Abstract
Purpose:
The aim of this study was to prospectively identify predictors of radiation exposure during ureteroscopy.
Patients and Methods:
Eighty-five consecutive patients who presented for ureteroscopies and laser lithotripsy were considered. Fluoroscopy time (FT) was obtained from radiology reports for each patient, and clinical data were obtained from chart review. Nine patients were excluded (three unconfirmed FTs, four staghorn calculi, one ectopic kidney, and one multiple ureteral strictures). Seventy-six patients were included in the study. Univariate and multivariate linear regression were used to identify factors that determined FT.
Results:
The patient cohort was 65.8% male with a mean age of 52.7 years. Mean FT was 183 s, and mean surgical time was 68.4±29 minutes. Mean stone size was 10±5 mm in the greatest dimension. A large proportion of patients (50%) had renal stones, multiple stones were present in 31.6% of cases, and 22.3% of stones were radiolucent. Cases were equally distributed between surgeons A and B, and 46% of patients had preoperative stents. On multivariate analysis, increased FT was independently associated with surgeon A (104 additional seconds per case, P<0.001), longer duration of surgery (14 s per 10 minutes, P<0.001), and male patients (54 s per procedure, P=0.02). Age, stone characteristics, presence of ureteral stent, and stone-free status did not correlate with FT.
Conclusions:
Surgeon behavior, longer duration of surgery, and male gender were significant predictors of FT and, hence, radiation exposure during ureteroscopy. In the present study, stone characteristics were not found to be predictors of FT.
Introduction
Patients with stone disease are also at increased risk of secondary effects from radiation exposure. Among patients who are undergoing endovascular procedures for vascular disease, several instances of skin desquamations and depilation have been reported with longer fluoroscopic procedures. 5 Although urologic procedures need lower doses of radiation, they are nonetheless important. The typical dose of radiation a patient is exposed to during ureteroscopy has been reported to range from 2.5 to 100 mSv. 6 Patient factors such as obesity and stone factors such as renal location, size, and opacity have been shown to increase radiation exposure during endourologic procedures. 1,7 –9
Patients who form stones may need multiple endourologic procedures and imaging studies. 10 Limiting radiation exposure when possible can, therefore, be important in preventing secondary malignancy. 11 The aim of the present study was to prospectively identify predictors of radiation exposure during ureteroscopy.
Patients and Methods
Patient population
Data were obtained prospectively from 85 consecutive unilateral ureteroscopies and holmium laser lithotripsies performed for patients who presented with stone disease. These consecutive ureteroscopies were performed between February 2008 and December 2009 by two attending endourologists (MA and SA) representing surgeons A and B. Fluoroscopy times (FT) were recorded from the C-arm timer, which were used for the official radiology reporting of FTs. Nine procedures in total were excluded: Three did not have FT documented by either the radiologist or the urologist, four had staghorn calculi, one patient had a solitary kidney and multiple ureteral strictures necessitating balloon dilations, and another had an ectopic pelvic kidney. In the final analyses, there were a total of 76 ureteroscopies.
Patient and stone characteristics were obtained from hospital and office charts. The preoperative size of stone was determined as the maximum stone diameter on preoperative CT scans. Anesthesia records were used to record duration of ureteroscopy. Patients were considered stone free (no stones or residual fragments <2–3 mm) based on intraoperative assessment and postoperative ultrasonography and/or abdominal plain radiography.
Operative technique
Both endourologists used the same technique and instruments during rigid and flexible ureteroscopy. For ureteral stones, flexible nephroureteroscopy was routinely performed to inspect the renal collecting system for stone fragments and confirm stone-free status. All cases had equal involvement of urology residents at the postgraduate year 4 level. Selected patients had preoperative ureteral stents, which was dependent on the clinical situation and discretion of the referring urologist. All patients had postoperative indwelling ureteral stents. Postoperative ureteral stents were placed under fluoroscopic guidance. During the present study, an experienced radiology technician was involved in each case, and continuous fluoroscopy with last-image hold was used with the surgeon controlling the foot pedal.
Statistical analyses
Baseline patient, stone, and perioperative characteristics were collected, and the absolute values with means and standard deviations were calculated. Univariate and multivariate linear regression analyses were then used to determine predictors of FT. The following variables were used: Patient sex, age, stone side, size, location (proximal-midureteral, distal ureteral, lower caliceal, and other renal [pelvis, upper, midcaliceal]), number (multiple vs single), radiolucency (lucent vs opaque), presence of preoperative stent, length of surgery, surgical outcome (residual stone vs stone-free), and surgeon (surgeon A vs B). Assumptions of a normal distribution of residuals, as well as lack of heteroscedasticity, multicollinearity, and nonlinearity were satisfied (data not shown). R2 values were calculated to describe the variability of our sample accounted for by the variables included in our multivariate analyses. A critical P value of 0.05 was used. Statistical analyses were performed using Stata v.10.1 (StataCorp, College Station, TX).
Results
Baseline patient, stone and perioperative characteristics of the 76 patients included in this study are summarized in Table 1. Study patients were predominantly male (65.8%) with 52.7±14.7 years of age. Mean stone size was 10±5 mm, 50% of which were renal and 42% were right sided. Multiple stones were present in 31.6% of the cohort, and 22.3% were radiolucent. Mean fluoroscopy time and length of surgery were 183±114 seconds and 68.4±29 minutes, respectively. Preoperative stents were present in 35 (46%) cases, and the stone-free rate was 81.6%.
Fisher exact test.
