Organ and effective doses in pediatric patients undergoing helical multislice computed tomography examination

Abstract

As multidetector computed tomography (CT) serves as an increasingly frequent diagnostic modality, radiation risks to patients became a greater concern, especially for children due to their inherently higher radiosensitivity to stochastic radiation damage. Current dose evaluation protocols include the computed tomography dose index (CTDI) or point detector measurements using anthropomorphic phantoms that do not sufficiently provide accurate information of the organ-averaged absorbed dose and corresponding effective dose to pediatric patients. In this study, organ and effective doses to pediatric patients under helical multislice computed tomography (MSCT) examinations were evaluated using an extensive series of anthropomorphic computational phantoms and Monte Carlo radiation transport simulations. Ten pediatric phantoms, five stylized (equation-based) ORNL phantoms (newborn, 1-year, 5-year, 10-year, and 15-year) and five tomographic (voxel-based) UF phantoms (9-month male, 4-year female, 8-year female, 11-year male, and 14-year male) were implemented into MCNPX for simulation, where a source subroutine was written to explicitly simulate the helical motion of the CT x-ray source and the fan beam angle and collimator width. Ionization chamber measurements were performed and used to normalize the Monte Carlo simulation results. On average, for the same tube current setting, a tube potential of 100 kVp resulted in effective doses that were 105% higher than seen at 80 kVp, and 210% higher at 120 kVp regardless of phantom type. Overall, the ORNL phantom series was shown to yield values of effective dose that were reasonably consistent with those of the gender-specific UF phantom series for CT examinations of the head, pelvis, and torso. However, the ORNL phantoms consistently overestimated values of the effective dose as seen in the UF phantom for MSCT scans of the chest, and underestimated values of the effective dose for abdominal CT scans. These discrepancies increased with increasing kVp. Finally, absorbed doses to select radiation sensitive organs such as the gonads, red bone marrow, colon, and thyroid were evaluated and compared between phantom types. Specific anatomical problems identified in the stylized phantoms included excessive pelvic shielding of the ovaries in the female phantoms, enhanced red bone marrow dose to the arms and rib cage for chest exams, an unrealistic and constant torso thickness resulting in excessive x-ray attenuation in the regions of the abdominal organs, and incorrect positioning of the thyroid within the stylized phantom neck resulting in insufficient shielding by clavicles and scapulae for lateral beam angles. To ensure more accurate estimates of organ absorbed dose in multislice CT, it is recommended that voxel-based phantoms, potentially tailored to individual body morphometry, be utilized in any future prospective epidemiological studies of medically exposed children.

Radiation Dose to the Fetus for Pregnant Patients Undergoing Multidetector CT Imaging: Monte Carlo Simulations Estimating Fetal Dose for a Range of Gestational Age and Patient Size

“…A previously validated multidetector CT source model was created by using Monte Carlo simulation-based techniques (14)(15)(16)(17)(18). The CT scanner modeled was a LightSpeed 16 (GE Medical Systems).…”

CT exposure in adult and paediatric patients: a review of the mechanisms of damage, relative dose and consequent possible risks

“…Axelsons et al [15] or Giacco et al [16] used a physical anthropomorphic phantom, while other investigators used Monte Carlo voxelized phantoms [17]. In both methodologies, unlike what happens in the adult, the phantoms are size and age specific [18].…”

Relationship between the radiation doses at nonenhanced CT studies using different tube voltages and automatic tube current modulation during anthropomorphic phantoms of young children

“…The dose calculations based on CTDI or DLP are readily available indicators of the radiation dose in CT studies. The organ dose and the effective dose can be estimated from the CTDI or the DLP using conversion factors derived from the Monte Carlo simulation of photon interactions within a simplified mathematical model of the human body 16. Accordingly, new series of Monte Carlo calculations have been carried out at National Radiological Protection Board for a family of 6 geometric mathematical phantoms, representing ages from newborn to adult.…”