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DOI:10.2214/AJR.04.1838
AJR 2006; 186:1120-1124
© American Roentgen Ray Society

Original Research

Radiation Dose Associated with Unenhanced CT for Suspected Renal Colic: Impact of Repetitive Studies

Sharyn I. Katz1,2, Sanjay Saluja1, James A. Brink1 and Howard P. Forman1

1 Department of Radiology, Yale–New Haven Hospital, Yale University School of Medicine, New Haven, CT 06520-8042.
2 Present address: Department of Radiology, Hospital of the University of Pennsylvania, 1 Silverstein Bldg., 3400 Spruce St., Philadelphia, PA 19104.

Received December 2, 2004; accepted after revision February 21, 2005.

 
Address correspondence to S. Katz (sharyn.katz{at}uphs.upenn.edu).


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of our study was to assess the dose of ionizing radiation delivered through the use of unenhanced CT for suspected renal colic by determining the incidence of repeated unenhanced CT examinations and the cumulative radiation dose delivered.

MATERIALS AND METHODS. All unenhanced CT examinations for suspected renal colic performed at our institution over a 6-year period were included, and patient age, sex, and multiplicity of examinations were determined. For the adult patient, this protocol prescribes a fixed tube current of 200 mA, 140 kVp, and a nominal slice width of 5 mm. The dose–length product (DLP) was estimated for 15 randomly chosen single-detector CT (SDCT) and MDCT adult flank pain examinations using manufacturer's software. The mean DLPs for SDCT and MDCT were computed and converted to effective doses. Total effective doses were calculated for patients who underwent more than three examinations, and values were compared with established standards.

RESULTS. A total of 5,564 examinations were performed on 4,562 patients. Of these patients, 2,795 (61%) were women (mean age, 45.5 ± 16.2 [SD] years) and 1,731 (38%) were men (mean age, 44.7 ± 16.4 years), with 144 patients (3%) of pediatric age. The mean effective doses for a single study were 6.5 mSv for SDCT and 8.5 mSv for MDCT. A subset of 176 patients (4%) had three or more examinations, with estimated effective doses ranging from 19.5 to 153.7 mSv. All patients with multiple examinations had a known history of nephrolithiasis.

CONCLUSION. Patients with a history of nephrolithiasis and flank pain are at increased risk for serial CT with potentially high cumulative effective doses.

Keywords: CT • genitourinary imaging • kidney • radiation dose • renal colic • urinary tract


Introduction
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 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
In a milieu of increasing utilization of radiology, great concern exists for the cumulative doses of ionizing radiation delivered to the patient population, which have increased by 20–25% between 1980 and 1990 [1]. This concern is reflected by the growing body of literature dedicated to estimating and limiting ionizing radiation doses in diagnostic medicine, including several publications documenting the likelihood of iatrogenically induced malignancies from the use of diagnostic imaging [2–4]. More recently, reports have been published of the theoretic increased risk of fatal cancer in adults [5] and children [6] as a result of a single CT examination, based on data from similar low levels of exposure to ionizing radiation during the atomic bombings of Nagasaki and Hiroshima [7]. In light of these theoretic risks, it seems prudent to limit the excessive use of ionizing radiation so long as diagnostic accuracy and efficacy are not compromised.

Flank pain is a common patient complaint that has been estimated to affect up to 12% of the population during their lifetime [8]. Flank pain frequently results in a CT examination to evaluate the presence, size, and location of an obstructing stone. Unenhanced abdominopelvic CT was first proposed for the workup of flank pain in 1995 by Smith et al. [9]. As a result of its superior sensitivity and specificity [8, 10, 11], CT has largely replaced excretory urography in the workup of acute flank pain. In a study of patients presenting to emergency departments with abdominal or flank pain, Nagurney et al. [12] found that the most valuable tests for evaluating nontraumatic abdominal pain in the emergency department were abdominopelvic CT and urinalysis.

In addition to determining the stone size and level of obstruction in nephrolithiasis, CT also detects nonurinary diagnoses that may present with flank pain, such as ovarian disorders, pyelonephritis, appendicitis, and diverticulitis [13]. Taking only minutes to perform, CT for the evaluation of flank pain does not necessitate the use of IV contrast material, thus obviating the risks associated with IV contrast material, such as allergy and nephrotoxicity.


Figure 1
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  		Fig. 1 —Bar graph shows subset of 176 patients who had three or more unenhanced CT examinations for suspected renal colic during 6-year study period. One patient underwent 18 CT examinations.

