- Email: [email protected]
Factors Affecting Patient Radiation Exposure During Percutaneous Nephrolithotomy
Factors Affecting Patient Radiation Exposure During Percutaneous Nephrolithotomy John G. Mancini, Eliza M. Raymundo, Michael Lipkin, Dorit Zilberman, Daniel Yong, Lionel L. Bañez, Michael J. Miller, Glenn M. Preminger and Michael N. Ferrandino* From the Duke University Medical Center, Durham, North Carolina
Purpose: We identified patient and stone characteristics that may contribute to increased radiation exposure during percutaneous nephrolithotomy and offer technique modifications to limit the radiation dose. Material and Methods: We reviewed the records of 96 patients who underwent percutaneous nephrolithotomy in the last 2 years. The effective radiation dose was calculated using accepted conversion tables. We performed multivariate linear regression to determine the association of the effective radiation dose with specific patient, stone and procedural characteristics. Results: Mean ⫾ SD patient age was 51.5 ⫾ 13.4 years and 62.5% of the patients were female. Median body mass index was 32.0 ⫾ 9.7 kg/m2 (range 16.2 to 59.6) and the median stone burden was 4 cm2. Increased body mass index (p ⬍0.001), higher stone burden (p ⫽ 0.013), stone nonbranched configuration (p ⫽ 0.002) and a greater number of percutaneous access tracts (p ⫽ 0.040) were significantly associated with an increased effective radiation dose. Specifically obese patients with a body mass index of 30 to 39.9 kg/m2 had a more than 2-fold increase in the mean adjusted effective radiation dose and morbidly obese patients with a body mass index of 40 kg/m2 or greater had a greater than 3-fold increase vs that in normal weight patients with a body mass index of less than 25 kg/m2 (6.49 and 9.13 mSv, respectively, vs 2.66, p ⬍0.001). Other stone specific parameters, including site and composition, percutaneous access site and estimated blood loss were not associated with the effective radiation dose. Conclusions: Patients with higher body mass index, greater stone burden, nonbranched stones and multiple nephrostomy access tracts are at risk for increased radiation exposure during percutaneous nephrolithotomy. Urologists must seek alternative strategies to minimize radiation exposure, such as tighter collimation to the region of interest, judicious use of magnification and the acquisition of as few images as possible during stone removal.
Abbreviations and Acronyms BMI ⫽ body mass index CT ⫽ computerized tomography ERD ⫽ effective radiation dose FST ⫽ fluoroscopic screening time PNL ⫽ percutaneous nephrolithotomy Submitted for publication May 3, 2010. Study received Duke University Medical Center institutional review board approval. * Correspondence: Division of Urology, Duke University Medical Center, DUMC 3167, Trent Dr., Durham, North Carolina 27710 (telephone: 919-681-5506; FAX: 919-681-5507; e-mail: [email protected]).
Key Words: kidney; kidney calculi; nephrostomy, percutaneous; radiation dosage; body mass index PERCUTANEOUS nephrolithotomy, which is most commonly done for large or complex renal calculi, offers the benefit of an improved stone-free rate over that of other minimally invasive procedures. In the United States fluoroscopy is typ-
ically used during the procedure to provide real-time visualization to establish and dilate a percutaneous tract to the renal collecting system, provide information on renal anatomy, properly place surgical devices and determine
0022-5347/10/1846-2373/0 THE JOURNAL OF UROLOGY® © 2010 by AMERICAN UROLOGICAL ASSOCIATION EDUCATION
Vol. 184, 2373-2377, December 2010 Printed in U.S.A. DOI:10.1016/j.juro.2010.08.033
AND
RESEARCH, INC.
www.jurology.com
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FACTORS AFFECTING RADIATION EXPOSURE DURING NEPHROLITHOTOMY
the remaining stone burden. However, fluoroscopy is associated with exposure to ionizing radiation. Recent studies show that patients, including those with urolithiasis, may be exposed to increased ionizing radiation and, thus, potentially be at risk for solid or hematological malignancy. As a result, it is widely accepted that every reasonable effort should be made to limit patient radiation exposure. Applying this principle does not apply only to diagnostic and followup imaging but also to imaging used to treat stone disease. Factors that affect radiation exposure during PNL have not been fully elucidated. We investigated patient, stone and procedural characteristics that may contribute to increased radiation exposure and offer ways in which the radiation dose can be limited.
