Evaluating Parameters Affecting Fluoroscopy Time During Percutaneous Nephrolithotomy: Focus on the Predictive Role of Guy's,S.T.O.N.E.,and CROES Scoring Systems

Abstract

Objective:

To identify parameters predicting prolonged fluoroscopy time (FT) during percutaneous nephrolithotomy (PCNL) and to evaluate the predictive ability of Guy's stone score, S.T.O.N.E. nephrolithometry, and CROES nomogram.

Patients and Methods:

We evaluated 153 consecutive patients who underwent a PCNL by a single experienced surgeon. Several preoperative, intraoperative, and postoperative parameters, including the three scoring systems, were assessed for their ability to predict FT.

Results:

FT was significantly associated with stone burden, stone location, number of stones, number of punctures, and number of tracts. It was also significantly associated with all scoring systems. Fluoroscopy use was significantly higher in patients who were effectively treated in these with longer hospitalization, probably reflecting case complexity. No association was found with postoperative complications.

Conclusions:

FT during PCNL can be affected by several parameters. Identifying the ones that can affect FT can lead to fluoroscopy and radiation exposure decrease. Guy's stone score, S.T.O.N.E. nephrolithometry, and CROES nomogram can all predict complexed cases and the need for increased FT.

Introduction

Radiation exposure represents a risk factor for secondary malignancies, and the risk is linearly related to exposure time.¹ Patients with urolithiasis belong to a high-risk group for high exposure since all modern methods for stone fragmentation are performed under fluoroscopic guidance. Subsequently, concerns have risen regarding the cumulative effects of ionized radiation to both patients and medical staff, leading several groups to publish protocols for decreasing radiation exposure and the consequences of it.^2,3

Percutaneous nephrolithotomy (PCNL) is probably the method that is correlated with highest radiation exposure compared to ureteroscopy and shockwave lithotripsy.^2

–6 Fluoroscopic imaging is assisting the surgeon in several steps of the procedure, but mainly to evaluate the anatomy of the collecting system and to choose the optimal calix for puncturing. Alternatively, fluoroscopy is increasing patients' exposure to radiation.

Several grading systems have been proposed lately to categorize the complexity of a renal stone. Their introduction was motivated by the need for a more objective discrimination between simple and complex cases and for better patient counseling as well. The most known of them are the Guy's stone score,⁷ S.T.O.N.E. nephrolithometry,⁸ and CROES nomogram.⁹ All of them have proven to be significant in predicting stone-free rates after PCNL.^10
–12 However, their use as a predictive tool for intraoperative fluoroscopy has never been evaluated.

Fluoroscopy time (FT) is commonly used as a surrogate for the estimation of radiation exposure during endourologic procedures.^5,6 However, FT during PCNL can widely vary depending on several factors. The aim of the present study is to evaluate several clinical, imaging, and intraoperative factors and to reveal parameters that may significantly predict FT during PCNL. The main study's endpoint is to evaluate the ability of Guy's stone score, S.T.O.N.E. nephrolithometry, and CROES nomogram to predict FT. In addition, we examined the association of FT with postoperative outcomes, in terms of stone-free and complication rates.

Patients and Methods

After the approval by the Scientific Committee of our institution, we conducted an analysis of 153 consecutive patients who underwent a PCNL in our institution between October 2012 and February 2015. All procedures were performed by a single experienced surgeon. Patients with incomplete data (n=40) were excluded from the study. Patients with anatomical anomalies (n=3) and aged <18 years (n=2) were excluded as well. Several clinical and imaging parameters were recorded to evaluate their association with FT during the operation. Clinical notes during admission, the intraoperative notes, and the Picture Archiving and Communication System (PACS) of our hospital network were all reviewed for the extraction of needed information. A preoperative computed tomography (CT) scan was utilized in all patients for grading renal stones according to the Guy's stone score, S.T.O.N.E. nephrolithometry, and CROES nomogram. CT scans were also used for the evaluation of stone characteristics (burden, side, location, and density). Stone burden was calculated in mm² using the following formula: length×width×π×0.25, where π is a mathematical constant equal to 3.14. The transverse axial images were used to evaluate several body characteristics, including anterior to posterior diameter of the abdomen, anterior abdominal fat length, and stone to skin distance. All the above parameters were measured at the level of the stone. The Fetal Urology Society classification system was used to estimate the grade of hydronephrosis.¹³ Stone radiopacity was estimated based on the preoperative plain abdominal X-ray. Information regarding the number of punctures, the number of tracts, the location of the tract, and the use of flexible nephroscope was extracted by the intraoperative images saved in the PACS. FT was recorded at the end of the procedure and it was saved in the PACS.