 
Given these many benefits, utilization of CT in the evaluation of acute flank pain has naturally increased at our institution. We have observed that patients with a history of flank pain return with similar symptoms and undergo repetitive CT examinations. Because the estimated relative rate of recurrence of urinary calculi is estimated to be 35.3% over 10 years [14], we sought to evaluate the number of unenhanced CT examinations performed for suspected renal colic, to estimate the effective dose for these patients, and to identify those undergoing serial unenhanced CT examinations for suspected renal colic.


Materials and Methods
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 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Our unenhanced CT protocol for suspected renal colic in an adult patient consists of contiguous axial images from the top of the kidneys to the base of the bladder obtained with the patient in the prone position and without the use of IV contrast material. For the adult patient, this protocol prescribes a tube current of 200 mA, 140 kVp, and a nominal slice width of 5 mm. All examinations were performed on either a single-detector CT (SDCT) scanner (CTi, GE Healthcare) or an MDCT scanner (QXi, GE Healthcare).

The radiology information system database was queried for all unenhanced CT examinations for suspected renal colic performed between January 2, 1996, and March 16, 2002, using search options in the database software. The age, sex, and multiplicity of unenhanced CT examinations for suspected renal colic for each patient were then determined. For patients with three or more unenhanced CT examinations, the clinical history was obtained from the radiology reports of the CT examinations. Pediatric patients were defined as less than 18 years old.

To estimate the ionizing radiation dose delivered with a single unenhanced CT examination for suspected renal colic, the dose–length product (DLP) for one examination was estimated. The DLP is the product of the weighted CT dose index (CTDIw)—a phantom derived value provided by the CT manufacturer that describes the estimated dose delivered (in milligrays) to any point within the volume of the imaged object—and the length of the imaged object (in centimeters). The DLP is given in units of milligray times centimeter. Fifteen adult unenhanced CT examinations for suspected renal colic were randomly selected from studies performed on each of the SDCT and MDCT scanners, for a total of 30 examinations. Patients were assigned a unique number in our database. The patients in both the MDCT and the SDCT groups were randomized using the Excel (Microsoft) randomization function.

At our institution, the estimated DLP for every CT examination is recorded from the scanner console and entered into a logbook by the CT technologists. The mean DLPs for SDCT and MDCT examinations for flank pain were calculated and converted to estimates of effective dose using the methods of Jessen et al. [15]. Effective dose is a unit that takes into consideration the biologic sensitivity of imaged organs; it can be estimated by multiplying the DLP by conversion factors specific to each region of the body imaged. The conversion factor for abdominal CT is 0.012 mSv/mGy · cm, and the conversion factor for pelvic CT is 0.016 mSv/mGy · cm. Because the abdominal and pelvic components of unenhanced CT for suspected renal colic involve similar longitudinal distances, the conversion factor for these examinations was taken as the average of the abdominal and pelvic conversion factors (0.014 mSv/mGy · cm).


Figure 2
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  		Fig. 2 —Estimated effective dose delivered with unenhanced CT examinations for suspected renal colic. Bar graph shows range of estimated effective doses reaches maximum of 150 mSv. MDCT (black bars) delivers slightly higher dose than single-detector CT (gray bars).

 
Once the effective dose for a single unenhanced CT examination was estimated, the total effective dose for multiple examinations was estimated by multiplying the effective dose for a single examination by the number of CT examinations performed on a particular patient.


Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
A total of 5,564 unenhanced CT examinations for suspected renal colic were performed on a total of 4,562 patients during the 6-year period. Of these patients, 2,765 (61%) were women (mean age, 45.5 ± 16.2 [SD] years) and 1,731 (38%) were men (mean age, 44.7 ± 16.4 years). The adults ranged in age from 18 to 99 years old. A total of 144 examinations were performed on pediatric patients ranging in age from 2 to 17 years old.

A total of 176 patients (4%) had three or more unenhanced CT examinations for suspected renal colic, and 19 patients had six or more (Fig. 1). One patient had 18 unenhanced CT examinations performed during the study period. All patients who had three or more unenhanced CT examinations had a history of nephrolithiasis, as indicated on the history provided for the first examination in the sequence.

The mean DLPs for SDCT and MDCT for a single examination for flank pain were estimated to be 460 and 610 mGy · cm, respectively. When converted to effective dose, this corresponds to 6.5 mSv for SDCT and 8.5 mSv for MDCT.

Using these estimated effective doses for a single unenhanced CT examination for suspected renal colic, the estimated effective dose delivered to most of the patients was low, ranging between 6.5 and 13.0 mSv for SDCT and between 8.5 and 17.0 mSv for MDCT (Fig. 2). Most of these patients underwent only one or two unenhanced CT examinations. However, those patients who underwent three or more unenhanced CT examinations were exposed to a range of effective doses estimated to be 20—154 mSv.


