (98–1501)	0.212
Preoperative stent present (%)	11 (12.6)	38 (36.9)	<0.001
Hydronephrosis present^c (%)	35 (46.1)	38 (58.5)	0.177
Stone location (%)
Renal	47 (54.0)	28 (27.2)	<0.001
Proximal ureter	24 (27.6)	26 (25.2)
Midureter	6 (6.9)	9 (8.7)
Distal ureter	10 (11.5)	40 (38.8)

Size of treated stone or of largest treated stone among patients with >1 treated stone.

Density (Hounsfield units) of treated stone or of largest treated stone among patients with >1 treated stone.

Only includes patients who did not have a preoperative stent placed.

SWL=shockwave lithotripsy; URS=ureteroscopy.

Radiation exposure

In the univariate analyses, there was a significantly greater TFT associated with SWL than with URS (476.5 vs 313.8 s, P=0.006 and 496.3 vs 261.8 s, P<0.001, for renal and ureteral stones, respectively). The ERDs for the SWL and URS groups, however, were similar (7.32 vs 6.00 mSv, P=0.262 and 7.23 vs 6.07 mSv, P=0.198, for renal and ureteral stones, respectively) (Table 3).

Table 3.

Univariate Analyses

	SWL	URS	P value
Renal stones
Effective radiation dose (mSv, range)	7.32 (1.9–30.7)	6.00 (1.5–17.1)	0.262
Total fluoroscopic time (seconds, range)	476.5 (162–1788)	313.8 (114–744)	0.006
Ureteral stones
Effective radiation dose (mSv, range)	7.23 (1.3–24.9)	6.07 (1.17–19.3)	0.198
Total fluoroscopic time (seconds, range)	496.3 (90–1740)	261.8 (101–741)	<0.001

SWL=shockwave lithotripsy; URS=ureteroscopy.

In the multivariate analyses of patients who were undergoing procedures for renal stones, our main independent variable (procedure type, SWL vs URS) was a significant predictor (β=4.06, P=0.026) of ERD while controlling for the covariates of BMI, number of stones treated, and stone size. Both BMI and stone size were also predictors of ERD in this model (β=0.212, P=0.045 and β=0.452, P=0.004, for BMI and stone size, respectively). The other covariates (stone density, presence of preoperative stent, and hydronephrosis) were not predictors and did not influence the effect of the variable procedure type on our outcome (ERD) and were therefore not included in our final model (Table 4).

Table 4.

Multivariate Analyses

	β	P value
Renal stones (n=75)
SWL vs URS	2.06	0.026
BMI	0.212	0.045
Number of stones treated	0.919	0.139
Stone size	0.452	0.004
Ureteral stones (n=115)
SWL vs URS	0.425	0.674
BMI	0.010	0.891
Number of stones treated	0.240	0.507
Stent	2.53	0.013

SWL=shockwave lithotripsy; URS=ureteroscopy; BMI=body mass index.

Among patients with ureteral stones, the variable procedure type (SWL vs URS) was not a significant predictor of ERD (β=0.425, P=0.674). The covariates of BMI and number of stones did alter the effect of procedure type and were therefore included in the model, but were not significant predictors. The variable presence of preoperative stent was, however, a significant predictor of ERD (β=2.53, P=0.013) in our final model. Surgeon volume was also not a significant predictor (F=0.891, P=0.678) (Table 4).

Discussion

The purpose of this study was to determine whether patients who were undergoing either SWL or URS were exposed to different amounts of ionizing radiation during the treatment of urolithiasis. While there are published data regarding radiation exposure incurred during diagnostic⁷ and follow-up imaging,^8,9 there is less information on effective dose levels from intervention with SWL¹⁰ or URS. Fluoroscopy, a source of ionizing radiation, is invaluable during the treatment of stones whether by SWL, where it is used to target the stone, direct the shockwaves, and to assess for fragmentation, or by URS, where it is used to direct instrument passage and to aid in intraoperative instrument navigation. While certain patient and stone characteristics may guide patients toward one or the other approach, many can choose either treatment as a reasonable option. Because those with urolithiasis will often have recurrent stones and require re-treatment, knowledge of radiation risk during treatments would be helpful in determining cumulative exposure. While there have been several studies reporting radiation exposure among patients undergoing SWL^10

–16 or URS,¹⁷ we are not aware of any studies comparing radiation exposure between these two modalities.

