Perioperative Patient Radiation Exposure in the Endoscopic Removal of Upper Urinary Tract Calculi

Abstract

Background and Purpose:

The efficacy of computed tomography (CT) in detailing upper urinary tract calculi is well established. There is no established acceptable annual recommended limit for medical exposure, yet the global accepted upper limit for occupational radiation exposure is <50 millisieverts (mSv) in any one year. We sought to appreciate the CT and fluoroscopic radiation exposure to our patients undergoing endoscopic removal of upper tract calculi during the periprocedure period.

Patients and Methods:

All patients undergoing upper urinary endoscopic stone removal between 2005 and 2009 were identified. To calculate the cumulative radiation exposure, we included all ionizing radiation imaging performed during a periprocedure period, which we defined as ≤90 days pre- and post-therapeutic procedure.

Results:

A total of 233 upper urinary tract therapeutic patient stone procedures were identified; 127 patients underwent ureteroscopy (URS) and 106 patients underwent percutaneous nephrolithotomy (PCNL). A mean 1.58 CTs were performed per patient. Ninety (38.6%) patients underwent ≥2 CTs in the periprocedure period, with an average number in this group of 2.49 CT/patient, resulting in approximately 49.8 mSv of CT radiation exposure. Patients who were undergoing URS were significantly more likely to have multiple CTs (P=0.003) than those undergoing PCNL. Median fluoroscopic procedure exposures were 43.3 mGy for patients who were undergoing PCNL and 27.6 mGy for those patients undergoing URS.

Conclusions:

CT radiation exposure in the periprocedure period for patients who were undergoing endoscopic upper tract stone removal is considerable. Added to this is the procedure-related fluoroscopic radiation exposure. Urologic surgeons should be aware of the cumulative amount of ionizing radiation received by their patients from multiple sources.

Introduction

T he sensitivity of CT scans in detailing upper urinary tract calculi has been well established.¹ In addition to the initial detection of calculi, CT is frequently used to monitor spontaneous stone passage, stone migration, and post-treatment outcomes. Such postdiagnosis use results in additional radiation exposure.²

There are currently no established guidelines for annual patient radiation exposure.³ There is, however, an accepted recommended radiation dose limit per adult for occupational exposure, which is no more than 20 millisieverts (mSv) per year averaged over a 5-year period with no more than 50 mSv in any one year, as established by the International Commission on Radiological Protection (ICRP).⁴ There is a wide range of variability in the effective dose radiation exposure between CT scanners and settings, even among contemporary machines.^5,6 The estimated effective radiation dose of a conventional CT of the abdomen and pelvis without contrast, however, is approximately 20 mSv.³ The mSv increases further when contrast images and multiphase scans are obtained. In comparison, plain film roentograms (two-film kidneys, ureters, and bladder) result in approximately 0.1 to 0.2 mSv effective dose radiation exposure, and ultrasonography carries no ionizing radiation at all.

Escalating radiation exposure in diagnostic imaging has generated significant concern for associated lifetime attributable risk of radiation-induced malignancies.^2,6

–11 In a recent report, 72 million CTs were performed in the United States in 2007, nearly a third to image the abdomen and pelvis. Although problematic to quantify the potential cancer risk associated with CT, it is projected that approximately 29,000 future radiation-induced malignancies will be attributed to radiation exposure from CTs performed in 2007 alone.¹⁰ It is impractical to set a patient radiation exposure limit in imaging, because the amount of radiation exposure should be minimal yet achieve the intended result (definitive diagnosis, outcome assessment, etc.).

Our objective was to evaluate the overall radiation exposure in our selected group of patients who were undergoing endoscopic management of upper urinary tract calculi during a periprocedure period.

Patients and Methods

Between January 2005 and July 2009, all patients undergoing endoscopic treatment for upper urinary tract calculi were entered into an institutional database. The number and type of CTs during the periprocedure period (defined as ≤90 days pre- and post-treatment) were quantified for each patient. Demographic information, including patient age, sex, and procedure type, was recorded. Patients underwent either ureteroscopy (URS) or percutaneous nephrolithotomy (PCNL). We do not routinely perform CTs after either endoscopic procedure at our institution. Patients with multiple procedures during a single periprocedure period were considered as a single stone event. Separate stone events were recorded for patients whose multiple procedures were more than 6 months apart. Univariate and multivariate analyses were performed to determine risk factors for increased number of CTs using SYSTAT software.

