The Role of Plain Radiography in Predicting Renal Stone Fragmentation by Shockwave Lithotripsy in the Era of Noncontrast Multidetector Computed Tomography

Abstract

Purpose:

The objective of this study is to determine if plain radiography has a role in prediction of stone fragmentation by shockwave lithotripsy (SWL).

Patients and Methods:

Our study included 106 patients undergoing SWL for renal stones. Based on plain radiography criteria, stones were classified according to density, homogeneity, and outline. We compared the success of SWL in fragmentation of stones with a density equal to or less than bone, nonhomogeneous stones, and stones with irregular outline to its success in stones with a density more than bone, homogeneous stones, and stones with smooth outline.

Results:

In plain radiography, stone density equal to or less than bone, nonhomogeneous stones, and stones with irregular outline showed successful SWL fragmentation in 88.8%, 91.2%, and 90.6%, respectively. Stones with a density more than bone, homogeneous stones, and stones with smooth outline showed successful SWL fragmentation in 52.9%, 52.6%, and 57.1%, respectively. CT attenuation value was significantly less in stones successfully fragmented by SWL compared with nonfragmented stone (649±169 and 1465±256, respectively). In homogeneous stones with smooth outline having a density more than bone, we found a significantly lower CT attenuation in patients with successful stone fragmentation by SWL compared with those with failed stone fragmentation (690.9±171 and 1462±212, respectively).

Conclusion:

In relatively large solitary renal pelvic stones, plain radiography can predict the success of stone fragmentation by SWL. Nonhomogeneous stones with irregular outline and a density equal to or less than bone are expected to be successfully fragmented by SWL. Noncontrast CT is only needed, to predict success of SWL, in cases of homogeneous stones with smooth outline and density more than bone.

Introduction

Extracorporeal shockwave lithotripsy (SWL) remains the gold standard treatment for patients with renal stones ≤2 cm.¹ Stone fragility is the most important factor determining the success of SWL. Failure of SWL to fragment stones results in undesirable exposure of renal parenchyma to shockwaves in addition to the useless medical costs.²

Radiologic criteria of stones, including density, homogeneity, and regularity of its outline, obtained from plain radiography have been used previously for the prediction of stone fragility.³ Its clinical application was limited, however, because density measurement is subjective. Recently, noncontrast CT (NCCT) has been used widely in the prediction of stone fragmentation by SWL. The advantage of NCCT, however, has to be balanced against the increased cost and the higher radiation dose given to the patient during CT investigation.^4,5

The recent improvement in X-ray technology with the use of computed radiography (CR) has drawn back the attention to the importance of plain radiography for the prediction of stone fragmentation by SWL. The objective of this study is to determine the role of plain radiography in prediction of stone fragmentation by SWL and determine the cases in which NCCT is mandatory.

Patients and Methods

In this retrospective case control study, 106 patients, 60 males and 46 females, with a mean age 38.4±14.2 years, undergoing SWL for single renal pelvic stones at Minia University Hospital were included. Patients were included if the maximal linear diameter of their stones, measured in NCCT, ranged from 10 to 15 mm. Patients with radiolucent stones, multiple stones, and those with anatomic abnormalities of the ureter or pelvicaliceal system were excluded from the study. In addition, we excluded patients with bilateral urinary stones, those with stones in the only functioning kidney, patients with elevated serum creatinine level, and all other patients needing ureteral Double-J stent insertion before SWL.

All patients were evaluated by detailed history, physical examination, plain radiography using CR, intravenous urography, and NCCT. The region of interest was created overlying the whole stone on the slice in which it was seen at its largest diameter. Three different points were determined at the densest areas of the stone and the Hounsfield unit (HU) was measured of an area 1 mm on average. The mean HU value was recorded on the basis of these results. Radiologic characteristics of the stones in plain radiographs were determined by a single radiologist and revised by three urologists at the SWL unit. All of them were blinded for the results of NCCT and SWL.

According to stone radiologic characteristics, in plainradiographs—namely, stone density compared with that of the 12th rib or a transverse process of a lumber vertebra—stones were classified to be stone with density>bone or stone with density≤bone). Regarding stone homogeneity, stones were classified into homogeneous and nonhomogeneous stones. In addition, stones were classified into stones with regular outline and irregular outline. According to these stone radiologic characteristics, in plainradiographs, patients were classified into two groups (Table 1). Group 1 included 60 patients who had nonhomogeneous stones, and their stones had irregular outline and density≤bone. Patients in group 2, 32 patients, had homogeneous stone, and their stones had regular outline and density>bone.

Table 1.

Radiographic Characteristics of Stones

Stone characters on plain radiography	No. of patients (%)
Density
Density≤bone	72 (67.9%)
Density>bone	34 (32.1%)
Homogeneity
Nonhomogeneous	68 (64.2%)
Homogeneous	38 (35.8%)
Outline
Irregular	64 (60.4%)
Smooth	42 (39.6%)

All patients were treated with Siemens Lithostar lithotripter modularis litho vario. Fragmentation was performed under fluoroscopic imaging. A maximum of 3000 shockwaves were used in each treatment session. Success and failure, in this study, was considered after the first session with a maximum of 3000 shockwaves. Clinical success was defined as stone-free status or the presence of clinically insignificant asymptomatic stone fragments ≤4 mm, onradiography, at 3-month follow-up. If a stone was not fragmented at all or necessitated more than one session for complete fragmentation, this was considered as failure.

We performed statistical analysis of our data using a computer software SPSS program (SPSS^® 16). Pearson correlation coefficient test and independent sample t test were used to compare continuous variables. Chi-square test and Mann Whitney U tests were used to compare categorical variables. P value <0.05 was considered significant.

Results

Our study included 106 patients with a mean age 38.4±14.2 years (60 males and 46 females). All patients had solitary renal pelvic stones. The mean of the maximal stone diameter in all patients was 12.4±2.8 mm; in group 1 it was 12.2±2.7 mm, and in group 2 it was 12.6±2.4 mm with no significant difference between the two groups (P=0.454). In addition, there was no significant difference in the mean age and body mass index between the two groups (P=0.633 and P=0.442, respectively).

Evaluation of the outcome of SWL in this study is based on stone fragmentation after the first session. The number of shockwaves used in this session, in all patients, was 3000 shockwaves. Success was achieved in 82 (77.4%) patients.

The mean stone density in NCCT, in all patients, was 945.9±278.1 HU. In patients with a stone density on plain radiography≤bone, the mean stone density in NCCT was 679±256 HU, which is much lower than in patients with a stone density on plain radiography>bone (1147±243 HU), and the difference was significant (P<0.001). In patients with homogeneous stones on plain radiography, the mean stone density in NCCT (1024±338 HU) was significantly higher than in patients with nonhomogeneous stones on plain radiography (899±303 HU) (P<0.001).

Patients who had stone density≤bone (72 patients), nonhomogeneous stones (68 patients), and stones with irregular outline (64 patients) showed successful stone fragmentation in 88.8%, 91.2%, and 90.6%, respectively. Patients who had stones with density>bone (34 patients), homogeneous stones (38 patients), and stones with smooth outline (42 patients) showed successful stone fragmentation in 52.9%, 52.6%, and 57.1%, respectively (Table 2).

Table 2.

Shockwave Lithotripsy Outcome in Different Stone Characters on Plain Radiography

	SWL outcome
Radiographic characters on plain radiography	Success pts. no. (%)	Failure pts. no. (%)	P value
Density≤bone	64 (88.8%)	8 (11.2%)	0.001
Density>bone	18 (52.9%)	16 (47.1%)	0.090
Nonhomogeneous	62 (91.2%)	6 (8.8%)	0.007
Homogeneous	20 (52.6%)	18 (47.4%)	0.091
Irregular outline	58 (90.6%)	6 (9.4%)	0.002
Regular outline	24 (57.1%)	18 (42.9%)	0.131

SWL=shockwave lithotripsy.

Patients in the first group showed a significantly higher rate of successful fragmentation (96.7%) compared with patients in the second group (46.9%) (P<0.001) (Table 3). In patients in the second group, we found significantly lower CT attenuation values in patients with successful stone fragmentation by SWL compared with those who had failure (Table 4). Using a CT attenuation value cutoff point of 976 HU, we found that CT HU can predict stone fragmentation with 91.7% sensitivity and 95.1% specificity. Stones with attenuation value ≤976 had significantly higher fragmentation rate (95.2%) than stones with attenuation value >976 (9.1%) (P<0.001).

Table 3.

Successful Fragmentation Rate in Both Groups

Groups (no. of patients)	Successful fragmentation rate	P value
Group 1: Nonhomogeneous stones with irregular outline and density≤bone (60)	96.7%	<0.001
Group 2: Homogeneous stones with smooth outline and density>bone (32)	46.9%

Table 4.

Correlation Between Radiographic Characteristics of Stones and Mean CT Attenuation Values of Stones in Group 2

	Mean of CT attenuation values in HU for stone
Radiographic characters of stone by plain radiography	Success	Failure	P value
Homogeneous	689.9±171.8	1462.2±212.8	<0.01
Density>bone	689.78±182.3	1524.13±113.6	<0.01
Smooth	679.08±113.6	1462.67±212.8	<0.01

CT=computed tomography; HU=Hounsfield unit.

Discussion

The success of SWL, in renal stone fragmentation, depends on stone size, number and composition, in addition to stone location and anatomy of the pelvicaliceal system.^3,6 This study was conducted on renal pelvic stones ranging from 10 to 15 mm. Stones smaller than 10 mm are difficult, in our opinion, to determine, with consensus, morphology on plain radiography. Stones larger than 15 mm may need more than one session of SWL to have complete fragmentation.^7,8 In our study, successful fragmentation of stones after the first session was achieved in 77.4%. This percent of success is consistent with literature reportinga successful stone fragmentation rate of renal pelvic stones, less than 20 mm, ranging between 60% and 90%.^4,9
–11

Stone composition is considered the most important factor affecting stone fragmentation by SWL.⁷ Ringdén and Tiselius calculated the hardness factor for different renal stones managed with SWL and reported a hardness factor for cystine, 2.4; calcium phosphate, 2.2; calcium oxalate monohydrate, 1.3; calcium oxalate dehydrate, 1.0; uric acid/urate, 1.0, and magnesium ammonium phosphate, 1.0.⁹ Stone composition is suspected, on plainradiographs, by the degree of radiopacity compared with bone, homogeneity, and the outline of the stone. Calcium phosphate is densely opaque (more than bone) and homogeneous, calcium oxalate stones are moderately opaque (nearly equal to bone) and have spiky irregular outline, magnesium ammonium phosphate (struvite) stones are faintly opaque (less than bone) and heterogeneous or stratified, Cystine stones are faintly opaque, homogeneous with regular outline, uric acid and xanthine stones are mostly radiolucent.¹²

Identification of stone composition on plain radiographs is not accurate, and most stones are mixed in composition. NCCT is more reliable than plain radiography in detecting small and faint stones. CT analysis of stone density is not likely to be more accurate than standard radiography in characterizing stone composition, however.¹³ This makes most studies use stone density on plain radiography or NCCT, and not stone composition, as a predictor of stone fragmentation by SWL.

NCCT is a quantitative way to assess stone density compared with plain radiography, which is subjective. In this study, we measured stone density by NCCT in different plain radiography characters of stones and found that the mean stone density in NCCT was 679±256 HU in stones with a density on plain radiography≤bone, which is much lower than in stones with a density on plain radiography>bone (1147±243 HU), and the difference was significant (P<0.001). This is consistent with the fact that bone density in NCCT ranges from+700 HU in cancellous bones to+3000 HU in denser bones.¹⁴ eIn homogenous stones on plain radiography, the mean stone density in NCCT was 1024±3 38 HU, which is significantly higher than in nonhomogeneous stones that have a density in NCCT 899±303 HU (P<0.001).

Few previous studies used other plain radiography characters of stone, in addition to stone density, to predict SWL stone fragmentation.^3,10 In this study, we evaluated the use of all plain radiography characters of stones—namely, density, homogeneity, and outline to predict SWL fragmentation. Stones with a density≤bone (with a mean density in NCCT 679±256 HU) showed successful fragmentation in 88.8%, which is much higher than stones with a density>bone (with a mean density in NCCT 1147±243 HU) that had a successful fragmentation in 52.9%. This is consistent with Bon and colleagues³ who reported that stones denser than bone on plain radiography had a less chance of fragmentation by SWL than stones with a density less than bone. In another way, this finding is confirmed by many studies concluding that when NCCT density of stones is less than 750 HU, have a density less than bone on plain radiography, according to our data, the success of SWL treatment is almost always guaranteed.^4,7,15

Bon and colleagues³ concluded that stones with a rough outline have more chance of fragmentation by SWL compared with stones with a smooth outline. This is confirmed, in our study, by high stone fragmentation rate in stones with an irregular outline (90.6%) compared with stones with a smooth outline (57.1%). Stone homogeneity is another character on plain radiography that is equally important to stone density and stone outline, in prediction of stone fragmentation by SWL. It is not widely studied in the literature and, we found, in our study, that nonhomogeneous stones have a higher fragmentation rate by SWL (91.2%) compared with homogeneous stones (52.6%).

In our study, when we collected all stone characters on plain radiography in one group, we found that stone fragmentation by SWL is almost certain in nonhomogeneous stones with irregular outline and a density≤bone (96.7%) and is uncertain in homogeneous stones with a smooth outline and a density>bone (46.9%). This percentage of success is similar to the study by Bon and colleagues³ reporting a stone fragmentation rate of 79.4% in rough less dense stones, whereas smooth dense stones had a fragmentation rate of 33.6%. Because of the high percent of successful SWL fragmentation of nonhomogeneous stones with irregular outline and a density≤bone, on plainradiography, NCCT is not needed to predict fragmentation in these stones.

According to our data, homogeneous stones with a smooth outline and a density>bone have almost a 50% chance of fragmentation by SWL. In these cases, NCCT is able to predict successful fragmentation by accurately measuring stone attenuation value. The attenuation value, in NCCT, of homogenous stones with a smooth outline and a density>bone in the failure group is almost double the attenuation value in the success group. When the attenuation value, in NCCT, of homogeneous stones with a smooth outline and a density>bone is less than 976 HU, its successful fragmentation by SWL is almost certain (95.2% chance of fragmentation), but its successful fragmentation is unlikely (9.1% chance of fragmentation), when it is more than 976 HU. We concluded that it is necessary to have NCCT for prediction of successful stone fragmentation by SWL in these cases. This conclusion is supported by Joseph and coworkers⁴ and Tarawneh and associates⁷; they concluded that stones with densities exceeding 950 HU have little chance of fragmentation by SWL.^4,7

Conclusion

We recommend radiography as an initial study in all patients undergoing stone fragmentation by SWL. If the stone is large enough to determine its radiologic morphology, its characters, on plain radiography, can predict the success of its fragmentation by SWL in many cases. Patients with nonhomogeneous stones with irregular outline and a density≤bone are expected to have a successful fragmentation after SWL. NCCT is not recommended in those patients because it will add cost with no added benefit. On the other hand, NCCT is essentially recommended in homogeneous stones with smooth outline and density more than bone to determine CT attenuation value, which is well predictive of successful stone fragmentation.

Disclosure Statement

No competing financial interests exist.

Footnotes

Abbreviations Used

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