Program Information
A Monte Carlo Study to Determine SSDE Coefficients for Pediatric Head CT (TG-293)
A M. Hernandez1*, J M. Boone2 , (1) Department of Radiology, Biomedical Engineering Graduate Group, University of California Davis, Sacramento, CA, (2) Departments of Radiology and Biomedical Engineering, University of California Davis, Sacramento, CA,
Presentations
TH-EF-601-2 (Thursday, August 3, 2017) 1:00 PM - 3:00 PM Room: 601
Purpose: The size specific dose estimate (SSDE) developed by AAPM TG-204 for body CT has been used successfully to better estimate absorbed dose to patients of different sizes using the CTDIvol and the water-equivalent diameter (WED) determined from the patient’s CT images. This study extends the SSDE concept to pediatric head CT, the principal charge to AAPM TG 293.
Methods: A range of pediatric patient head dimensions was determined from a number of data sources. Using this range, Monte Carlo simulations (MCNP6) were performed across several parameters. The absorbed dose was estimated from monoenergetic source photons ranging from 10 keV to 140 keV (1 keV intervals) in mathematical ellipsoid phantoms differing in size. Polyenergetic dose values were determined by spectrally weighting the monoenergetic data using the TASMICS spectral model. Air Kerma was also simulated in the center and peripheral locations for the 16 cm diameter PMMA head CT phantom, and CTDIvol was computed. Curves of f-factors, representing the ratio of the absorbed dose/CTDIvol as a function of effective phantom diameter was generated for four typical tube potentials (80, 100, 120, 140 kV) used in CT.
Results: The measured effective diameter of the pediatric patients ranged from 10.9 cm to 18.2 cm and the average skull thickness was 4 mm. SSDE coefficients (f-factors) were determined using these dimensions and resulted in values ranging from 1.14 to 0.85 for the 10.9 cm and 18.2 cm effective diameters, respectively.
Conclusion: The differences in these f-factor curves based on tube potential varied by only 7.2%, and so it is likely that a single set of size-dependent factors can be used across all CT spectra. The data produced in this study will be combined with data sets from other groups, to form the final recommendations of AAPM TG-293.
Funding Support, Disclosures, and Conflict of Interest: This communication was funded in part by NIH grants P30 CA093373 and R01 CA181081. Comments made are solely the responsibility of the authors and do not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.
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