MATERIALS AND METHODS After receiving approval from the Duke University Medical Center institutional review board we retrospectively reviewed the electronic medical records of 272 patients who underwent PNL in the last 2 years. We identified 96 patients with sufficient data available to calculate the effective radiation dose. All percutaneous procedures were done by a single surgeon (GMP) and percutaneous access was achieved in the operating room by a single interventional radiologist (MJM). Pulsed fluoroscopy was performed by a radiology technician using a digital portable OEC 9800® Elite C-arm with automatic exposure control. The device captured radiation exposure data as a dose report, including a calculated dose area product in rads/cm2. We then calculated ERD using accepted conversion tables.1 Stone burden, defined as the greatest cross-sectional area of the stone viewed anteroposterior in cm2, was derived from preoperative excretory urogram or noncontrast CT. Preoperative radiographic imaging was also used to determine stone site and configuration, including whether a stone was branched and whether a renal infundibulum was crossed. The number of percutaneous access tracts required during the procedure was recorded. When the patient presented to the operating room with a percutaneous nephrostomy tube already in place, a value of 0 was recorded in the data field. We used multivariate linear regression models to determine independent associations between ERD and certain candidate predictors chosen a priori, including age, gender, race, BMI, stone burden, composition, site and configuration, number of access tracts, drainage type and hydronephrosis. Continuous variables showing nonnormal distribution, such as ERD, BMI and stone burden, were entered in the regression models after log transformation. After determining relationships between log transformed ERD and candidate predictors we back transformed values to properly interpret results. We calculated the mean adjusted ERD and 95% CI for certain stratifications, including BMI (less than 25, 25 to 29.9, 30 to 39.9, or 40 kg/m2 or greater), stone configuration (branched vs nonbranched), stone burden (5 or less, 5.01 to 10 or greater than 10 cm2) and access number (0, 1, or 2 or greater). Significance for trend was evaluated using the Wald test. All statistical analysis was done with STATA®, version 10.0 with p ⬍0.05 considered statistically significant.
RESULTS Table 1 lists study sample baseline characteristics in the 36 men (37.5%) and 60 women (62.5%) with a mean age of 51.5 ⫾ 13.4 years (range 17 to 86). Mean BMI ⫾ SD was 32.04 ⫾ 9.68 kg/m2 (95% CI 16.2– 59.6). In 20 (20.8%), 26 (27.1%) and 50 patients (52.1%) BMI was less than 25, 25 to 29.9 and 30 kg/m2 or greater, respectively. The study included 17 patients (17.7%) with morbid obesity, defined as BMI 40 kg/m2 or greater. The mean ⫾ stone burden was 5.50 ⫾ 4.50 cm2 (median 4, range 0.09 to 23.1). Stones were branched and crossed a caliceal infundibulum in 37 (38.5%) and 44 patients (45.8%), respectively. Patients had a mean of 1.18 ⫾ 0.70 percutaneous access tracks (range 0 to 3) and received a mean ERD of 8.66 ⫾ 9.22 SV (range 0.46 to 51.60). Median estimated blood loss was 50 cc (range 15 to 1,000). Several patient, stone and procedural factors were significantly associated with increased ERD. When treated as a continuous log transformed variable, BMI was significantly and positively associated with ERD (p ⬍0.001). When stratified by BMI category, obese patients with a BMI of 30 to 39.9 kg/m2 had a greater than 2-fold increase and morbidly obese patients with a BMI of 40 kg/m2 or greater had a greater than 3-fold increase in mean adjusted ERD vs normal weight patients with a BMI of less than 25 kg/m2 (6.49 and 9.13 mSv, respectively, vs 2.66, p ⬍0.001, table 2 and see figure). Table 1. Baseline patient and procedural characteristics No. Pts (%) Race: White Black Other Stone composition: Calcium oxalate Calcium phosphate Magnesium ammonium phosphate Uric acid Ammonium urate Cystine Not determined Stone site: Renal pelvis ⫹ multiple poles Renal pelvis Renal pelvis ⫹ lower pole Renal pelvis ⫹ upper/mid pole Isolated pole Ureter Hydronephrosis Drainage: Double-J® ureteral stent 6Fr ureteral catheter PNL PNL ⫹ 6Fr ureteral catheter PNL ⫹ Double-J ureteral stent
79 (82.3) 14 (14.6) 3 (3.1) 33 (34.4) 29 (30.2) 17 (17.7) 7 (7.3) 4 (4.2) 3 (3.1) 2 (2.1) 36 (37.5) 22 (22.9) 18 (18.8) 6 (6.3) 8 (8.3) 6 (6.3) 20 (20.8) 36 (37.5) 25 (26.0) 16 (16.7) 14 (14.6) 5 (5.2)
FACTORS AFFECTING RADIATION EXPOSURE DURING NEPHROLITHOTOMY
Factor
Mean Adjusted ERD (95% CI) (mSv)
2
BMI (kg/m ): Less than 25 25–29.9 30–39.9 40 or Greater Stone configuration: Branched Nonbranched Stone burden (cm2): 5 or Less 5.01–10 Greater than 10 No. access tracts: 0 1 2 or Greater
p Trend 0.000
2.66 (1.81–3.90) 5.44 (3.89–7.63) 6.49 (4.85–8.68) 9.13 (6.02–13.85) 0.002 2.80 (1.80–4.36) 8.30 (6.12–11.25) 0.040 4.42 (3.40–5.75) 6.72 (4.85–9.32) 9.39 (4.96–17.79) 0.024 3.36 (2.01–5.62) 5.28 (4.28–6.51) 7.52 (5.03–11.26)
Certain stone specific parameters, including stone nonbranched configuration (p ⫽ 0.002) and increased stone burden (p ⫽ 0.013), were significantly related to increased ERD. However, stone site and composition were not associated with increased ERD. From a procedural perspective a greater number of percutaneous access tracts was significantly associated with increased ERD (p ⫽ 0.040). Percutaneous access site and estimated blood loss were not associated with ERD.