In our study, among patients being treated for renal stones, SWL was found to be associated with a higher ERD. While this difference was statistically different, clinically the difference was modest. Among patients being treated for ureteral stones, no difference in ERD was found. The overall ERDs in this study were higher than those seen in other studies. Perisinakis and associates¹⁵ reported mean ERD doses from SWL of 0.76 to 1.82 mSv while our mean dose of 7.23 mSv was much higher. One explanation for this large difference may lie in the means by which ERD was measured. In their study, it was measured using anthropomorphic phantoms and thermoluminescence dosimetry while in our study, the ERD was calculated from lithotriptor recordings of current, voltage, time, skin-to-source difference, and field size. Baldock and colleagues¹¹ also reported value with annual doses from SWL of 0.73 to 4.8 mSv.

In addition to ERD, we also measured TFT. Unlike ERD, we found that there were significant differences in TFT between patients undergoing SWL and URS for either renal or ureteral stones. We believe this is primarily because of two main factors. First, the devices used to perform fluoroscopy for treatment modality were different. The OEC 9800® Elite C-arm uses a 12-inch image-intensifier while the LithoTron lithotripter uses only a 6-inch intensifier. As a result, a single image from the OEC 9800 exposes the patient to more radiation per unit of exposure time because the cross-sectional area it captures is larger than for LithoTron.

Second, fluoroscopic images during URS can be acquired in a broader and quicker fashion as the C-arm is allowed to swing. As such, the C-arm operator can scan larger areas of the body in shorter periods. With fluoroscopy during SWL, however, it is often the table itself on which the patient lies, not the C-arm, that must be moved to bring the stone into the fluoroscopic field. Such table movement tends to be slower and made in each dimension, x,y, and z, one at a time. This combined with the smaller field of view (ie, the 6- rather than 12-inch intensifier) usually necessitates a longer time to “find” the stone target. Furthermore, both machines use automatic exposure control and with patients of varying BMIs, the amount of ERD will also vary as the devices attempt to produce quality images.¹⁸ While we used different fluoroscopic devices and different calculations for ERD, our TFTs were measured directly. Furthermore, the mean ERD results are still roughly consistent with published data.

In our multivariate analysis of patients undergoing a treatment for renal stones, we found that SWL was associated with a significantly higher amount of ERD than was SWL, even after controlling for BMI, number of stones treated, and stone size. This may be a manifestation of the SWL surgeon's tendency to use fluoroscopy repeatedly during SWL to assess for fragmentation, while with URS, the surgeon may assess this visually through the ureteroscope. In this multivariate model, the variables of BMI and stone size were also significant predictors of ERD. For BMI, the reasons for this are intuitive, because more radiation is needed to penetrate larger patients to produce images of adequate quality. This is also consistent with the study by Mancini and coworkers¹⁸ who described a proportional relationship between BMI and ERD among a cohort of patients undergoing percutaneous nephrolithotripsy.

Larger stones were also associated with higher levels of ERD. We believe this to be counterintuitive, because one might expect smaller stones to be associated with higher doses of radiation as more fluoroscopic time would be used to find the smaller targets. We believe this finding is also related to the surgeon's tendency to see fragmentation that may, in fact, be easier to visualize when the stone is larger. Because fragmentation is much more difficult to see in smaller stones, the surgeon may be less likely to spend time finding it.

In the multivariate analysis of ureteral stones, we did not find a difference in ERD between SWL and URS. In this model, BMI and number of stones treated were also not significant predictors of ERD. The presence of a stent at the time of surgery, however, was associated with decreased ERD. For SWL, we believe that the presence of a stent helps identify the ureteral stone location more rapidly and thus allows the surgeon to use less fluoroscopy. For either renal or ureteral stones, the presence of hydronephrosis on the preoperative CT scan was not related to ERD. This is likely because all patients for whom clinically relevant obstruction was present were likely selected out before the study by having a stent placed preoperatively. Those remaining patients in our study who did not have a stent at the time of their procedure, especially those undergoing URS, likely did not have obstruction significant enough to complicate a URS procedure and thus need additional fluoroscopy to facilitate the procedure. Among patients undergoing SWL, it seems less likely that the presence of hydronephrosis would influence ERD when it cannot be appreciated on fluoroscopic imaging.

Conclusions

Many patients with upper tract urinary calculi may choose between SWL and URS for treatment. Each, however, uses fluoroscopy, which is a form of ionizing radiation with its attendant risk of subsequent malignancy. This study showed that among patients with renal stones, SWL is associated with a modestly elevated level of radiation exposure compared with URS, but no difference was found among patients with ureteral stones. Overall, our ERD were higher than those previously reported for each modality. While these levels are still very low, this information is relevant to recurrent stone formers who need repeated interventions and will thus have elevated cumulative exposures over time. Finally, despite our assessment of radiation exposure in this study, more data are needed to correlate ERD with subsequent cancer risk.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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