Accepted radiation effective dose standards were used in calculating the radiation exposure secondary to CT scanning of each patient.^3,5 The “effective dose” is a dose descriptor that reflects the differences in biologic sensitivity of different tissues. It is a single dose parameter that reflects the risk of a nonuniform exposure in terms of an equivalent whole-body exposure and can therefore be used to quantify the overall radiation exposure associated with each CT.⁵ The units used to describe the effective dose are sieverts or mSv. The effective radiation dose of a conventional CT of the abdomen or pelvis without contrast varies considerably but is approximately 10 mSv each, and 20 mSv when both the abdomen and pelvis are scanned.³ For CTs performed with and without contrast, radiation exposure was doubled because of two passes through the CT scanner.

The recommended annual dose limit for occupational exposure by the ICRP is 50 mSv, based on atomic bomb exposure and survival data.⁴ While no radiation exposure limit exists for medical indications, this surrogate was chosen because of the lack of an established patient exposure limit and the recurrent nature of calculus disease. This same value was recently used by Ferrandino and associates¹² in a study that evaluated all imaging related to stone disease within 1 year of the acute stone event.

To calculate the total radiation exposure, we included all CTs performed within 90 days before or after the procedure, and fluoroscopic data from the therapeutic procedure and from the percutaneous access for PCNLs. This was done in an effort to be inclusive of all pre- and post-operative CTs relevant to the particular stone event identified without overlapping with other subsequent stone episodes or CTs performed for other indications. A distinction was made between patients who underwent one CT during the periprocedure period, and patients who had two or more CTs. Also, we collected procedure-related fluoroscopic radiation data to obtain the patients' additional radiation exposure.

The data on radiation emission were recorded on a Philips BV Pulsera Fluoroscopic C-arm and by convention was reported in milligray (mGy). All PCNL patients underwent percutaneous access in our interventional radiology department using ultrasonography and fluoroscopy. The radiation exposure during this procedure was quantified.

Results

Complete periprocedure data were collected for 233 patient stone events during the study period. Of these, 127 (54.5%) underwent URS and 106 (45.5%) PCNL. The type of procedure was chosen by the treating surgeon based on the individual clinical situation and the stone(s) characteristics (size, location, density, etc.) on imaging. No significant differences were observed in age or sex between the two groups (Table 1).

Table 1.

Patient Demographics

	URS	PCNL	All
Procedure (n)	127 (54.5%)	106 (45.5%)	233
Mean age (y)	56.1	60.0	57.9
Males	85 (66.9%)	53 (50.0%)	138 (59.2%)
Females	42 (33.1%)	53 (50.0%)	95 (40.8%)

URS=ureteroscopy; PCNL=percutaneous nephrolithotomy.

A total of 241 endoscopic procedures were performed for 233 stone events. A single endoscopic procedure was performed in 212 of 233 patient stone events. Eight patients underwent two procedures for a single stone event during the study period; five patients had two, and one patient had three separate stone events. There was no significant correlation between the number of procedures and the number of CTs performed per stone event (Pearson chi-square test, P=0.666). There was a statistically significant correlation between surgeon and both the total number and type of procedure performed (Pearson chi-square test, P=0.001). Analysis of surgeon preference, however, did not reveal any statistical difference with regard to number of CTs obtained per patient (Pearson chi-square test, P=0.112).

A total of 367 CTs were performed in the management of 233 patient stone events, resulting in a mean 1.58 CTs per patient stone event (Table 2). A total of 143 (61.4%) patients had only one CT and 90 (38.6%) patients had two or more CTs. Subset analysis of patients with at least two CTs revealed that 75.5% had two CTs, 12.2% had three, 5.6% had four, and 6.7% had five or more CTs during the periprocedure period. Eighteen (4.9%) CTs were performed with intravenous contrast.

Table 2.

Radiation Exposure

	URS	PCNL	All
1 CT scan (n)	67 (52.8%)	76 (71.7%)	143 (61.4%)
≥2 CT Scans (n)	60 (47.2%)	30 (28.3%)	90 (38.6%)
Mean # CTs (all patients)	1.77 (range 1–6)	1.34 (range 1–7)	1.58 (Range 1–7)
Mean effective radiation dose exposure for all patients (mSv)	35.4 mSv (range 20–120 mSv)	26.8 mSv (range 20–140 mSv)	31.6 mSv (range 20–140 mSv)
Mean # CTs (patients ≥2 CTs)	2.63 (range 2–6)	2.20 (range 2–7)	2.49 (range 2–7)
Mean effective radiation dose exposure for patients ≥2 CTs (mSv)	52.6 mSv	44.0 mSv	49.8 mSv
Median intraoperative radiation exposure (mGy)	22.8 mGy (range 1.4–121 mGy)	27.6 mGy (range 3.5–217 mGy)	23.9 mGy (range 1.4–217 mGy)
Median interventional radiology radiation exposure for PCNL access	NA	15.7 mGy (range 8.8–30.2 mGy)	na

URS=ureteroscopy; PCNL=percutaneous nephrolithotomy; CT=computed tomography.