DISCUSSION The risks of ionizing radiation are considered minimal but may be potentially devastating. These risks include skin injury, cataracts and solid or hematological malignancy. The risk of malignancy has garnered increased attention. Recently a group study reported that an estimated 29,000 future cancers could be attributable to ionizing radiation produced by CT in 2007 in the United States.2 Although these risk estimates are based on a conservative model and are not universally accepted, a strong, meaningful association certainly exists between ionizing radiation and the risk of malignancy. The risk of malignancy due to low doses of ionizing radiation is small compared to the individual baseline risk of cancer but with the 3-fold increase in CT since 1993 (up to 70 million scans yearly3) many cancers are possibly being created. CT is the most significant contributor to patient radiation exposure since standard CT of the abdomen and pelvis produces an ERD of approximately 8 and 6 mSv, respectively.3 Other imaging modalities, including fluoroscopy used during PNL with a mean ERD of approximately 9 mSv, also produces significant radiation.4 Recent reports suggested that patients with nephrolithiasis are at risk for increased radiation
exposure since radiographic imaging is indispensable to diagnosis, treatment and followup in these individuals. While quantifying exposure during a single stone episode, a group found that patients received a median effective dose of 5.3 mSv (range 1.18 to 37.66) during diagnosis and treatment with minimal CT use.5 Another investigation showed that radiation exposure associated with an acute stone episode and 1-year followup at 2 large medical centers was 29.7 mSv.6 Of 108 patients 22 (20%) received more than 50 mSv, which the International Commission on Radiation Protection considers a significant radiation dose.6 Currently no guidelines indicate what level of radiation exposure is acceptable in light of the valuable information gained from imaging techniques. The International Commission on Radiation Protection recommends that exposure be less than 50 mSv in 1 year or 100 mSv in a 5-year period.7 However, these recommendations are for occupational exposure only and do not provide more than a framework for clinicians. Radiation risks incurred during medical imaging are offset by the benefits achieved by rapid, accurate diagnosis when used judiciously. Also, estimated effective dose calculations, such as that in our study, are only approximations of the actual effective dose, which can only be determined by obtaining multiple specific organ doses. Factors that affect radiation exposure were previously identified for certain minimally invasive stone procedures but never fully characterized for PNL. In a retrospective review of almost 300 patients a group found that stone burden, patient weight and ureteral vs renal stone site were associated with increased radiation exposure during shock wave lithotripsy.8 Another group evaluated radiation exposure during ureteroscopy and found that radiation exposure was higher when flexible endoscopes were used compared to semirigid devices and highest when the 3 devices were used.9 The investi16 14 Mean-adjusted ERD
Table 2. Factors independently associated with ERD in patients with PNL
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12 10 8 6 4 2 0 <25
25 - 29.9
30 - 39.9
≥40
BMI (kg/m2)
Mean adjusted effective radiation dose by BMI category
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FACTORS AFFECTING RADIATION EXPOSURE DURING NEPHROLITHOTOMY
gators proposed that this effect is likely related to increased case complexity and noted that radiation exposure is higher during therapeutic than diagnostic procedures. During PNL the radiation dose to personnel is 0.002 to 0.1 mSv per case but patient exposure data are lacking.10,11 Recently FST was evaluated during PNL.12 An increased stone burden and multiple percutaneous access tracts were associated with increased FST, consistent with our findings. Greater stone burden may make percutaneous access more difficult and time-consuming. Also, it is not surprising that a greater number of access tracts is associated with increased ERD since percutaneous access is the part of the case during which fluoroscopic imaging is most used. Our current calculations revealed that nonbranched stone configuration was associated with increased ERD. This may seem counterintuitive since one may think that the more complex the stone, the more difficult the access. Our findings suggest this may not always be the case. The best explanation of this disparity is that nonbranched stones may be more likely to obstruct the ureteropelvic junction, making antegrade passage of a guidewire down the ureter more difficult and, thus, requiring greater ERD. Also, a branched stone that extends into a calix may be easier to target during initial percutaneous access, requiring less fluoroscopic imaging time. An interesting finding in the current study is the association between BMI and increasing ERD. In contrast to the mentioned study of FST, in which mean BMI was 25.21 kg/m2, our study population had a mean BMI of 32.0 kg/m2, and in 52.1% and 17.7% of our patients BMI was 30 or greater and 40 kg/m2 or greater, respectively. These findings represent a distribution among BMI ranges that is certainly more typical of a Western stone former population. Moreover, these data are particularly important since obese patients with stones are at the intersection of several important factors known to increase cancer risk. Obesity is a documented risk factor for malignancy with a clear association established for cancer of the endometrium, kidney, gallbladder, breast and colon.13 A group estimated that in the United States 14% of all cancer deaths in men and 20% in women may be attributable to overweight or obese status.14 Also, increased ERD is often required to achieve radiographic imaging results similar to those in the nonobese population. We recently found that an increased radiation dose is required during CT to identify small stones in obese patients.15 The current findings agree with this point, documenting increased radiation exposure in obese patients during PNL. The association between obesity and recurrent stone disease is well established,16 and likely results in more frequent radiographic imaging in obese patients. Thus, since they
