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The International Commission on Radiological Protection (ICRP) recently reduced the dose limit for the eye lens for occupational exposure from 150 mSv yr−1 to 20 mSv yr−1, as averaged over defined periods of five years, with no annual dose in a single year exceeding 50 mSv, emphasizing the importance of the accurate estimation of lens dose. In the present study, for more accurate lens dosimetry, detailed eye models were developed for children and adolescents (newborns and 1, 5, 10, and 15 year olds), which were then incorporated into the pediatric mesh-type reference computational phantoms (MRCPs) and used to calculate lens dose coefficients (DCs) for photon and electron exposures. Finally, the calculated values were compared with those calculated with the adult MRCPs in order to determine the age dependence of the lens DCs. For photon exposures, the lens DCs of the pediatric MRCPs showed some sizable differences from those of the adult MRCPs at very low energies (10 and 15 keV), but the differences were all less than 35%, except for the posterior-anterior irradiation geometry, for which the lens dose is not of primary concern. For electron exposures, much larger differences were found. For the anterior-posterior (AP) and isotropic irradiation geometries, the largest differences between the lens DCs of the pediatric and adult phantoms were found in the energy range of 0.6–1 MeV, where the newborn lens DCs were larger by up to a factor of ∼5 than the adult. The lens DCs of the present study, which were calculated for the radiosensitive region of the lens, also were compared with those for the entire lens in the AP irradiation geometry. Our results showed that the DCs of the entire lens were similar to those of the radiosensitive region for 0.02–2 MeV photons and >2 MeV electrons, but that for the other energy ranges, significant differences were noticeable, i.e. 10%–40% for photons and up to a factor of ∼5 for electrons.

In 2016, the International Commission on Radiological Protection (ICRP) launched Task Group 103 (TG 103) for the explicit purpose of developing a new generation of adult and pediatric reference computational phantoms, named ‘mesh-type reference computational phantoms (MRCPs)’, that can overcome the limitations of voxel-type reference computational phantoms (VRCPs) of ICRP Publications 110 and 143 due to their finite voxel resolutions and the nature of voxel geometry. After completing the development of the adult MRCPs, TG 103 has started the development of pediatric MRCPs comprising 10 phantoms (male and female versions of the reference newborn, 1-year-old, 5-year-old, 10-year-old, and 15-year-old). As part of the TG 103 project, within the present study, the skeletal systems, one of the most important and complex organ systems of the body, were developed for each phantom age and sex. The developed skeletal systems, while closely preserving the original bone topology of the pediatric VRCPs, present substantial improvements in the anatomy of complex and/or small bones. In order to investigate the dosimetric impact of the developed skeletons, the average absorbed doses and the specific absorbed fractions for radiosensitive skeletal tissues (i.e. active marrow and bone endosteum) were computed for some selected external and internal exposure cases, which were then compared with those calculated with the skeletons of pediatric VRCPs. The comparison result showed that the dose values of the pediatric MRCPs were generally similar to those of the pediatric VRCPs for highly penetrating radiations (e.g. photons >200 keV); however, for weakly penetrating radiations (e.g. photons ⩽200 keV and electrons), significant differences up to a factor of 140 were observed.

As a geometrical format for computational human phantoms, tetrahedral mesh (TM) is known to have significant advantages over polygonal mesh (PM), including higher compatibility with Monte Carlo radiation transport codes, higher computation speed, and the capability of modeling heterogeneous density variation in an organ of the phantom. In the present study, a computer program named POLY2TET was developed to convert the format of computational human phantoms from PM to TM and generate a sample source code or input file, as applicable, for the converted phantom to be used in some general-purpose Monte Carlo radiation transport codes (i.e. Geant4, PHITS, and MCNP6). The developed program was then tested using four existing high-fidelity PM phantoms. The computation speed, memory requirement, and initialisation time of the generated TM phantoms were also measured and compared with those of the original PM phantoms in Geant4. From the results of our test, it was concluded that the developed program successfully converts PM phantoms into the TM format. The organ doses calculated using the generated TM phantom for the three Monte Carlo codes all produced essentially identical dose values to those for the original PM phantoms in Geant4. The comparison of computation speed showed that compared to the original PM phantoms in Geant4, the TM phantoms in the three Monte Carlo codes were much faster in transporting the particles considered in the present study, i.e. by up to ∼2600 times for electron beams simulated in PHITS. The comparison of the memory requirement showed that the TM phantoms required more memory than the original PM phantoms, but, except for MCNP6, the memory required for the TM phantoms was still less than 12 GB, which typically is available in personal computers these days. For MCNP6, the required memory was much higher, i.e. 60-70 GB.