The data were analyzed to determine risk factors for increased radiation exposure. Patients who were undergoing URS were significantly more likely to have multiple CT scans than those undergoing PCNL: Mean 1.77 and 1.34 CTs, respectively (Pearson chi-square test, P=0.003). This results in a mean effective radiation dose from CT of 35.4 and 28.6 mSv for the URS and PCNL groups, respectively. Of more concern were the 90 patients who underwent two or more CTs in the periprocedure period, who were exposed to a mean effective radiation dose of 49.8 mSv from CT alone. In a multivariate logistic regression analysis of all patients, only increasing age (odds ratio [OR]: 1.03, 95% confidence interval [CI] 1.01–1.05, P=0.006) and procedure type (OR: 2.59, 95% CI 1.47–4.57, P=0.001) were significant predictors of having multiple CTs, with the URS group undergoing more scans. Sex differences were not statistically significant with regard to number of CTs performed (Pearson chi-square test, P=0.721).

Fluoroscopic data were also analyzed. The median PCNL access fluoroscopic radiation exposure was 15.7 mGy. The median intraoperative fluoroscopic radiation exposure was calculated to be 23.9 mGy per endoscopic procedure (Table 2). Intraoperatively, patients who were undergoing URS and PCNL were exposed to median values of 22.7 mGy and 27.6 mGy, respectively. On an additive basis, patients who were undergoing PCNL procedures had a median exposure of 43.3 mGy including percutaneous access and intraoperative therapy.

Discussion

CT offers unparalleled sensitivity and specificity in the detection and description of upper urinary tract calculus disease, although with considerable radiation exposure. Radiation exposure has been associated with the development of induced malignancies,^6,8,10 and many other disease states. In regard to malignancy, a direct relationship cannot be accurately quantified because of the many variables involved as well as the long latency period between radiation exposure and the development of cancer. As use of CT continues to increase, so too does the potential incidence of radiation-induced malignancy. Although multiple theories exist, the accumulated radiation exposure is likely to result in malignancy based on a linear, no-threshold model, which is the model on which the ICRP has based its recommendations.^4,10 The ICRP consensus proposes that at the relatively low radiation doses used in medical imaging, the cancer risk associated with radiation exposure is directly proportional to the radiation dose. The ICRP's risk projection models are based on organ-specific radiation doses and the National Research Council's “Biological Effects of Ionizing Radiation” report, using analyses of current data on Japanese atomic bomb survivors.¹¹ While there is disagreement regarding comparisons of atomic survivor radiation exposure (gamma rays) with medical imaging X-ray data, such discord is beyond the scope of this article. Interestingly, there is evidence that suggests that exposure to low-energy X-rays may result in more chromosomal aberrations per unit dose than gamma rays, potentially underestimating the risk of X-rays in comparison with gamma rays.¹³

Recent publications have implicated the significance of the risk of radiation-induced cancers secondary to CT. Berrington de Gonzalez and associates¹⁰ estimate that the approximately 70 million CTs performed in 2007 alone will result in 29,000 future cancers. Scans of the abdomen and pelvis would be implicated in nearly half of these radiation-induced malignancies. These numbers excluded patients in their last 5 years of life and those CTs performed with an indication for known malignancy. Smith-Bindman and colleagues⁶ calculated the associated lifetime attributable risk of cancer from CT. They estimated that a single noncontrast CT of the abdomen/pelvis would result in a radiation-induced cancer in 1:1400 women, or 1:1330 men, at age 60. The risk increases inversely with decreasing age, as radiation-induced cancer would develop in 1:500 women and 1:660 men when performing the same CT on a 20-year-old patient. Radiation-induced malignancy risk was based on sex, age at exposure, amount of radiation exposure, and life expectancy.

Radiation dosing variables for medical imaging are specifically defined. Although the Sv has the same units as the Gy, joules per kilogram, it measures a different quantity. Gy measures the absorbed dose of energy, while the Sv measures the effective dose (by convention, radiologists and physicists use mSv when describing the doses that affect biologic entities). The concept of effective dose was designed for radiation protection of occupationally exposed personnel. It is a dose descriptor that reflects the difference in biologic sensitivity of different tissues. Specifically, the effective dose is the sum radiation calculated by measuring 20 organ specific doses and then multiplying each by their respective tissue weighting factor; the tissue weighting factor correlates to the organ's radiosensitivity. Used in context, mSv is a single dose parameter that reflects the risk of a nonuniform exposure in terms of an equivalent whole-body exposure. Because of these conventions, combining the fluoroscopic radiation data (in mGy) with the CT radiation data (in mSv) is not scientifically sound, because they are measuring different qualities of radiation.^3,5

Limitations on patient radiation exposure would be difficult to establish in view of the efficacy of CT in diagnosis and in planning therapy. Consequently, as a surrogate, we and others have used the recommended yearly dose limit for occupational exposure by the ICRP of 50 mSv.^4,12 This proxy is used with the understanding that the benefits of medical imaging, and their inherent radiation exposure, must be balanced with the necessity for diagnostic and therapeutic use.