Mann-Whitney U test.
Statistical significance P≤0.05. Total percentages may not add up to 100% because of rounding of decimals.
SD=standard deviation.
On univariate analysis, a distal ureteral stone was associated with 99 seconds less FT (P=0.006), and a residual stone at the end of the case was associated with 90 seconds more FT (P=0.007). These variables, however, ceased to be significant on subsequent multivariate analysis (Table 2). On multivariate analysis, the most significant independent predictor of FT was the surgeon. When compared with surgeon B, patients who underwent ureteroscopy by surgeon A were exposed to an additional 104 seconds of fluoroscopy per ureteroscopy (P<0.001), after correcting for patient characteristics (sex, age, and presence of preoperative stent), stone characteristics (side, size, location, number, and radiolucency), length of surgery, and stone-free rates (R2=0.52) (Table 2). Furthermore, male sex was associated with an extra 54 seconds of FT (P=0.03), and each additional 10 minutes of surgery was associated with 14 seconds of FT (P=0.003) (Table 2).
Linear regression (statistical significance P≤0.05) (R2=0.52)
Discussion
On multivariate analysis, duration of surgery was found to be an independent predictor of FT. Longer duration of surgery can be expected to increase fluoroscopy use, because a more difficult case may necessitate additional fluoroscopy and operative time. In a previous study using rigid ureteroscopy for distal ureteral stones, Peschel and colleagues 12 found that stones larger than 5 mm necessitated longer operative and fluoroscopy times than stones less than 5 mm. In the present study, stone size did not predict FTs (Table 2). This may be because in the present study, the mean stone size was 10.0±5.0 mm (Table 1). Furthermore, 90% of all stones were between 5 and 18 mm with very few small stones less than 5 mm. Perhaps a larger sample size and greater range in stone size would show a correlation between stone size and FT.
In the present study, male patient sex was associated with longer FTs. The significantly prolonged FT in male patients may be related to increased use of flexible ureteroscopes with male patients in the management of proximal ureteral stones. 6 This finding concurs with recent work by Ngo and associates. 13 Interestingly, significantly shorter FTs in female patients during ureteroscopy may be associated with similar radiation doses to the female gonads. Krupp and coworkers 14 developed a cadaveric model of radiation exposure to the gonads during flexible ureteroscopy. They observed that female gonads received significantly higher effective radiation doses when compared with male gonads (3.4 mGy and 1.9 mGy for left and right female gonads compared with 0.36 mGy and 0.39 mGy for left and right male gonads) using a fixed FT of 145 seconds. Therefore, it is important to minimize FTs, especially in female patients.
In terms of variation of FTs with stone location, Hellawell and colleagues 1 found that lower caliceal renal stones were associated with higher FTs than ureteral stones, whereas in another study, Bagley and Cubler-Goodman 6 found that renal stone location was not associated with increased FT. In the present study, distal ureteral stones were associated with decreased FT on univariate analysis when compared with mid and proximal ureteral stones. This finding, however, became insignificant on multivariate analysis (Table 2). It is likely that the present study is underpowered to detect differences in FTs among different locations. Furthermore, flexible nephroureteroscopy was performed at the end of ureteroscopy for ureteral stones to inspect the renal collecting system for retropulsed stone fragments and confirm stone-free status. This may have blunted the difference in FT between ureteral and renal stones.
On multivariate analysis, surgeon A was associated with 105 seconds more FT per ureteroscopy case (P<0.001). Because both surgeons used the same technique and instruments in the same institution, this difference is likely because of intraoperative preference in the use of fluoroscopy during ureteroscopy. Surgeon B has a particular interest in radiation safety and documents FT in the operative record. This may have contributed to the significantly lower FT during each case. This Hawthorne-like effect has been documented in other clinical contexts. 15,16 Ngo and coworkers 13 evaluated the impact of providing surgeon feedback on fluoroscopy use during ureteroscopy. After implementing a feedback protocol, they observed a 24% decrease in FT used during ureteroscopy (2.74 min vs 2.08 min, P=0.002). The authors attributed the decreased use of fluoroscopy to surgeon awareness.
In another attempt to minimize radiation exposure during ureteroscopy, Green and associates 17 found that by implementing reduced fluoroscopic protocol by relying on visual and tactile cues, they significantly reduced the mean FT from 86.1 seconds (range 22–300 s) to 15.5 seconds (range 0–54 s) (P<0.001). They have even described 10 ureteroscopies performed without the aid of fluoroscopy or ultrasonography. 18 Therefore, surgeon behavior remains the most significant modifiable factor in fluoroscopy use during ureteroscopy.
There are several potential limitations to the present study. First, statistical tests had limited power because of a small number of cases (n=76). Therefore, it is possible that stone characteristics play a more significant role in determining FT. A larger sample size may elucidate further stone-related factors determining FT. Another limitation is that when considering ureteral stones, the present study did not take into account whether ureteral stone fragments migrated into the kidney necessitating renoscopy and stone extraction using a flexible ureteroscope. This may have needed more FT and added a confounding factor for ureteral stones.
Conclusions
Length of surgery, male sex, and surgeon are independent predictors of FT during ureteroscopy. Of these variables, surgeon behavior remains the most significant modifiable factor in determining radiation exposure during ureteroscopy.
Footnotes
Acknowledgments
This work was supported in part by grants from Northeastern American Urological Aassociation Young Investigator Award and the Montreal General Hospital Foundation Award to Sero Andonian.
Disclosure Statement
No competing financial interests exist.