Very recently, Task Group 103 of the International Commission on Radiological Protection (ICRP) completed the development of the pediatric Mesh-type Reference Computational Phantoms (MRCPs) comprising ten phantoms (newborn, 1-year-old, 5-year-old, 10-year-old, and 15-year-old males and females). The pediatric MRCPs address the limitations of ICRPPublication 143's pediatric reference computational phantoms, which are in voxel format, stemming from the nature of the voxel geometry and the limited voxel resolutions. The pediatric MRCPs were constructed by converting the voxel-type reference phantoms to a high-quality mesh format with substantial enhancements in the detailed anatomy of the small and complex organs and tissues (e.g., bones, lymphatic nodes, and extra-thoracic (ET) region). Besides, the pediatric MRCPs were developed in consideration of the intra-organ blood contents and by modelling the micron-thick target and source regions of the skin, lens, urinary bladder, alimentary tract organs, and respiratory tract organs prescribed by the ICRP. For external idealized exposures, the pediatric MRCPs provide very similar effective dose coefficients to those from the ICRP-143 phantoms but significantly different values for weakly penetrating radiations (e.g., the difference of ~20,000 times for 10-keV electron beams). This paper introduces the developed pediatric MRCPs with a brief explanation of the construction process. Then, it discusses their computational performance in Geant4, PHITS, and MCNP6 in terms of memory usage and computation speed and their impact on dose calculations by comparing their calculated values of effective dose coefficients for external exposures with those of the voxel-type reference phantoms.

For use in electron paramagnetic resonance dosimetry with tooth enamel, in the present study, very detailed mesh-type tooth models composed of 198 individual tooth models (i.e. newborn: 20; 1 year: 28; 5 years: 48; 10 years: 38; 15 years: 32; and adult: 32) were developed for each sex. The developed tooth models were then implanted in the International Commission on Radiological Protection pediatric and adult mesh-type reference computational phantoms and used to calculate tooth enamel doses, by Monte Carlo simulations with Geant4, for external photon exposures in several idealized irradiation geometries. The calculated dose values were then compared to investigate the dependency of the enamel dose on the age and sex of the phantom and the sites of the teeth. The results of the present study generally show that, if the photon energy is low (i.e.0.1 MeV), the enamel dose is significantly affected by the age and sex of the phantom and also the sites of the teeth used for dose calculation; the differences are frequently greater than a few times or even orders of magnitude. However, with a few exceptions, the enamel dose was hardly affected by these parameters for energies between 0.1 and 3 MeV. For energies3 MeV, moderate differences were observed (i.e., up to a factor of two), due to the existence of dose build-up in the head of the phantom for high-energy photons. The calculated dose values were also compared with those of the previous studies where voxel and mathematical models were used to calculate the enamel doses. The results again show significant differences at low energies, e.g., up to ∼3500 times at 0.015 MeV, which are mainly due to the differences in the level of tooth-modeling detailedness.

This paper proposes a low latency and low complexity signal detection method for overloaded MU-MIMO OFDM (Multi-User Multi-Input Multi-Output) signals using a convex optimization approach for uplink IoT (Internet of Things) environments, in which there are a large number of IoT terminals and a base station having smaller number of antennas than that of the terminals. Simulation results demonstrate the validity of the proposed signal detection method.