Development of alimentary tract organs for ICRP pediatric mesh-type reference computational phantoms.

In line with the activities of Task Group 103 under the International Commission on Radiological Protection (ICRP), the present study was conducted to develop a new set of alimentary tract organs consisting of the oral cavity, oesophagus, stomach, small intestine, and colon for the newborn, 1 year-old, 5 year-old, 10 year-old, and 15 year-old males and females for use in the pediatric mesh-type reference computational phantoms (MRCPs). The developed alimentary tract organs of the pediatric MRCPs, while nearly preserving the original topology and shape of those of the pediatric voxel-type reference computational phantoms (VRCPs) of ICRPPublication 143, present considerable anatomical improvement and include all micrometre-scale target and source regions as prescribed in ICRPPublication 100. To investigate the dosimetric impact of the developed alimentary tract organs, organ doses and specific absorbed fractions were computed for certain external exposures to photons and electrons and internal exposures to electrons, respectively, which were then compared with the values computed using the current ICRP models (i.e. pediatric VRCPs and ICRP-100 stylised models). The results showed that for external exposures to penetrating radiations (i.e. photons >0.04 MeV), there was generally good agreement between the compared values, within a 10% difference, except for the oral mucosa. For external exposures to weakly penetrating radiations (i.e. low-energy photons and electrons), there were significant differences, up to a factor of ∼8300, owing to the geometric difference caused by the anatomical enhancement in the MRCPs. For internal exposures of electrons, there were significant differences, the maximum of which reached a factor of ∼73 000. This was attributed not only to the geometric difference but also to the target mass difference caused by the different luminal content mass and organ shape.

Dose conversion coefficients for neutron external exposures with five postures: walking, sitting, bending, kneeling, and squatting.

In a previous study, posture-dependent dose coefficients (DCs) for photon external exposures were calculated using the adult male and female mesh-type reference computational phantoms (MRCPs) of the International Commission on Radiological Protection (ICRP) that had been transformed into five non-standing postures (i.e. walking, sitting, bending, kneeling, and squatting). As an extension, the present study was conducted to establish another DC dataset for external exposures to neutrons by performing Monte Carlo radiation transport simulations with the adult male and female MRCPs in the five non-standing postures. The resulting dataset included the DCs for absorbed doses (i.e., organ/tissue-averaged absorbed doses) delivered to 29 individual organs/tissues, and for effective doses for neutron energies ranging from 10-9 to 104 MeV in six irradiation geometries: antero-posterior (AP), posteroanterior (PA), left-lateral (LLAT), right-lateral (RLAT), rotational (ROT), and isotropic (ISO) geometries. The comparison of DCs for the non-standing MRCPs with those of the standing MRCPs showed significant differences. In the lateral irradiation geometries, for example, the standing MRCPs overestimate the breast DCs of the squatting MRCPs by up to a factor of 4 due to the different arm positions but underestimate the gonad DCs by up to about 17 times due to the different leg positions. The impact of different postures on effective doses was generally less than that on organ doses but still significant; for example, the standing MRCPs overestimate the effective doses of the bending MRCPs only by 20% in the AP geometry at neutron energies less than 50 MeV, but underestimate those of the kneeling MRCPs by up to 40% in the lateral geometries at energies less than 0.1 MeV.

Development of detailed pediatric eye models for lens dose calculations.

The International Commission on Radiological Protection (ICRP) recently reduced the dose limit for the eye lens for occupational exposure from 150 mSv yr-1to 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.

Development of paediatric mesh-type reference computational phantom series of International Commission on Radiological Protection.

Very recently, Task Group 103 of the International Commission on Radiological Protection (ICRP) completed the development of the paediatric mesh-type reference computational phantoms (MRCPs) comprising ten phantoms (newborn, one year-old, five year-old, ten year-old, and fifteen year-old males and females). The paediatric MRCPs address the limitations of ICRPPublication 143's paediatric reference computational phantoms, which are in voxel format, stemming from the nature of the voxel geometry and the limited voxel resolutions. The paediatric 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 region). Besides, the paediatric 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 idealised exposures, the paediatric MRCPs provide very similar effective dose coefficients (DCEs) 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 paediatric 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 DCEs for external exposures with those of the voxel-type reference phantoms.

Detailed tooth models for ICRP mesh-type reference computational 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 energies >3 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.

Development of skeletal systems for ICRP pediatric mesh-type reference computational phantoms.

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 ICRPPublications 110and143due 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.

New small-intestine modeling method for surface-based computational human phantoms.

