Draft report: Paediatric Mesh-type Reference Computational Phantoms
Abstract
Following the issuance of the 2007 Recommendations in Publication 103, the Commission released the adult (male and female) and paediatric (newborn, 1-, 5-, 10- and 15- year-old male and female) reference computational phantoms in Publications 110 and 143, respectively, for use in effective dose calculations. These phantoms are voxel models represented in the form of a 3D array of cuboidal voxels, which were constructed from computed tomography images of people and adjusted to be consistent with the reference anatomical parameters given in Publication 89. The voxel phantoms provide anatomical improvements over the mathematical equation-based stylised phantoms used for the previous dose coefficient (DC) calculations prior to the 2007 Recommendations. Nevertheless, the voxel phantoms, due to the nature of voxel geometry and finite voxel resolutions, have limitations in representing small and thin organs and tissues, necessitating additional supplementary stylised models such as those defined for the respiratory tract airways, the alimentary tract organ walls and stem cell layers, lens of the eye and the skin basal layer.
To address the limitations of the voxel phantoms, Task Group 103 was charged with developing mesh-type reference computational phantoms (MRCPs) by converting the voxel phantoms into a high-quality/fidelity mesh format with anatomical improvements for the complex organs and tissues which were not fully represented in the voxel phantoms. The MRCPs for adult male and female were then developed and recently released in Publication 145. Following the release of the adult MRCPs, the current publication describes the construction of the paediatric MRCPs, the counterparts of the Publication 143 voxel phantoms. The paediatric MRCPs, like the adult MRCPs, were developed to have all the source and target tissues required for calculation of effective dose, including the micrometre-scale regions, assimilating the supplementary stylised models. These phantoms can be directly used in general-purpose Monte Carlo codes such as Geant4, PHITS and MCNP6, fully maintaining the high fidelity of the mesh geometry in Monte Carlo dose calculations.
To investigate the impact of the paediatric MRCPs, the DCs of organ dose and effective dose and specific absorbed fractions (SAFs) for some selected external and internal exposures were calculated and compared with the values calculated using the Publication 143 phantoms and the Publications 66 and 100 mathematical models for the respiratory and alimentary tracts and the reference values of Publication 1XX. While some differences in the DCs and SAFs were observed for anatomically improved organs and weakly penetrating radiations, they were found not to be much different, indicating that the reference DCs obtained from the Publication 143 voxel phantoms for both external and internal exposures remain valid in the current ICRP dosimetry system. The Publication 143 voxel phantoms remain the primary ICRP/ICRU reference models for the calculation of reference DCs based on Publication 103 methodology. The paediatric MRCPs will be used for all calculations of reference DCs following the next set of general recommendations and provide a resource for wider use in radiological protection applications