During the periprocedure period, 38.6% of all patients undergoing endoscopic stone surgery were exposed to a mean of 2.49 CTs, resulting in an approximate 49.8 mSv effective radiation dose exposure from CT alone. The increased use of CT in patients who are undergoing URS (47.2% of whom underwent ≥2 CT), resulting in a mean effective radiation dose of 52.6 mSv, which may be attributed to relatively smaller stones and ureteral position, necessitating more vigilant radiographic monitoring for potential migration, spontaneous passage, etc. An additional consideration is that patients with urolithiasis-induced colic often present to different emergency departments and are routinely reimaged.

A small percentage of CTs (4.9%) were performed with intravenous contrast, doubling the effective dose. Indications for pre- and postcontrast CT were to better evaluate additional clinical findings. While this provides more thorough anatomic and functional information, it comes at the realization of exposing the patient to double the dose of radiation.

Considerable variability in radiation dose exposure between similar CT scanners limits the accuracy of direct comparison, but allows for different protocols to be used. Because of this, we have used an established standard estimate of CT scan radiation dose exposure that has been used previously in similar reports.^3,12 A comparison between contemporary CT scanners revealed a mean 13-fold variation between highest and lowest radiation dose exposures for each CT study, highlighting the difficulty in standardizing the radiation dose exposure.⁶ Specifically, routine CT of the abdomen and pelvis without intravenous contrast had a mean effective dose range of 7 to 39 mSv.

Several studies have evaluated the use of low-dose radiation protocols in assessing upper urinary tract calculi and have shown comparable sensitivity and specificity in diagnosing calculus disease.^14
–16 A recent study by Jellison and coworkers¹⁷ showed that a low dose CT protocol using only 0.95 mSv had similar sensitivity and specificity (97% and 84%, respectively) in identifying ureteral calculi when compared with regular radiation dose setting of 19 mSv (98% and 83%, respectively), representing a 95% reduction. Although the study design used cadaveric models with numerous phleboliths inserted as distractors, the inter-reader reproducibility was excellent. Similarly, Kluner and associates¹⁶ used an ultralow dose protocol of 0.5 mSv effective dose in the identification of in situ renal calculi and found 97% sensitivity and 95% specificity.

Strategies for reducing radiation with CT and fluoroscopy should be implemented. These include milliampere (mA) attenuation, reformatting of images, dual energy CT scans,¹⁸ and maintenance of the ALARA (as low as reasonably achievable) principles. The ALARA principles include appropriate patient and study selection, in addition to dosimetry optimization. For CT, consideration should be made to find the lowest deliverable mA to facilitate optimal clinical decisions by the treating urologist. Radiation from fluoroscopy is also quite variable and is estimated to produce a radiation dose of <10 mGy to >500 mGy per minute, with spot films increasing this dose by a factor of 10 to 60 times.^3,5 By maintaining a low tube current, using pulsed-fluoroscopy, avoiding the use of spot films, limiting the beam-on time, and using ultrasonography where possible, the ALARA principles are optimized. Our department has discussed with our emergency department how to best treat patients who present with a suspicion of upper urinary tract calculus disease. Similarly, our faculty is actively engaged, with our radiology colleagues, in low dose protocols and in limiting radiation exposure to patients with symptoms that are suspicious for obstructive urolithiasis.

Limitations of our study include assigning a recognized set mean mSv dose for noncontrast conventional abdomen and pelvic CTs (20 mSv) because of the wide variability among scanners and imaging techniques.^3,6,12 In addition, fluoroscopic data were recorded in mGy only, limiting the ability to combine the data, although increasing mSv and mGy result in increased radiation exposure, regardless of the definitions.

Conclusions

Our findings indicate significant radiation exposure for at least 38.6% of our patients, whose exposure from CT alone approached the recommended annual occupational limit. Additional radiation exposure from fluoroscopy during the procedure(s), recorded as 22.7 mGy and 43.3 mGy for URS and PCNL, respectively, may be enough to have total radiation exposure exceed the ICRP limit in this subgroup.

CT radiation exposure is considerable in the periprocedure period for patients who are undergoing therapeutic endoscopic upper urinary tract stone removal. Radiation exposure is further increased by the use of procedure-related fluoroscopy. Urologic surgeons should be aware of CT and fluoroscopic radiation exposure and, where appropriate, should consider strategies to reduce the amounts of ionizing radiation because urinary stone disease is often recurrent and the effects of radiation cumulative.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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