The procedure was initiated with the patient in the prone position. Using the flexible cystoscope, an open-ended 5F/70 cm ureteral catheter was inserted in the ipsilateral ureter. Through the latter, a retrograde pyelography was performed for opacification of the collecting system. Fluoroscopy was used in the anteroposterior axis and 300 oblique axis toward the surgeon for identification of the designated posterior calix for puncturing the collecting system. Using the bull's-eye technique, an 18-gauge/15-cm, diamond-shaped trocar needle was used for puncturing the fornix of the chosen calix. After confirmation of correct positioning into the calix, a 0.035-angled glidewire was inserted and forwarded to the ureter. With the assistance of a 5F-angled ureteral catheter, the glidewire was exchanged with an extra stiff wire. The tract was developed with a 20 atm balloon dilator and the insertion of a 30F sheath. Lithotripsy was performed by the Olympus LUS-2 ultrasonic lithotripter. The flexible nephroscope was used whenever it was considered necessary. The procedure was ended by putting a Council 18F nephrostomy catheter.

The mobile General Electric OEC 9900 Elite C-arm with automatic exposure settings (kV/mA, brightness) was used to apply low-dose continuous fluoroscopy with a rate of 30 pulses/second. Fluoroscopy settings were constant for all cases. The C-arm was manually moved by a designated operator under surgeon's guidance, while fluoroscopy use was self-controlled by the surgeon with a foot pedal.

Length of hospitalization and complications after PCNL were recorded. The Clavien–Dindo system was used to categorize postoperative complications.¹⁴ Postoperative success status was defined as no significant fragment >4 mm found in the postoperative CT scan.

Statistical analysis was performed by SPSS software version 17 (SPSS, Inc., Chicago, IL). Numerical variables are presented as the mean±standard deviation (SD). A univariate analysis of several demographics, stone characteristics, and intraoperative and postoperative characteristics was conducted to evaluate their association with FT. The Pearson correlation test was used for continuous variables and the Spearman correlation test was used for categorical variables to produce the r correlation coefficient. The latter is ranged between −1 and 1. Values close to 1 or −1 indicate a positive or negative linear relation, respectively. All p-values were two-tailed with statistical significance set at 0.05.

Results

The study cohort consisted of 108 patients. Of them, 43 (39.8%) patients were graded as Guy's I, 35 (32.4%) as Guy's II, 20 (18.5%) as Guy's III, and 10 (9.3%) as Guy's IV. Mean S.T.O.N.E. and CROES scores were 7.97±1.80 and 198.2±64.0, respectively. Clinical characteristics are seen in Table 1. A single tract was utilized in most of the cases (103, 95.4%) to perform the procedure. A single puncture was performed in 88 (81.5%) patients, while a second and a third puncture was needed in 14 (13.0%) and 6 (5.6%) patients, respectively. The punctured calix was a lower one in 102 (94.4%) cases, while a middle and an upper calix was chosen equally in the rest of the 6 (5.6%) patients. A flexible nephroscopy was performed in 102 (94.4%) patients. The postoperative success rate was 89.8%. Complications were observed in 36 (33.3%) of the patients. More specifically, a grade 1 complication was observed in 18 (16.7%) patients, a grade 2 in 11 (10.2%) patients, and a grade 3a in 7 (6.5%) patients. The mean hospitalization time was 5.83±2.29 days.

