Techniques for performing a CT exam on a teenager need to be determined by size and body mass, rather than by age, to reduce radiation dose, researchers from the Medical University of South Carolina (MUSC) in Charleston advise in a poster presented at the recent RSNA meeting in Chicago.
While investigating radiation dose delivered to pediatric patients undergoing body CT examinations, MUSC researchers discovered that smaller teenagers received a radiation dose comparable to that of an adult -- despite their smaller body habitus.
"This discovery was unexpected and alarming, considering that more than 50% of teenage children who receive CT exams at MUSC don't have the body mass of a fully formed adult," said Dr. Dobrinka Dimitrova, a radiology resident, during her scientific poster presentation.
For the past 18 months, Dimitrova and colleagues have been working under the tutelage of medical physicist Walter Huda, Ph.D., to accurately determine the effective dose delivered to children undergoing CT body examinations.
The researchers evaluated x-ray technique factors for pediatric patients receiving CT on a 16-slice scanner (Sensation, Siemens Healthcare, Malvern, PA). Knowledge of kV, mAs, and pitch ratio permitted calculation using an ImPACT spreadsheet (ImPACT, London) of the effective dose for an adult scanned with these techniques. The spreadsheet makes use of Monte Carlo dose computations performed by the U.K.'s National Radiological Protection Board.
Pediatric effective doses were obtained by taking into account differences in scan length, patient weight, and relative values of energy imparted to children relative to adults. Representative effective doses for body CT exams of children ranging in age from newborn to 18 years were computed using a new formula developed by Huda.
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Huda/MUSC formula for determining pediatric CT dose Ep = Ea x (Lp/La) x (mAsp/mAsa) x (Ma/Mp) x (Ren)
This equation assumes that adult and pediatric scans are performed on the same body region, using the same CT scanner operated with the same technical parameters. |
The researchers estimated effective dose by taking into account how much radiation is received by any individual organ, as well as the organ's relative radiosensitivity. A total of 300 CT exams were evaluated, with infants younger than 12 months and newborns representing the largest number (13.3%) for a single age group and teenagers (ages 15-18) representing 27.3% of the total, Dimitrova explained.
The researchers had previously determined from their original calculations that effective dose to the youngest children was approximately double that of adults at the same technique factors, and MUSC made major changes to its CT protocols based on this data.
By using lower-dose techniques for younger patients, they were able to achieve an effective dose of 4 mSv to 6 mSv for newborns to 10-year-olds. The effective dose for 5-year-old children was 5 mSv for a whole-body scan, 3 mSv for an abdomen/pelvis CT scan, and 1.5 mSv for a chest CT scan.
But the new research indicates that children ages 12 to 18 years also need low-dose protocols tailored to their body habitus.
"Above the age of 12 years, kV and mAs are standardized to fully formed adult bodies," Dimitrova said. "Because children over the age of 12 have different growth rates, a 15-year-old girl, for example, may have a body size comparable to a 10-year-old. She may receive a lot higher dose than needed because the set parameters are for adults."
"With this knowledge, we are trying to customize the mAs and kV that we use in all the trauma scans and all the abdomen and pelvis scans for teenagers ages 12 to 18 based on their body size," Dimitrova said.
MUSC has established a protocol whereby CT technologists are expected to consult with a radiologist whenever a pediatric CT scan is ordered by a physician. The radiologist determines if the order is appropriate, and if he or she thinks that a different procedure should be performed, the radiologist contacts the referring physician with a more appropriate recommendation.
"Radiologists are responsible for knowing the radiation doses and educating the clinicians. They have no way of knowing effective dose and radiation risk levels," Dimitrova told AuntMinnie.com. "Some of the CT scans I've investigated have as much as 25 mSv of radiation dose. Two of these will put a pediatric patient into a category of high incidence of solid tumors or lymphoma. Radiologists need to be the gatekeepers to protect children from unnecessary and excessive radiation dose."
By Cynthia Keen
AuntMinnie.com staff writer
December 11, 2008
Related Reading
Studies address need to reduce CT dose in emergency settings, October 17, 2008
Study: Radiologists dial back on pediatric CT settings, October 4, 2008
Cincinnati meeting starts long road to harmonizing CT dose displays, September 17, 2008
Lower tube voltage leads to reduced dose in pediatric CT, June 14, 2007
Part IV: Imaging the pediatric patient: CT applications in pediatric imaging, May 12, 2004
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