Discussion
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 Abstract
 Introduction
 Materials and Methods
 Results
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References
 
During the 6-year study period, most of the patients were women and most underwent one or two unenhanced CT examinations, resulting in low estimated effective doses of 6–17 mSv. The range of estimated effective doses reflects the slightly higher dose imparted by MDCT compared with SDCT, a result of known inherent MDCT dose inefficiencies [16]. A small but significant subset of the patient population (4%) was estimated to receive from 20 to as high as 154 mSv because of the repetitive use of CT to evaluate acute flank pain. The patients in this subset were all known to have chronic nephrolithiasis.

Although the exact biologic impact of an exposure of this magnitude is not known, it has been estimated that the risk of fatal cancer is 0.05% (1 in 2,000) for 10 mSv of ionizing radiation [5, 17]. This estimate is determined by linear extrapolation from the risk of 5% per each sievert established by the International Commission on Radiological Protection in 1991 (ICRP 60) [17]. Those patients who undergo three or more unenhanced CT examinations would thus assume an estimated incremental risk of fatal cancer as high as 1 in 133 for a patient who underwent 18 CT examinations. Moreover, these estimated risks account for only unenhanced CT examinations for suspected renal colic performed at our institution for the 6-year study period and do not include CT examinations performed outside that time, performed for other purposes, or performed at outside institutions. This risk estimation also does not include other radiology studies that use ionizing radiation.

Supporting such extrapolated risks, data have been published on 35,000 of the survivors from the 1945 atomic bombings of Nagasaki and Hiroshima who were exposed to low doses of ionizing radiation [7]. This subset of atomic bomb survivors were exposed to ionizing radiation doses ranging from 5 to 200 mSv and were found to have a statistically significant increased risk for developing fatal cancer. This dose range parallels that used in our studied patient population. Although controversial, data suggest that the cumulative effect of fractionated low-dose exposures to ionizing radiation has a similar biologic impact to a single acute dose of the same magnitude [5, 18].

Although the benefits of the medical use of ionizing radiation through imaging are plentiful and increasing, it is prudent to identify sources of excessive use. In our study, patients who had chronic renal stones underwent serial CT examinations, with one patient undergoing 18 examinations in a 6-year period. Several strategies should be considered in the management of this patient group. The sensitivity of sonography alone is limited, largely because of poor visualization of the ureters. The sensitivity and specificity of sonography alone have been reported to be 61–93% and 95–100%, respectively [19, 20], in the setting of acute flank pain. However, the combination of sonography and unenhanced abdominal radiography (kidneys, ureters, and bladder [KUB]) may be considered in the evaluation of patients with known chronic renal stones, particularly if their stones are shown to be radiopaque on radiography.

If a combination of KUB and sonography is used in lieu of CT, the radiation dose would be decreased by an order of magnitude (Table 1). In a study by Varanelli et al. [21], 89% and 97% of patients with obstructive nephrolithiasis have CT evidence of pelviectasis and ureterectasis, respectively, after 8 hr of symptoms of acute flank pain. Al-though CT has superior accuracy in leading to the correct diagnosis in the setting of flank pain, Catalano et al. [22] compared helical CT and sonography in the clinical setting of acute flank pain. The combination of KUB and sonography, when compared with CT, has a 77.1% versus 92.4% sensitivity, 92.7% versus 96.4% specificity, 95.3% versus 98% positive predictive value, 68% versus 86.9% negative predictive value, and 82.5% versus 93.7% accuracy in patients who underwent all three techniques [22]. Thus, although CT is a more accurate technique for determining the presence of nephrolithiasis, the combination of KUB and sonography yielded comparable results with no clinically important misdiagnoses; all stones that were missed on sonography passed spontaneously. Because the combination of KUB and sonography has an adequate negative predictive value and CT rarely adds clinically important information in cases having an initial negative result, CT may be reserved for those patients with suspected renal colic but negative findings on KUB and sonography [22].


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  		TABLE 1: Estimated Effective Doses Delivered in Examinations for Evaluation of Acute Flank Pain

 

Ripolles et al. [23] also found that the combination of KUB and sonography compared favorably with unenhanced CT for the detection of nephrolithiasis. In their study, all stones missed on sonography and KUB were passed spontaneously; and the best sensitivity and specificity, 100% and 90%, respectively, were achieved when assessing for lithiasis and secondary signs of obstruction. In other reports, the sensitivity for sonography and KUB ranged from 84% to 97%, with specificity ranging from 85% to 90% [24, 25] for the assessment of suspected renal colic.