are at higher baseline risk for malignancy and have a higher rate of stone formation, which subsequently requires more frequent radiographic imaging with higher doses of ionizing radiation required to achieve adequate image quality, obese patients are particularly at greater risk. We also evaluated FST in our cohort. As expected, it was independently and directly associated with ERD (p ⬍0.001). However, the role of FST as a surrogate for ERD is limited and may not provide accurate assessment of radiation risk. This was especially true in our obese patients since FST did not significantly differ among BMI groups (p ⫽ 0.545, table 3). This indicates that time alone does not account for the increased ERD in the obese population. Rather, it is due to increased output from the fluoroscopy device via the automatic exposure control used to obtain images of adequate quality. Our current goal was to identify patient, stone and procedural characteristics that increase patient radiation exposure during percutaneous stone removal to enhance our understanding and ultimately enable us to develop new ways to decrease radiation exposure. Success in decreasing the radiation dose has been achieved by implementation of a radiation awareness program, in which focus was placed on obtaining fewer images (snapshots) during shock wave lithotripsy.17 Also, after starting a radiation safety education initiative a group noted significant decreases in dose area product and fluoroscopy time for several of their most common pediatric procedures.18 With our current understanding there are many ways to globally decrease radiation exposure in patients, beginning by eliminating unnecessary studies and optimizing modalities that do not use ionizing radiation, such as ultrasound and magnetic resonance imaging, especially in populations at risk. In patients with nephrolithiasis low dose CT with an associated ERD of approximately 1 to 5 mSv appears promising for diagnosis and followup.19 Specifically for PNL a number of improvements have been made by manufacturers of fluoroscopic imaging equipment in recent years with the goal of producing the highest quality image with the lowest possible radiation dose. The most modern devices use low attenuation materials such as carbon fiber Table 3. Effective dose, fluoroscopic screening time and effective dose rate by BMI group BMI (kg/m2)
Median ERD (mSv)
Median Fluoroscopy (mins)
Median ERD Rate (mSv/min)
Less than 25 25–29.9 30–39.9 40 or Greater p Value
2.63 4.89 6.65 8.84 0.001
8.28 7.42 8.67 6.69 0.545
0.38 0.71 0.71 1.43 ⬍0.001
FACTORS AFFECTING RADIATION EXPOSURE DURING NEPHROLITHOTOMY
in table tops and cassettes, and have improved energy spectra produced by high frequency generators and additional filters to decrease the entrance skin dose. They also have features such as last image hold, pulsed fluoroscopy and digital imaging that can contribute to decreasing patient exposure. With digital imaging pulsed fluoroscopy is possible with the image refreshed in continuous mode at 30 frames per second. This rate can be further adjusted down until in 1 study equivalent image quality was noted with a marked dose reduction at a pulse rate of 15 frames per second.20 These features are inherent to the device and many are automatically selected. Also, several factors are under operator control and can be further maximized to limit radiation exposure, including acquiring as few images as possible, precisely collimating the beam to the region of interest and limiting magnification use. Ideally the fluoroscopy device is operated by the surgeon using a foot pedal. If this is not possible, a well trained, experienced fluoroscopy technician should operate the unit, and good communication between surgeon and technician is of great importance. Finally, using ultrasound during PNL was reported but it is typically performed in conjunction
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with fluoroscopy.21 Thus, it is unclear to what extent using ultrasound decreases overall exposure. These and other methods of decreasing radiation exposure should be further investigated and potentially used to protect our patients.
CONCLUSIONS Patients with higher BMI, greater stone burden, nonbranched stones and multiple nephrostomy access tracts are at risk for increased radiation exposure during PNL. Particularly obese patients represent a unique challenge since they are at higher baseline risk for malignancy, have a higher stone formation rate and subsequently undergo more frequent radiographic imaging with higher doses of ionizing radiation required to achieve adequate image quality. Since radiographic imaging is inseparable from long-term stone management, urologists must seek alternative imaging strategies to minimize radiation exposure even during stone removal procedures. It is the responsibility of urologists to understand radiation safety to protect patients, themselves and the operating team from potential radiation injury.
REFERENCES 1. Le Heron JC: Estimation of effective dose to the patient during medical x-ray examinations from measurements of the dose-area product. Phys Med Biol 1992; 37: 2117.
8. Carter HB, Naslund EB and Riehle RA Jr: Variables influencing radiation exposure during extracorporeal shock wave lithotripsy. Review of 298 treatments. Urology 1987; 30: 546.
2. Berrington de Gonzalez A, Mahesh M, Kim KP et al: Projected cancer risks from computed tomographic scans performed in the United States in 2007. Arch Intern Med 2009; 169: 2071.
9. Bagley DH and Cubler-Goodman A: Radiation exposure during ureteroscopy. J Urol 1990; 144: 1356.
3. Mettler FA Jr, Huda W, Yoshizumi TT et al: Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology 2008; 248: 254. 4. Safak M, Olgar T, Bor D et al: Radiation doses of patients and urologists during percutaneous nephrolithotomy. J Radiol Prot 2009; 29: 409. 5. John BS, Patel U and Anson K: What radiation exposure can a patient expect during a single stone episode? J Endourol 2008; 22: 419. 6. Ferrandino MN, Bagrodia A, Pierre SA et al: Radiation exposure in the acute and short-term management of urolithiasis at 2 academic centers. J Urol 2009; 181: 668. 7. Wrixon AD: New ICRP recommendations. J Radiol Prot 2008; 28: 161.