Table 1.

Patients Characteristics

No. of patients	108
Gender
Male, n (%)	76 (70.4)
Female, n (%)	32 (29.6)
Age (mean±SD)	53.2±15.4
Side
Right, n (%)	53 (49.1)
Left, n (%)	55 (50.9)
Number of stones
1, n (%)	55 (50.9)
2, n (%)	23 (21.3)
>2, n (%)	30 (27.8)
Staghorn
Partial, n (%)	19 (17.6%)
Complete, n (%)	11 (10.2%)
Location
Single position, n (%)	44 (40.7)
Multiple positions, n (%)	64 (59.3)
Stone burden, mean±SD	557.5±566.3
Hounsfield Units, mean±SD	1112.3±354.9

SD=standard deviation.

Mean FT was 424.4±217.7 seconds. As seen in Table 2, FT was significantly and positively associated with stone burden (p=0.001, r=0.307), stone location (p=0.010, r=0.248), and number of stones (p=0.039, r=0.199). Guy's stone score (p=0.014, r=0.236), S.T.O.N.E. nephrolithometry (p<0.001, r=0.372), and CROES nomogram (p=0.006, r=−0.265) were all associated with intraoperative fluoroscopy use. FT was also significantly associated with several intraoperative and postoperative parameters (Table 3). FT was significantly higher in longer procedures (p=0.005, r=0.489), in cases that>1 puncture (p<0.001, r=0.557) and >1 tract (p=0.003, r=0.286) were performed. FT was significantly lower in patients with postoperative effective results (p=0.013, r=0.238). FT was by a margin significantly higher in patients with longer hospitalization time (p=0.048, r=0.191).

Table 2.

Association of FT to Clinical Characteristics of Study Cohort

	FT (seconds)	p
Gender	r=0.015	0.878^a
Male (mean±SD)	422.0±227.6
Female (mean±SD)	434.1±241.4
Age	r=0.006	0.953^b
Side	r=0.102	0.294^a
Right (mean±SD)	445.3±230.6
Left (mean±SD)	406.5±231.3
Stone burden	r=0.307	0.001^b,c
Number of stones	r=0.199	0.039^a,c
≤2 (mean±SD)	387.4±186.7
>2 (mean±SD)	529.6±301.5
Staghorn	r=0.128	0.188^a
No (mean±SD)	395.9±194.7
Partial (mean±SD)	490.6±261.7
Complete (mean±SD)	524.0±358.9
Radiopacity	r=0.037	0.705^a
Radiopaque (mean±SD)	423.4±233.8
Radiolucent (mean±SD)	442.6±213.0
Location	r=0.248	0.010^a,c
Single position (mean±SD)	349.5±170.4
Multiple positions (mean±SD)	477.9±252.5
Hounsfield units	r=0.032	0.739^b
Hydronephrosis grade	r=0.028	0.770^a
≤2 (mean±SD)	399.9±168.3
>2 (mean±SD)	439.0±257.4
Anterior–Posterior diameter	r=0.038	0.699^b
Stone–skin distance	r=0.098	0.312^b
Abdominal fat length	r=0.002	0.981^b
Guy' stone score	r=0.236	0.014^a,c
Grade I (mean±SD)	380.3±191.3
Grade II (mean±SD)	413.0±195.1
Grade III (mean±SD)	475.9±272.8
Grade IV (mean±SD)	563.8±351.9
S.T.O.N.E. nephrolithometry	r=0.372	<0.001^b,c
CROES nomogram	r=−0.265	0.006^b,c

Spearman correlation.

Pearson correlation.

Statistical significance.

FT=fluoroscopy time; r=correlation coefficient.

Table 3.

Association of FT to Intraoperative Variables and Postoperative Outcomes

	FT (seconds)	p
Procedure duration	r=0.489	0.005^a,b
No. of punctures	r=0.557	<0.001^b,c
≤1 (mean±SD)	352.1±146.8
>1 (mean±SD)	748.8±257.1
No. of tracts	r=0.286	0.003^b,c
≤1 (mean±SD)	404.3±206.4
>1 (mean±SD)	863.1±293.3
Tract location	r=0.141	0.142^c
Lower calix (mean±SD)	416.5±225.2
Middle calix (mean±SD)	605.5±286.5
Upper calix (mean±SD)	553.7±356.2
Flexible nephroscope	r=0.110	0.256^c
No (mean±SD)	505.8±216.3
Yes (mean±SD)	420.9±231.6
Stone free	r=0.238	0.013^b,c
No (mean±SD)	663.1±346.2
Yes (mean±SD)	398.6±199.1
Postoperative complications	r=0.118	0.222^c
No (mean±SD)	406.7±224.6
Yes (mean±SD)	463.2±241.2
Clavien–Dindo grade	r=0.095	0.327^c
0 (mean±SD)	406.7±224.6
1 (mean±SD)	512.7±279.7
2 (mean±SD)	451.2±172.3
3a (mean±SD)	355.0±216.5
Length of hospitalization	r=0.191	0.048^a,b

Pearson correlation.

Statistical significance.

Spearman correlation.

Discussion

Patients suffering from urolithiasis have an increased risk for significant exposure to ionizing radiation. Diagnosis, management, and follow-up of the disease are based on imaging. Although CT represents the gold standard for the diagnosis of urolithiasis, it represents the greatest contributor in radiation exposure. It was estimated that a single conventional abdominal and pelvis scan exposes the patient to 8 and 6 mSv, respectively.¹⁵ Specifically for patients with an acute stone episode, it was reported that the median radiation dose was 29.7 mSv within the first year following the episode.¹⁶ Of great interest, 20% of studied patients exceed the upper proposed annual limit for radiation exposure. The exposure is even greater if the patient is receiving a kind of treatment. PCNL represents the appropriate treatment for large renal stones. The effective radiation dose during PCNL has been estimated to be around 7–9 mSv.^17,18 Taking into consideration that the safety limit proposed by the International Commission on Radiological Protection for occupational exposure is 50 mSv yearly, one can understand that special concern should be applied when we are implicated with the management of urolithiasis patients.¹⁹

Fluoroscopic imaging represents a mainstay for performing PCNL. FT is one of the few controllable variables that impact radiation exposure and this is the reason that FT is usually used as a surrogate for estimating radiation doses. There are a number of parameters that contribute to radiation exposure outside of FT. For instance, KVP, mAs, pulse rate, and automatic brightness control. However, FT is the one that can be directly controlled and monitored throughout the procedure. Factors that affect FT during the procedure have not been adequately characterized. However, it is of great importance to identify parameters that may increase fluoroscopy use to decrease exposure of patients and medical stuff. Majidpour²⁰ reported that the surgeon receives the highest amount of radiation during PCNL followed by the assistant and the nurse who receive an insignificant amount. Exposure for the surgeon is higher to the legs and lower to the eyes.

The goal of the present study was to identify clinical, imaging, and intraoperative parameters that increase FT during PCNL to enhance our knowledge on this important issue and to assist future research. Several groups have proposed techniques that may decrease FT and concomitant radiation dose. Pulsed fluoroscopy and air pyelogram instead of contrast pyelogram are some of them and they may achieve a 65% and 35% decrease in FT, respectively.^21,22 The implementation of a reduced fluoroscopy protocol was associated with an 80% decrease in FT.³ The reduced fluoroscopy protocol was including among others a laser-guided C-arm, fluoroscopy appliance during end expiration, and a single pulse/second fluoroscopy mode. The C-arm industry itself has contributed in reducing radiation exposure by applying several low-dose functions, including automatic brightness control, for enhancing image quality by continuously adjusting mAs and kVp settings in real intraoperative time.²³

In the present study, FT was significantly associated with several preoperative parameters, including stone burden, number of stones, and multiple locations, and these come in agreement with the findings of previous reports.^17,24

There is a controversy regarding the impact of body–mass index on FT. Mancini et al.¹⁷ reported that increased body–mass index was significantly associated with an increased effective radiation dose, but this was in contrary with the results of Tepeler et al.²⁴ who reported that the body–mass index had no impact on FT. The above conflicting results probably reflect the discordance between FT and radiation exposure. In our study, there was no information regarding body–mass index. Due to this, we supposed that measuring the anterior to posterior diameter of the abdomen, anterior abdominal fat length, and stone-to-skin distance can even better express body characteristics and they may more precisely evaluate the association with FT. Following statistical analysis, no association with FT was observed.

Procedure duration and number of tracts have been identified before as parameters increasing FT.²⁴ Our findings are constant with the above report. A fact that is well known in surgeons performing PCNL is that there are cases, whereas several punctures may be needed, even in experienced hands, to obtain the tract. Based on the results of the present study, we found that whenever more than two trials were needed for puncturing the designated calix, FT was significantly higher. This is of great value showing that a single puncture tract should always be the primary goal since it can decrease radiation exposure to both the surgeon and patient.

Several prognostic tools, including the Guy's stone score, S.T.O.N.E. nephrolithometry, and CROES nomogram, have been introduced lately in an effort to better address stone complexity. All of them can significantly predict stone-free outcomes assisting treatment planning and patient counseling. However, can these tools be used for the prediction for increased intraoperative FT? There is only one published study so far dealing with this question. Noureldin et al.²⁵ assessed the impact of S.T.O.N.E. nephrolithometry in predicting FT by analyzing 103 PCNL procedures. Their results have similarities with ours since the authors reported that FT was significantly associated with the number of punctures, the number of tracts, and operative time, while on multivariate analysis, the number of punctures, estimated blood loss, and operative time were found to be independent predictors. They reported that there was no association between the FT and S.T.O.N.E. nephrolithometry score. We found that all of the evaluated scoring systems were associated with prolonged FT.

Translating the results of the present study into the clinical practice, we can report that several parameters may predict prolonged FT during PCNL, including procedure duration, number of punctures, and number of tracts. Apart from patient counseling and treatment planning, the implementation of scoring systems in clinical practice may also predict the need for prolonged intraoperative FT. Measures to decrease FT, for example, lowering pulses/second rate, should be used in patients with renal stones of high complexity and unfavorable scores from scoring systems. In addition, precise preoperative planning may be helpful to decrease the above parameters. For cases that last longer, more than one puncture was needed for obtaining the tract, and when more than one tract was made, FT may be significantly increased and, thus, preventive measurements, like the ones described by Blair et al.,³ should be applied. Given the potential discordance between radiation exposure and FT, it would be of great interest for the correlation between radiation dose area product and preoperative nomograms to be assessed. Future studies would assist toward this.

The present study has some limitations arising by its retrospective mode. Several parameters that could affect FT, including body–mass index, were not evaluated. Although digital storage of patients' files that is used in our institution facilitates accurate data collection, several parameters were difficult to be collected. Prospective studies analyzing the present findings and comparing PCNL to other interventions, such as retrograde intrarenal surgery and shockwave lithotripsy, would justify and confirm the present results.

Several parameters can predict prolonged intraoperative use of fluoroscopy during a PCNL procedure. Number of punctures, number of tracts, and operation time are all intraoperative variables that could be potentially positively affected after a precise preoperative treatment plan. This could lead to a significant decrease of FT and radiation exposure. Guy's stone score, S.T.O.N.E. nephrolithometry, and CROES nomogram can reliably grade stone complexity and predict prolonged intraoperative FT.

Conclusions

Identifying parameters that may increase FT either preoperatively or intraoperatively may increase our awareness and lead to decreased radiation exposure. Guy's stone score, S.T.O.N.E. nephrolithometry, and CROES nomogram can all predict complexed cases and the need for increased FT.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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