We propose, therefore, that the combination of KUB and sonography has a role as a first imaging examination in patients known to have chronic stone formation who have a high pretest probability of nephrolithiasis and thus are less likely to have a missed alternative diagnosis. Most of the studies in the literature that favorably compare KUB and sonography with CT have been performed in Europe, where there is more experience using these techniques for this purpose. We therefore do not propose eliminating CT in this patient population but rather propose to reserve CT for those patients with suspected renal colic who first have negative or equivocal results for KUB and sonography.

Alternatively, several low-dose protocols have been proposed for the CT evaluation of renal colic [26–30], with estimated effective doses as low as 1.5 mSv while maintaining acceptable sensitivity and specificity. Techniques for decreasing dose in CT primarily include decreasing tube current and increasing pitch. This low effective dose of 1.5 mSv was achieved using 70 mA, 120 kVp, and pitch of 2; this protocol maintained a sensitivity and specificity of 96% and 97% for the detection of renal stones [26]. When compared with our study, this protocol would result in a decrease in radiation dose by an order of magnitude. Hamm et al. [26] recommended that in obese patients, with a body index greater than 31 kg/m2, a conventional protocol should be implemented to achieve adequate image quality. Interestingly, Heneghan et al. [27] directly compared conventional and reduced radiation dose techniques for the detection of nephrolithiasis in unenhanced helical CT and showed scans of high accuracy at a tube current of 100 mA. A measure of dose reduction can also be achieved using recently available CT manufacturer's software that automatically corrects for body thickness, thus avoiding excess radiation in thin patients [31].

It has also been proposed that MR urography be used in the evaluation of suspected renal colic, using heavily T2-weighted sequences with or without gadolinium-enhanced fat-saturated spoiled gradient-echo sequences [32–34]. In a study of 49 patients, Sudah et al. [33] showed a sensitivity of 94–100% and specificity of 100% for MR urography, which compared favorably with unenhanced CT for the detection of ureteral stones in the setting of acute flank pain. The advantage of this approach would be the absence of ionizing radiation while acquiring cross-sectional images that would allow evaluation of the renal parenchyma and collecting system and detect nonurinary sources of flank pain.

The limitations of MR urography are largely derived from the inability of MRI to directly visualize calcification; the stone is seen as a filling defect, for which the differential diagnosis includes blood clot, sloughed papillae, and urothelial neoplasm [33, 35]. In the study by Sudah et al. [33], the stone size and the caliceal stone burden were underestimated on MRI as compared with CT, an observation that was attributed, in part, to the suboptimal signal-to-noise ratio and to the spatial resolution with the clinically available field strengths.

Finally, MRI has some practical limitations: availability, cost, and length of the examination. Given these limitations, MR urography, as currently practiced, has not been put forth as a first-line imaging examination for the general patient population with acute flank pain but rather as an acceptable alternative for cooperative children and pregnant women [33–35]. Perhaps with further investigation and experience with higher field strengths, MR urography may in the future play a more central role in the imaging of acute flank pain.

The limitations of this study include its retrospective nature and the use of estimates, rather than direct measurements, for DLP and effective dose. In addition, the estimated risks of fatal cancer are made either from extrapolation from risks known at higher doses of ionizing radiation or from data on atomic bomb survivors who received a single dose rather than the fractionated doses used in medical imaging. Although these estimates remain largely theoretic, there are data to support an actual increase in iatrogenically induced cancers in patients who are exposed to repeated diagnostic imaging [2–4]. Therefore, it seems prudent to limit avoidable iatrogenic radiation exposure.

In this study, most of the patients who underwent CT for the evaluation of acute flank pain were women who received an effective dose of 6–17 mSv (equivalent to 1–2 CT examinations). During a 6-year period, 4% of patients, all of whom had chronic nephrolithiasis, underwent three to 18 CT examinations with effective doses ranging from 20 to 154 mSv. Perhaps a combination of KUB and sonography may be used as a first imaging study in evaluating acute flank pain in this subset of the population who have a high pretest probability for symptomatic nephrolithiasis and who are undergoing serial CT. When CT is used, a low-dose protocol or dose-lowering manufacturer's software may be used to decrease the amount of ionizing radiation delivered and thus diminish the risk of iatrogenically induced malignancy.


Acknowledgments
 
We thank Janet Cameron for her secretarial support and Pradeep Mutalik for his assistance with the radiology information system database. We also thank Parvi Ramchandani for her critical reading of this manuscript.


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 Introduction
 Materials and Methods
 Results
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HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS