Pregnant-female Mesh-type Reference Computational Phantoms

Abstract

Following the issuance of the 2007 Recommendations, the Commission released the adult and paediatric voxel-type reference computational phantoms in Publications 110 and 143, respectively, to be used in calculations of reference dose coefficients. While providing anatomically improved representation of the anatomy over the mathematical equation-based stylised phantoms, the voxel phantoms still have limitations in representing small and thin organs, due mainly to the finite voxel resolution. Therefore, additional supplementary stylised phantoms have to be utilised to estimate the doses for the micrometre-thick source and target layers of the alimentary and respiratory tracts, skin, eye lens and urinary bladder. Acknowledging the limitations of the voxel geometry, the Commission decided to develop mesh-type reference computational phantoms (MRCPs). The adult and paediatric MRCPs were developed and released in Publications 145 and 156, respectively. This publication reports the development of the pregnant-female MRCPs representing the pregnant women with the male or female fetus at fetal ages of 8, 10, 15, 20, 25, 30, 35 and 38 weeks (corresponding to gestational ages of 10, 12, 17, 22, 27, 32, 37 and 40 weeks). These phantoms reflect changes in the maternal anatomy such as changes in maternal weight, blood volume, rib cage morphology, spinal curvature, pelvic inclination, breast morphology and the circumferences of chest, waist, arms and legs and the developing fetal anatomy such as crown-rump length, biparietal diameter, head circumference, femur length and foot length and developing organ systems. Like the adult and paediatric MRCPs, the pregnant-female MRCPs include the necessary source and target tissues defined by the Commission for equivalent dose calculations including the micrometre-thick source and target layers. These phantoms were developed in the tetrahedral mesh format and can be implemented in Monte Carlo codes such as Geant4, PHITS, MCNP6 and EGSnrc without voxelisation, fully maintaining the high fidelity of the mesh geometry. The pregnant-female MRCPs, unlike the adult and paediatric MRCPs, are the first reference computational phantoms for pregnant females and therefore will also be used in the calculation of Publication 103-based dose coefficients, in addition to all the future calculations of reference dose coefficients following the next set of general recommendations. To support the estimation of Publication 103-based internal dose coefficients, this publication provides the SAFs for the entire fetus and/or individual fetal organs as target regions for three selected fetal ages (i.e. 8, 25 and 38 weeks) which were calculated using the pregnant-female MRCPs. The fetal SAFs were found to show fetal-age-dependent dose trends (i.e. consistent increase or decrease with the fetal age), depending on the particle type (e.g. photons and electrons), energy and target-source distance.