10. Bush WH, Jones D and Brannen GE: Radiation dose to personnel during percutaneous renal calculus removal. AJR Am J Roentgenol 1985; 145: 1261. 11. Kumari G, Kumar P, Wadhwa P et al: Radiation exposure to the patient and operating room personnel during percutaneous nephrolithotomy. Int Urol Nephrol 2006; 38: 207. 12. Tepeler A, Binbay M, Yuruk E et al: Factors affecting the fluoroscopic screening time during percutaneous nephrolithotomy. J Endourol 2009; 23: 1825.
15. Mancini JG and Ferrandino MN: The impact of new methods of imaging on radiation dosage delivered to patients. Curr Opin Urol 2009. 16. Taylor EN, Stampfer MJ and Curhan GC: Obesity, weight gain, and the risk of kidney stones. JAMA 2005; 293: 455. 17. Griffith DP, Gleeson MJ, Politis G et al: Effectiveness of radiation control program for Dornier HM3 lithotriptor. Urology 1989; 33: 20. 18. Sheyn DD, Racadio JM, Ying J et al: Efficacy of a radiation safety education initiative in reducing radiation exposure in the pediatric IR suite. Pediatr Radiol 2008; 38: 669. 19. Kim BS, Hwang IK, Choi YW et al: Low-dose and standard-dose unenhanced helical computed tomography for the assessment of acute renal colic: prospective comparative study. Acta Radiol 2005; 46: 756.
13. Carroll KK: Obesity as a risk factor for certain types of cancer. Lipids 1998; 33: 1055.
20. Persliden J, Helmrot E, Hjort P et al: Dose and image quality in the comparison of analogue and digital techniques in paediatric urology examinations. Eur Radiol 2004; 14: 638.
14. Calle EE, Rodriguez C, Walker-Thurmond K et al: Overweight, obesity, and mortality from cancer in a prospectively studied cohort of U.S. adults. N Engl J Med 2003; 348: 1625.
21. Osman M, Wendt-Nordahl G, Heger K et al: Percutaneous nephrolithotomy with ultrasonography-guided renal access: experience from over 300 cases. BJU Int 2005; 96: 875.
Purpose: We identified patient and stone characteristics that may contribute to increased radiation exposure during percutaneous nephrolithotomy and offer technique modifications to limit the radiation dose. Material and Methods: We reviewed the records of 96 patients who underwent percutaneous nephrolithotomy in the last 2 years. The effective radiation dose was calculated using accepted conversion tables. We performed multivariate linear regression to determine the association of the effective radiation dose with specific patient, stone and procedural characteristics. Results: Mean ⫾ SD patient age was 51.5 ⫾ 13.4 years and 62.5% of the patients were female. Median body mass index was 32.0 ⫾ 9.7 kg/m2 (range 16.2 to 59.6) and the median stone burden was 4 cm2. Increased body mass index (p ⬍0.001), higher stone burden (p ⫽ 0.013), stone nonbranched configuration (p ⫽ 0.002) and a greater number of percutaneous access tracts (p ⫽ 0.040) were significantly associated with an increased effective radiation dose. Specifically obese patients with a body mass index of 30 to 39.9 kg/m2 had a more than 2-fold increase in the mean adjusted effective radiation dose and morbidly obese patients with a body mass index of 40 kg/m2 or greater had a greater than 3-fold increase vs that in normal weight patients with a body mass index of less than 25 kg/m2 (6.49 and 9.13 mSv, respectively, vs 2.66, p ⬍0.001). Other stone specific parameters, including site and composition, percutaneous access site and estimated blood loss were not associated with the effective radiation dose. Conclusions: Patients with higher body mass index, greater stone burden, nonbranched stones and multiple nephrostomy access tracts are at risk for increased radiation exposure during percutaneous nephrolithotomy. Urologists must seek alternative strategies to minimize radiation exposure, such as tighter collimation to the region of interest, judicious use of magnification and the acquisition of as few images as possible during stone removal.
Abbreviations and Acronyms BMI ⫽ body mass index CT ⫽ computerized tomography ERD ⫽ effective radiation dose FST ⫽ fluoroscopic screening time PNL ⫽ percutaneous nephrolithotomy Submitted for publication May 3, 2010. Study received Duke University Medical Center institutional review board approval. * Correspondence: Division of Urology, Duke University Medical Center, DUMC 3167, Trent Dr., Durham, North Carolina 27710 (telephone: 919-681-5506; FAX: 919-681-5507; e-mail: [email protected]).
Key Words: kidney; kidney calculi; nephrostomy, percutaneous; radiation dosage; body mass index PERCUTANEOUS nephrolithotomy, which is most commonly done for large or complex renal calculi, offers the benefit of an improved stone-free rate over that of other minimally invasive procedures. In the United States fluoroscopy is typ-
ically used during the procedure to provide real-time visualization to establish and dilate a percutaneous tract to the renal collecting system, provide information on renal anatomy, properly place surgical devices and determine
0022-5347/10/1846-2373/0 THE JOURNAL OF UROLOGY® © 2010 by AMERICAN UROLOGICAL ASSOCIATION EDUCATION
Vol. 184, 2373-2377, December 2010 Printed in U.S.A. DOI:10.1016/j.juro.2010.08.033
AND
RESEARCH, INC.
www.jurology.com
2373
2374
FACTORS AFFECTING RADIATION EXPOSURE DURING NEPHROLITHOTOMY
the remaining stone burden. However, fluoroscopy is associated with exposure to ionizing radiation. Recent studies show that patients, including those with urolithiasis, may be exposed to increased ionizing radiation and, thus, potentially be at risk for solid or hematological malignancy. As a result, it is widely accepted that every reasonable effort should be made to limit patient radiation exposure. Applying this principle does not apply only to diagnostic and followup imaging but also to imaging used to treat stone disease. Factors that affect radiation exposure during PNL have not been fully elucidated. We investigated patient, stone and procedural characteristics that may contribute to increased radiation exposure and offer ways in which the radiation dose can be limited.
