Postshockwave Lithotripsy Outcome Evaluation in Ureteral Stones: Comparison Between Noncontrast Computed Tomography and Plain Abdominal Radiography

Abstract

Background and Purpose:

There are no definite data indicating which modality to use to assess the efficacy of shockwave lithotripsy (SWL). Usually, plain abdominal radiography (PAR) is recommended in percutaneous nephrolithotomy (PCNL) afterward and in the follow-up of asymptomatic stones, whereas noncontrast CT (NCCT) is recommended in cases of residual fragments. We compared the efficacies of PAR and NCCT in terms of assessing the outcome of SWL treatment for radiopaque ureteral stones.

Patients and Methods:

Those patients with renal colic and a radiopaque ureteral stone of 5 to 20 mm that was detected on PAR were included in the study; body mass index (BMI) values were calculated and recorded. Patients whose PAR revealed opacities suspicious for ureteral stones were evaluated with NCCT at 3-mm slices. Stone status was assessed with PAR and NCCT on post-SWL day 3. Detection of no stone, a residual fragment of ≤4 mm, and a residual fragment of >4 mm was defined as success, clinically insignificant residual fragments, and failure, respectively.

Results:

On post-SWL day 3, both PAR and NCCT revealed stones in 31 patients, and no stones were seen in either modality in 29 patients. NCCT revealed stones whereas PAR had negative results for stones in two patients. These patients had upper ureteral stones of 7.5 mm (6–9 mm) before SWL. Mean stone size on NCCT after SWL was 2.5 mm (1–4 mm). Mean BMI of these two patients was 27.72, and mean BMI of the patients with upper ureteral stones that were revealed by both PAR and NCCT was 27.68; these two values were statistically similar.

Conclusion:

PAR is capable of detecting clinically significant residual fragments, and patients can be followed up with PAR alone after SWL treatment for radiopaque ureteral stones. This approach both decreases the cost and prevents excessive radiation exposure.

Introduction

P lain abdominal radiography (PAR) and intravenous urography (IVU) have been used in the diagnosis of urinary stones. These techniques also served to reveal the urinary tract anatomy and possible obstruction. Alternatively, a combination of PAR and ultrasonography (USG) was a technique eliminating the disadvantages related to intestinal air superposition, pregnancy, and renal insufficiency. Improvements in medical imaging techniques brought about developments in the diagnosis of urinary stones, and noncontrast CT (NCCT) has become the technique of choice.¹ NCCT provides a rapid and reliable diagnosis, has high sensitivity and specificity rates, eliminates the need for an intravenous contrast substance, and allows for the diagnosis of extraurinary pathologies.^1,2 High cost and high radiation associated with NCCT, however, limits its use.^2,3

There are no definite data indicating which modality to use to assess the efficacy of shockwave lithotripsy (SWL). Usually, PAR is recommended in percutaneous nephrolithotomy (PCNL) afterward and in the follow-up of asymptomatic stones, whereas NCCT is recommended in cases of residual fragments.^1,3 We compared the efficacies of PAR and NCCT in terms of assessing the outcome of SWL treatment for radiopaque ureteral stones.

Patients and Methods

Our study started with the approval of the Institutional Review Board. Those patients who presented with renal colic and a radiopaque ureteral stone of 5 to 20 mm that was detected on PAR were included in the study. Patients were informed and those who consented were enrolled. Our exclusion criteria were: Pregnancy or suspicion of pregnancy, history of ureteral stenosis or tumor, coagulation defects, solitary kidneys, acute renal insufficiency (serum creatinine>1.5 mg/dL), severe urinary tract infection, urosepsis, or existence of ureteral stents.

Body mass index (BMI) values were calculated and recorded. Urinalysis was performed with a stick test and microscopy. Peripheral blood samples were obtained from every patient for biochemical tests (hemogram, serum urea, creatinine, activated partial thomboplastin time, prothrombin time/international normalized ratio). Patients whose PAR revealed opacities that were suspicious for ureteral stones were evaluated with NCCT at 3-mm slices (Philips Brilliance CT 64-slice). Stones were classified according to their images on PAR as upper (from the ureteropelvic junction to the upper edge of the sacrum), mid (between the upper and lower edge of the sacrum), and lower (from the lower edge of the sacrum to the bladder) ureteral stones. Hounsfield unit was not assessed.

SWL was performed by the same urologist using a lithotripter with electromagnetic generator (Siemens Lithoskop,™ Germany). Stone status was assessed with PAR and NCCT on post-SWL day 3. Detection of no stone, a residual fragment of ≤4 mm, and a residual fragment of >4 mm was defined as success, clinically insignificant residual fragments, and failure, respectively. The patients were not evaluated in terms of the existence and severity of pain and their prognostic ability to diagnose residual fragments.

Statistical evaluation was performed with SPSS 16.0^® software. Data were tested as to whether they conformed to normal distribution, and independent samples t test and Mann Whitney U test were used accordingly. Categorical data were assessed with a chi-square test. A P value<0.05 was considered as the threshold for statistical significance.

