Tetrahedral-mesh counterparts of ICRP reference computational phantoms.

Background / Aims: This talk is to introduce the new adult male and female mesh-type reference computational phantoms (MRCPs) recently developed in the International Commission on Radiological Protection (ICRP), generally discussing their advantages over the current voxel-type reference computational phantoms (VRCPs)[1]. Methods: The MRCPs were constructed by converting the VRCPs to a high-quality tetrahedral-mesh format and addressing the limitations of the VRCPs mostly due to their limited voxel resolutions. The MRCPs were implemented in Monte Carlo codes (i.e., Geant4, MCNP6, and PHITS) to investigate computation speed and memory usage. In addition, dose coefficients (DCs) of the MRCPs for some external and internal exposures were calculated and compared with the current reference DCs produced with the VRCPs and supplemental stylized phantoms[2,3]. Furthermore, the MRCPs, using their high deformability over the VRCPs, were transformed to phantoms in different statures or postures, which were then used to calculate DCs for industrial radiography sources near the body. Results: The MRCPs include all the target and source regions needed for effective dose calculations, even micron-scale regions of the respiratory and alimentary tract organs, urinary bladder, skin, and eye lens, completely obviating the need of supplemental stylized phantoms. PHITS, compared to Geant4 and MCNP6, showed the best performance in both computation speed and memory usage for the MRCPs, which are even faster than the VRCPs. The DCs of the MRCPs were found to be very similar to the current reference DCs for penetrating radiations (e.g., photons and neutrons), but more reliable for weakly penetrating radiations (e.g., electrons and ions). The DCs for industrial radiography sources were found to be significantly influenced by different statures, demonstrating the capability of the MRCPs for personalized dosimetry. Conclusion: The MRCPs, as the next generation of ICRP reference comput

Ambient dose equivalent conversion coefficients for radionuclides exponentially distributed in the ground.

Conversion coefficients of radionuclide deposition density to the ambient dose equivalent rate at 1 m height above ground were calculated for exponentially distributed sources in the ground. First, Monte Carlo transport simulations assuming exponential distributions in the ground were performed to obtain ambient dose equivalent for mono-energetic gamma-ray sources having different relaxation depths; next, on the basis of the simulated data, conversion coefficients for radionuclides were composed considering recent nuclear decay data. The ambient dose equivalent rates were then compared to the effective dose rates for reference adults and a new-born baby as well as to air kerma rates quoted from previous studies. It was confirmed that the ambient dose equivalent sufficiently overestimates effective doses, independently of age, for sources exponentially distributed in the ground. Furthermore, the air kerma was found to also overestimate the effective doses for all ages in the same conditions. In order to verify the computed conversion coefficients, the ratio of ambient dose equivalent to air kerma obtained by simulation was compared to the ratios measured at hundreds of locations in Japan which have been contaminated with radioactive cesium after the accident at the nuclear power plant in Fukushima Prefecture, Japan, in 2011; a good agreement was observed

A software tool for modification of human voxel models used for application in radiation protection.

This note describes a new software tool called 'VolumeChange' that was developed to modify the masses and location of organs of virtual human voxel models. A voxel model is a three-dimensional representation of the human body in the form of an array of identification numbers that are arranged in slices, rows and columns. Each entry in this array represents a voxel; organs are represented by those voxels having the same identification number. With this tool, two human voxel models were adjusted to fit the reference organ masses of a male and a female adult, as defined by the International Commission on Radiological Protection (ICRP). The alteration of an already existing voxel model is a complicated process, leading to many problems that have to be solved. To solve those intricacies in an easy way, a new software tool was developed and is presented here. If the organs are modified, no bit of tissue, i.e. voxel, may vanish nor should an extra one appear. That means that organs cannot be modified without considering the neighbouring tissue. Thus, the principle of organ modification is based on the reassignment of voxels from one organ/tissue to another; actually deleting and adding voxels is only possible at the external surface, i.e. skin. In the software tool described here, the modifications are done by semi-automatic routines but including human control. Because of the complexity of the matter, a skilled person has to validate that the applied changes to organs are anatomically reasonable. A graphical user interface was designed to fulfil the purpose of a comfortable working process, and an adequate graphical display of the modified voxel model was developed. Single organs, organ complexes and even whole limbs can be edited with respect to volume, shape and location

Organ dose conversion coefficients for voxel models of the reference male and female from idealized photon exposures.

A new series of organ equivalent dose conversion coefficients for whole body external photon exposure is presented for a standardized couple of human voxel models, called Rex and Regina. Irradiations from broad parallel beams in antero-posterior, postero-anterior, left- and right-side lateral directions as well as from a 360° rotational source have been performed numerically by the Monte Carlo transport code EGSnrc. Dose conversion coefficients from an isotropically distributed source were computed, too. The voxel models Rex and Regina originating from real patient CT data comply in body and organ dimensions with the currently valid reference values given by the International Commission on Radiological Protection (ICRP) for the average Caucasian man and woman, respectively. While the equivalent dose conversion coefficients of many organs are in quite good agreement with the reference values of ICRP Publication 74, for some organs and certain geometries the discrepancies amount to 30% or more. Differences between the sexes are of the same order with mostly higher dose conversion coefficients in the smaller female model. However, much smaller deviations from the ICRP values are observed for the resulting effective dose conversion coefficients. With the still valid definition for the effective dose (ICRP Publication 60), the greatest change appears in lateral exposures with a decrease in the new models of at most 9%. However, when the modified definition of the effective dose as suggested by an ICRP draft is applied, the largest deviation from the current reference values is obtained in postero-anterior geometry with a reduction of the effective dose conversion coefficient by at most 12%