IMAGING

Radiation Dose Associated with Unenhanced CT for Suspected Renal Colic: Impact of Repetitive Studies
Katz SI, Saluja S, Brink JA, Forman HP
Department of Radiology, Yale-New Haven Hospital, Yale University School of Medicine, New Haven, CT, USA
AJR Am J Roentgenol. 2006; 186: 1120-4

• 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.
  
  
• Editorial Comment
  Recent studies have been shown that computed tomography (CT) contribute to 35-45% of total radiation dose to the patient population. Nowadays, radiologists’ aim must be to decrease radiation dose to the patient and also check very carefully all indications and recommend alternative imaging methods. Recently several CT protocol imaging have been developed in order to decrease the total amount of radiation dose that a patient receives during abdominal CT. This is a very important study, which discusses all the issues and possibilities regarding those patients that are submitted to repetitive abdominal CT for the evaluation of acute flank pain. The authors showed that a small but significant subset of the patient population (4%) was estimated to receive from 20 to as high as 154 mSv, which is totally undesirable. In order to decrease the amount of radiation dose they suggested that sonography associated with abdominal radiography (KUB), should be used 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. Another useful approach for these patients is to use the reduced radiation-dose technique. When using these optimized CT protocol, the tube current can be reduced to 70 mA in comparison with the higher tube current of multidetector conventional CT protocol (200 mA). By using this optimized low dose protocol, the estimated effective dose is reduced from 8.6 mSv to 1.5 mSv. This reduced radiation-dose protocol result in scans with high accuracy for detecting urinary calculi in patients evaluated for acute flank pain.

Dr. Adilson Prando
Chief, Department of Radiology
Vera Cruz Hospital
Campinas, São Paulo, Brazil