MATERIALS AND METHODS After receiving approval from the Duke University Medical Center institutional review board we retrospectively reviewed the electronic medical records of 272 patients who underwent PNL in the last 2 years. We identified 96 patients with sufficient data available to calculate the effective radiation dose. All percutaneous procedures were done by a single surgeon (GMP) and percutaneous access was achieved in the operating room by a single interventional radiologist (MJM). Pulsed fluoroscopy was performed by a radiology technician using a digital portable OEC 9800® Elite C-arm with automatic exposure control. The device captured radiation exposure data as a dose report, including a calculated dose area product in rads/cm2. We then calculated ERD using accepted conversion tables.1 Stone burden, defined as the greatest cross-sectional area of the stone viewed anteroposterior in cm2, was derived from preoperative excretory urogram or noncontrast CT. Preoperative radiographic imaging was also used to determine stone site and configuration, including whether a stone was branched and whether a renal infundibulum was crossed. The number of percutaneous access tracts required during the procedure was recorded. When the patient presented to the operating room with a percutaneous nephrostomy tube already in place, a value of 0 was recorded in the data field. We used multivariate linear regression models to determine independent associations between ERD and certain candidate predictors chosen a priori, including age, gender, race, BMI, stone burden, composition, site and configuration, number of access tracts, drainage type and hydronephrosis. Continuous variables showing nonnormal distribution, such as ERD, BMI and stone burden, were entered in the regression models after log transformation. After determining relationships between log transformed ERD and candidate predictors we back transformed values to properly interpret results. We calculated the mean adjusted ERD and 95% CI for certain stratifications, including BMI (less than 25, 25 to 29.9, 30 to 39.9, or 40 kg/m2 or greater), stone configuration (branched vs nonbranched), stone burden (5 or less, 5.01 to 10 or greater than 10 cm2) and access number (0, 1, or 2 or greater). Significance for trend was evaluated using the Wald test. All statistical analysis was done with STATA®, version 10.0 with p ⬍0.05 considered statistically significant.
RESULTS Table 1 lists study sample baseline characteristics in the 36 men (37.5%) and 60 women (62.5%) with a mean age of 51.5 ⫾ 13.4 years (range 17 to 86). Mean BMI ⫾ SD was 32.04 ⫾ 9.68 kg/m2 (95% CI 16.2– 59.6). In 20 (20.8%), 26 (27.1%) and 50 patients (52.1%) BMI was less than 25, 25 to 29.9 and 30 kg/m2 or greater, respectively. The study included 17 patients (17.7%) with morbid obesity, defined as BMI 40 kg/m2 or greater. The mean ⫾ stone burden was 5.50 ⫾ 4.50 cm2 (median 4, range 0.09 to 23.1). Stones were branched and crossed a caliceal infundibulum in 37 (38.5%) and 44 patients (45.8%), respectively. Patients had a mean of 1.18 ⫾ 0.70 percutaneous access tracks (range 0 to 3) and received a mean ERD of 8.66 ⫾ 9.22 SV (range 0.46 to 51.60). Median estimated blood loss was 50 cc (range 15 to 1,000). Several patient, stone and procedural factors were significantly associated with increased ERD. When treated as a continuous log transformed variable, BMI was significantly and positively associated with ERD (p ⬍0.001). When stratified by BMI category, obese patients with a BMI of 30 to 39.9 kg/m2 had a greater than 2-fold increase and morbidly obese patients with a BMI of 40 kg/m2 or greater had a greater than 3-fold increase in mean adjusted ERD vs normal weight patients with a BMI of less than 25 kg/m2 (6.49 and 9.13 mSv, respectively, vs 2.66, p ⬍0.001, table 2 and see figure). Table 1. Baseline patient and procedural characteristics No. Pts (%) Race: White Black Other Stone composition: Calcium oxalate Calcium phosphate Magnesium ammonium phosphate Uric acid Ammonium urate Cystine Not determined Stone site: Renal pelvis ⫹ multiple poles Renal pelvis Renal pelvis ⫹ lower pole Renal pelvis ⫹ upper/mid pole Isolated pole Ureter Hydronephrosis Drainage: Double-J® ureteral stent 6Fr ureteral catheter PNL PNL ⫹ 6Fr ureteral catheter PNL ⫹ Double-J ureteral stent
79 (82.3) 14 (14.6) 3 (3.1) 33 (34.4) 29 (30.2) 17 (17.7) 7 (7.3) 4 (4.2) 3 (3.1) 2 (2.1) 36 (37.5) 22 (22.9) 18 (18.8) 6 (6.3) 8 (8.3) 6 (6.3) 20 (20.8) 36 (37.5) 25 (26.0) 16 (16.7) 14 (14.6) 5 (5.2)
FACTORS AFFECTING RADIATION EXPOSURE DURING NEPHROLITHOTOMY
Factor
Mean Adjusted ERD (95% CI) (mSv)
2
BMI (kg/m ): Less than 25 25–29.9 30–39.9 40 or Greater Stone configuration: Branched Nonbranched Stone burden (cm2): 5 or Less 5.01–10 Greater than 10 No. access tracts: 0 1 2 or Greater
p Trend 0.000
2.66 (1.81–3.90) 5.44 (3.89–7.63) 6.49 (4.85–8.68) 9.13 (6.02–13.85) 0.002 2.80 (1.80–4.36) 8.30 (6.12–11.25) 0.040 4.42 (3.40–5.75) 6.72 (4.85–9.32) 9.39 (4.96–17.79) 0.024 3.36 (2.01–5.62) 5.28 (4.28–6.51) 7.52 (5.03–11.26)
Certain stone specific parameters, including stone nonbranched configuration (p ⫽ 0.002) and increased stone burden (p ⫽ 0.013), were significantly related to increased ERD. However, stone site and composition were not associated with increased ERD. From a procedural perspective a greater number of percutaneous access tracts was significantly associated with increased ERD (p ⫽ 0.040). Percutaneous access site and estimated blood loss were not associated with ERD.
DISCUSSION The risks of ionizing radiation are considered minimal but may be potentially devastating. These risks include skin injury, cataracts and solid or hematological malignancy. The risk of malignancy has garnered increased attention. Recently a group study reported that an estimated 29,000 future cancers could be attributable to ionizing radiation produced by CT in 2007 in the United States.2 Although these risk estimates are based on a conservative model and are not universally accepted, a strong, meaningful association certainly exists between ionizing radiation and the risk of malignancy. The risk of malignancy due to low doses of ionizing radiation is small compared to the individual baseline risk of cancer but with the 3-fold increase in CT since 1993 (up to 70 million scans yearly3) many cancers are possibly being created. CT is the most significant contributor to patient radiation exposure since standard CT of the abdomen and pelvis produces an ERD of approximately 8 and 6 mSv, respectively.3 Other imaging modalities, including fluoroscopy used during PNL with a mean ERD of approximately 9 mSv, also produces significant radiation.4 Recent reports suggested that patients with nephrolithiasis are at risk for increased radiation
exposure since radiographic imaging is indispensable to diagnosis, treatment and followup in these individuals. While quantifying exposure during a single stone episode, a group found that patients received a median effective dose of 5.3 mSv (range 1.18 to 37.66) during diagnosis and treatment with minimal CT use.5 Another investigation showed that radiation exposure associated with an acute stone episode and 1-year followup at 2 large medical centers was 29.7 mSv.6 Of 108 patients 22 (20%) received more than 50 mSv, which the International Commission on Radiation Protection considers a significant radiation dose.6 Currently no guidelines indicate what level of radiation exposure is acceptable in light of the valuable information gained from imaging techniques. The International Commission on Radiation Protection recommends that exposure be less than 50 mSv in 1 year or 100 mSv in a 5-year period.7 However, these recommendations are for occupational exposure only and do not provide more than a framework for clinicians. Radiation risks incurred during medical imaging are offset by the benefits achieved by rapid, accurate diagnosis when used judiciously. Also, estimated effective dose calculations, such as that in our study, are only approximations of the actual effective dose, which can only be determined by obtaining multiple specific organ doses. Factors that affect radiation exposure were previously identified for certain minimally invasive stone procedures but never fully characterized for PNL. In a retrospective review of almost 300 patients a group found that stone burden, patient weight and ureteral vs renal stone site were associated with increased radiation exposure during shock wave lithotripsy.8 Another group evaluated radiation exposure during ureteroscopy and found that radiation exposure was higher when flexible endoscopes were used compared to semirigid devices and highest when the 3 devices were used.9 The investi16 14 Mean-adjusted ERD
Table 2. Factors independently associated with ERD in patients with PNL
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12 10 8 6 4 2 0 <25
25 - 29.9
30 - 39.9
≥40
BMI (kg/m2)
Mean adjusted effective radiation dose by BMI category
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FACTORS AFFECTING RADIATION EXPOSURE DURING NEPHROLITHOTOMY
gators proposed that this effect is likely related to increased case complexity and noted that radiation exposure is higher during therapeutic than diagnostic procedures. During PNL the radiation dose to personnel is 0.002 to 0.1 mSv per case but patient exposure data are lacking.10,11 Recently FST was evaluated during PNL.12 An increased stone burden and multiple percutaneous access tracts were associated with increased FST, consistent with our findings. Greater stone burden may make percutaneous access more difficult and time-consuming. Also, it is not surprising that a greater number of access tracts is associated with increased ERD since percutaneous access is the part of the case during which fluoroscopic imaging is most used. Our current calculations revealed that nonbranched stone configuration was associated with increased ERD. This may seem counterintuitive since one may think that the more complex the stone, the more difficult the access. Our findings suggest this may not always be the case. The best explanation of this disparity is that nonbranched stones may be more likely to obstruct the ureteropelvic junction, making antegrade passage of a guidewire down the ureter more difficult and, thus, requiring greater ERD. Also, a branched stone that extends into a calix may be easier to target during initial percutaneous access, requiring less fluoroscopic imaging time. An interesting finding in the current study is the association between BMI and increasing ERD. In contrast to the mentioned study of FST, in which mean BMI was 25.21 kg/m2, our study population had a mean BMI of 32.0 kg/m2, and in 52.1% and 17.7% of our patients BMI was 30 or greater and 40 kg/m2 or greater, respectively. These findings represent a distribution among BMI ranges that is certainly more typical of a Western stone former population. Moreover, these data are particularly important since obese patients with stones are at the intersection of several important factors known to increase cancer risk. Obesity is a documented risk factor for malignancy with a clear association established for cancer of the endometrium, kidney, gallbladder, breast and colon.13 A group estimated that in the United States 14% of all cancer deaths in men and 20% in women may be attributable to overweight or obese status.14 Also, increased ERD is often required to achieve radiographic imaging results similar to those in the nonobese population. We recently found that an increased radiation dose is required during CT to identify small stones in obese patients.15 The current findings agree with this point, documenting increased radiation exposure in obese patients during PNL. The association between obesity and recurrent stone disease is well established,16 and likely results in more frequent radiographic imaging in obese patients. Thus, since they
are at higher baseline risk for malignancy and have a higher rate of stone formation, which subsequently requires more frequent radiographic imaging with higher doses of ionizing radiation required to achieve adequate image quality, obese patients are particularly at greater risk. We also evaluated FST in our cohort. As expected, it was independently and directly associated with ERD (p ⬍0.001). However, the role of FST as a surrogate for ERD is limited and may not provide accurate assessment of radiation risk. This was especially true in our obese patients since FST did not significantly differ among BMI groups (p ⫽ 0.545, table 3). This indicates that time alone does not account for the increased ERD in the obese population. Rather, it is due to increased output from the fluoroscopy device via the automatic exposure control used to obtain images of adequate quality. Our current goal was to identify patient, stone and procedural characteristics that increase patient radiation exposure during percutaneous stone removal to enhance our understanding and ultimately enable us to develop new ways to decrease radiation exposure. Success in decreasing the radiation dose has been achieved by implementation of a radiation awareness program, in which focus was placed on obtaining fewer images (snapshots) during shock wave lithotripsy.17 Also, after starting a radiation safety education initiative a group noted significant decreases in dose area product and fluoroscopy time for several of their most common pediatric procedures.18 With our current understanding there are many ways to globally decrease radiation exposure in patients, beginning by eliminating unnecessary studies and optimizing modalities that do not use ionizing radiation, such as ultrasound and magnetic resonance imaging, especially in populations at risk. In patients with nephrolithiasis low dose CT with an associated ERD of approximately 1 to 5 mSv appears promising for diagnosis and followup.19 Specifically for PNL a number of improvements have been made by manufacturers of fluoroscopic imaging equipment in recent years with the goal of producing the highest quality image with the lowest possible radiation dose. The most modern devices use low attenuation materials such as carbon fiber Table 3. Effective dose, fluoroscopic screening time and effective dose rate by BMI group BMI (kg/m2)
Median ERD (mSv)
Median Fluoroscopy (mins)
Median ERD Rate (mSv/min)
Less than 25 25–29.9 30–39.9 40 or Greater p Value
2.63 4.89 6.65 8.84 0.001
8.28 7.42 8.67 6.69 0.545
0.38 0.71 0.71 1.43 ⬍0.001
FACTORS AFFECTING RADIATION EXPOSURE DURING NEPHROLITHOTOMY
in table tops and cassettes, and have improved energy spectra produced by high frequency generators and additional filters to decrease the entrance skin dose. They also have features such as last image hold, pulsed fluoroscopy and digital imaging that can contribute to decreasing patient exposure. With digital imaging pulsed fluoroscopy is possible with the image refreshed in continuous mode at 30 frames per second. This rate can be further adjusted down until in 1 study equivalent image quality was noted with a marked dose reduction at a pulse rate of 15 frames per second.20 These features are inherent to the device and many are automatically selected. Also, several factors are under operator control and can be further maximized to limit radiation exposure, including acquiring as few images as possible, precisely collimating the beam to the region of interest and limiting magnification use. Ideally the fluoroscopy device is operated by the surgeon using a foot pedal. If this is not possible, a well trained, experienced fluoroscopy technician should operate the unit, and good communication between surgeon and technician is of great importance. Finally, using ultrasound during PNL was reported but it is typically performed in conjunction
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with fluoroscopy.21 Thus, it is unclear to what extent using ultrasound decreases overall exposure. These and other methods of decreasing radiation exposure should be further investigated and potentially used to protect our patients.
CONCLUSIONS Patients with higher BMI, greater stone burden, nonbranched stones and multiple nephrostomy access tracts are at risk for increased radiation exposure during PNL. Particularly obese patients represent a unique challenge since they are at higher baseline risk for malignancy, have a higher stone formation rate and subsequently undergo more frequent radiographic imaging with higher doses of ionizing radiation required to achieve adequate image quality. Since radiographic imaging is inseparable from long-term stone management, urologists must seek alternative imaging strategies to minimize radiation exposure even during stone removal procedures. It is the responsibility of urologists to understand radiation safety to protect patients, themselves and the operating team from potential radiation injury.
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