Results

A total of 63 patients were included in the study between December 2010 and April 2011. Average stone size was 8.46 mm (5–20 mm). Spontaneous passage of stone occurred in one patient. On post-SWL day 3, both PAR and NCCT revealed stones in 31 patients, and no stones were seen in either modality in 29 patients. NCCT revealed stones, whereas PAR had negative results for stones in two patients (Table 1).

Table 1.

Stone Sizes Before and 3 Days After Shockwave Lithotripsy and Distribution Among the Groups

Stone detection 3 days after SWL	n	Mean stone size prior to SWL (mm)	Mean stone size after SWL (mm)
PAR+/NCCT+	31	9.43 (5–20)	6.8 (2–11)
PAR−/NCCT−	29	7.65 (5–11)	–
PAR−/NCCT+	2	7.5 (6–9)	2.5 (1–4)
PAR+/NCCT−	0	–	–

SWL=shockwave lithotripsy; PAR=plain abdominal radiography; NCCT=noncontrast computed tomography.

The outcome of SWL treatment, stone detection by imaging modalities, and mean stone sizes according to the stone locations are outlined in Table 2. Two and three sessions of SWL treatments were performed in 19 and 14 patients, respectively. SWL was unsuccessful in eight patients, in whom ureteroscopic lithotripsy and Double-J stent insertion were performed in six and two, respectively.

Table 2.

Pre- and Post-Shockwave Lithotripsy Stone Sizes and Locations According to Detection by Imaging Modalities

		Pre-SWL		Post-SWL residual stone
	Stone detection	Number	Size (mm)		Number	Size (mm)
Upper ureteral stone	NCCT−/PAR−	19	5–13.5 (7.97)		–	–
	NCCT+/PAR+	20	5–20 (9.99)	Upper ureter	12	7.8 (5–11)
				Midureter	1	5
				Lower ureter	7	6.5 (2–15)
	NCCT+/PAR−	2	6–9 (7.5)	Upper ureter	1	4
				Midureter	–	–
				Lower ureter	1	1
Midureteral stone	NCCT−/PAR−	3	5–9 (7)		–	–
	NCCT+/PAR+	3	6–11 (8.6)	Upper ureter
				Midureter	1	4
				Lower ureter	2	7–9 (8)
	NCCT+/PAR−	0	–	–	–	–
Lower ureteral stone	NCCT−/PAR−	7	6–11 (7.06)	–	–	–
	NCCT+/PAR+	8	5–12 (8.3)	Lower ureter	8	6.8 (3.5–12)
	NCCT+/PAR−	0	–	–	–	–

SWL=shockwave lithotripsy; NCCT=noncontrast computed tomography; PAR=plain abdominal radiography.

In two patients, PAR results were negative, but NCCT had positive results for residual fragments. These patients had upper ureteral stones of 7.5 mm (6–9 mm) before SWL. Mean stone size on NCCT after SWL was 2.5 mm (1–4 mm). After SWL, the stone was at the upper part in one patient and 4 mm, and at the lower part in another patient and 1 mm (Table 2). Patients were stone free 4 and 5 days after one course of SWL without necessitating additional procedures. Mean BMI of these two patients was 27.72, and mean BMI of the patients with upper ureteral stones that were revealed by both PAR and NCCT was 27.68; these 2 values were statistically similar.

Average stone sizes according to the imaging technique and stone locations in paients in whom residual stones were detected on post-SWL day 3 are shown in Table 3. Stone sizes overall and according to each location were statistically similar between the two modalities.

Table 3.

Average Stone Sizes According to Imaging Technique and Stone Locations in Patients in Whom Residual Stones Were Detected on Post-Shockwave Lithotripsy Day

	n	PAR (mm)	NCCT (mm)
All stones	31	6.7 (4–11)	6.8 (1–11)
Upper ureter	13	7.4 (4–11)	7.5 (4–11)
Midureter	2	8 (7–9)	8 (7–9)
Lower ureter	16	6.5 (4–15)	6.2 (1–15)

PAR=plain abdominal radiography; NCCT=noncontrast computed tomography.

Discussion

The most important factors that determine success are stone size and location, according to the American Urological Association Urolithiasis Guidelines.⁴ These parameters were used with PAR and IVU previously; however, because of the high sensitivity and specificity, NCCT has been used frequently. It has been reported that stones were missed on PAR in 43% of patients who were evaluated with both PAR and NCCT.⁵ Ninety percent of urinary stones are radiopaque; therefore, large stones are easily recognized on PAR, whereas small stones can be missed because of intestinal air and superposition of bones (transverse process of the vertebra, sacrum, etc). Sensitivity and specificity of PAR has been reported as 45% and 77%, respectively, and sensitivity of NCCT as 90%.² Sensitivity and specificity of IVU and NCCT have been reported as 94% to 100% to 64% to 97%, and 92% to 100% to 92% to 94%.² In our study, sensitivity and specificity of PAR was 90% and 100% respectively when NCCT was accepted as standard.

Although stone detection rate is high with NCCT in patients with acute flank pain, stone sizes determined by PAR and NCCT are different from each other. In a study, NCCT yielded larger measurements than PAR, and in another study, NCCT has been shown to measure the stone size smaller by 12% than PAR.^6,7

Others concluded that although transverse and longutidinal measurements are minimally different between NCCT and PAR, cephalocaudal distance of the stone has been found to be larger with NCCT.^8,9 Some studies reported that stone measurements with both methods yielded similar values.¹⁰

In our study, average stone sizes obtained with two imaging methods were similar when evaluated both overall and according to the location. Because the treatment approach is changed according to the stone size, underestimation of the size is important, especially for those >5 mm, such that spontaneous passage rates of stones of 5 to 7 mm and >7 mm are 60% and 39%, respectively.⁷ Therefore, combined use of PAR and NCCT during the initial visit would prevent underestimation of the stone size, thereby identifying the patients in whom active treatment is needed. In another study, scout images obtained during NCCT failed to reveal stones that were detected on PAR in 51% of the cases.^11,12 We aimed to determine the exact stone sizes at the beginning of our study and used both PAR and NCCT; we included the cases with radiopaque ureteral stones of 5 to 20 mm.

An imaging study is needed for outcome assessment after the treatment. This study should ideally be effective and associated with low morbidty and low cost.¹ The best follow-up imaging modality after diagnosis is ill-defined, however.¹

The most important limitation to NCCT is the concern for radiation dose to which the patients are subjected, which is significantly higher than conventional methods. Patients receive radiation doses of 30 to 50 mGy and 2.5 to 3 mGy with NCCT and IVU, respectively (1). Single PAR and NCCT radiation doses are 0.7 to 1 mSv and 6.5 mSv.³ The greater the radiation dose gets, the higher the fatal cancer risk becomes, which is 0.05% per 10 mSv radiation dose.

Combination of PAR and USG was found to be better in detecting residual fragments and intrarenal and perirenal abnormalities than IVU alone, but hydronephrosis was defined better by IVU in a study that evaluated asymptomatic patients 1 month after SWL treatment. The authors concluded that PAR and USG would be sufficient for follow-up of asymptomatic patients 1 month after SWL, and IVU could be used to delineate the collecting system as needed.¹³ On the other hand, USG alone has been found be capable of detecting hydronephrosis, yet insufficient to reveal residual fragments after SWL for ureteral stones.¹⁴ Follow-up of asymptomatic patients should not be ignored, because lack of symptoms does not necessarily mean that there is no indication of intervention whatsoever.

Digital NCCT was found to be more successful than PAR and conventional NCCT in a study similar to ours, in which patients were evaluated with PAR, conventional and digital NCCT before and on post-SWL day 1. Because outcome assessment was performed early, however, their conclusion is practically indeterminate.¹⁵

The appropriate method to detect clinically significant residual fragments after PCNL was investigated, and NCCT, PAR, and USG revealed 100%, 20%, and 20% of the stones, respectively. When only the radiopaque stones were considered, detection rate with PAR reached 85%, and there was no significant difference between PAR and NCCT in this case.¹⁶

We evaluated SWL outcome with PAR and NCCT. Certainly, the follow-up with PAR is easier in a patient in whom the existence and location of a stone have been revealed with NCCT and/or PAR previously. Actually, we did not aim to find out a general sensitivity and specificity rate for PAR in stone detection; rather, we evaluated the efficiency of PAR for a particular clinical condition, such that post-SWL outcome evaluation in ureteral stones. Our aim was to question the need for NCCT, and to find out whether PAR misses the residual fragments necessitating intervention. Both PAR and NCCT gave the same results in 60 of the 62 cases. The two patients, in whom PAR missed and NCCT revealed residual fragments, had upper ureteral stones before SWL, and the sizes of residual fragments were 1 and 4 mm, which were clinically insignificant and left for spontaneous passage. When patients were grouped according to the detection of residual fragments by either PAR or NCCT, BMIs of all patient groups were found to be similar. In addition, there was no relationship between the stone location before SWL and no stone detection with PARs.

The very early follow-up imaging (3 days post-SWL) in our study might be debatable. It is known that the interval between two sessions of SWL treatments for kidney stones should be 5 days. This can be shorter with ureteral stones, because the renal parenchyma is not affected by SWL. We attempted to evaluate the success as soon as possible to be able to decide on the necessity for another session of SWL treatment.

Another point for discussion is why our study was performed among a limited group of patients—ie, those with ureteral stones. As already known, diagnosis of ureteral stones may sometimes necessitate more advanced methods than kidney stones for various reasons, and the optimal method for post-SWL outcome evaluation in ureteral stones is a controversial issue. On the other hand, the kidney stone fragments after disintegration with SWL treatment are passed through the ureter. Therefore, it is our thought that limiting our study to a subset of patients does not decrease its value; furthermore, our findings are capable of revealing the use of PAR in the specific indication we studied. Other studies concluded that follow-up for radiopaque stones after SWL should be performed with PAR, and NCCT should be reserved for patients in whom significant residual fragments were suspected and PAR results were negative to decrease radiaton exposure.^1,3 To our knowledge, our study is the first to compare PAR with NCCT in terms of outcome assessment after SWL.

Conclusion

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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