When converting voxel phantoms to a surface format, the small intestine (SI), which is usually not accurately represented in a voxel phantom due to its complex and irregular shape on one hand and the limited voxel resolutions on the other, cannot be directly converted to a high-quality surface model. Currently, stylized pipe models are used instead, but they are strongly influenced by developer's subjectivity, resulting in unacceptable geometric and dosimetric inconsistencies. In this paper, we propose a new method for the construction of SI models based on the Monte Carlo approach. In the present study, the proposed method was tested by constructing the SI model for the polygon-mesh version of the ICRP reference male phantom currently under development. We believe that the new SI model is anatomically more realistic than the stylized SI models. Furthermore, our simulation results show that the new SI model, for both external and internal photon exposures, leads to dose values that are more similar to those of the original ICRP male voxel phantom than does the previously constructed stylized SI model.

Incorporation of micro-CT-based detailed bone models into ICRP adult mesh-type reference computational phantoms.

Objective.The red bone marrow (RBM) and bone endosteum (BE), which are required for effective dose calculation, are macroscopically modeled in the reference phantoms of the international commission on radiological protection (ICRP) due to their microscopic and complex histology. In the present study, the detailed bone models were developed to simplify the dose calculation process for skeletal dosimetry.Approach.The detailed bone models were developed based on the bone models developed at the University of Florida. A new method was used to update the definition of BE region by storing the BE location indices using virtual sub-voxels. The detailed bone models were then installed in the spongiosa regions of the ICRP mesh-type reference computational phantoms (MRCPs) via the parallel geometry feature of the Geant4 code.Main results.Comparing the results between the detailed-bone-installed MRCPs and the original MRCPs with the absorbed dose to spongiosa and fluence-to-dose response function (DRF)-based methods, the DRF-based method showed much smaller but still significant differences. Compared with the values given in ICRPPublications116 and 133, the differences were very large (i.e. several orders of magnitudes), due mainly to the anatomical improvement of the skeletal system in the MRCPs; that is, spongiosa and medullary cavity are fully enclosed by cortical bone in the MRCPs but not in the ICRP-110 phantoms.Significance.The detailed bone models enable the direct calculation of the absorbed doses to the RBM and BE, simplifying the dose calculation process and potentially improving the consistency and accuracy of skeletal dosimetry.

Body-size-dependent phantom library constructed from ICRP mesh-type reference computational phantoms.

Recently, ICRP Task Group 103 developed new mesh-type reference computational phantoms (MRCPs) for the adult male and female by converting the current voxel-type reference computational phantoms (VRCPs) of ICRP Publication 110 into a high-quality/fidelity mesh format. Utilizing the great deformability/flexibility of the MRCPs compared with the VRCPs, in the present study, we established a body-size-dependent phantom library by modifying the MRCPs. The established library includes 108 adult male and 104 adult female phantoms in different standing heights and body weights, covering most body sizes representative of Caucasian and Asian populations. Ten secondary anthropometric parameters with respect to standing height and body weight were derived from various anthropometric databases and applied in the construction of the phantom library. An in-house program for automatic phantom adjustment was developed and applied for practical construction of such a large number of phantoms in the library with minimized human intervention. Organ/tissue doses calculated with three male phantoms in different standing heights (165, 175, and 190 cm) with a fixed body weight of 80 kg for external exposures to broad parallel photon beams from 0.01 to 104 MeV were compared, observing there are significant dose differences particularly for the photon energies <0.1 MeV in which the organ/tissue doses tended to increase with increasing standing height. In addition, the organ/tissue doses of three female phantoms in different body weights (45, 65, and 140 kg) with a fixed standing height of 165 cm were compared, showing a significant decreasing tendency with increasing body weight for the photon energies <10 MeV. For the higher energies, the opposite trend, interestingly, was observed; that is, the organ/tissue doses tended to increase with increasing body weight. The results, despite the limited number of exposure cases, suggest that the use of the body-size-dependent phantom library can improve the accuracy of individual dose estimates for many retrospective dosimetry studies by taking the body size of individuals into account.

DOSE COEFFICIENTS OF MESH-TYPE ICRP REFERENCE COMPUTATIONAL PHANTOMS FOR EXTERNAL EXPOSURES OF NEUTRONS, PROTONS, AND HELIUM IONS.

To overcome inherent limitations of the Voxel-type Reference Computational Phantoms (VRCPs) due to the limited voxel resolutions and the nature of voxel geometry, the International Commission on Radiological Protection (ICRP) has developed the adult male and female Mesh-type Reference Computational Phantoms (MRCPs). We previously used the MRCPs to calculate a complete set of dose coefficients (DCs) for idealized external exposures of photons and electrons (Yeom et al. NET in press). In the present study, we extended the previous study to include additional radiation particles (neutrons, protons, and helium ions) into the DC library by conducing Monte Carlo radiation transport simulations with the Geant4 code. The MRPC-based DCs were compared with the existing reference DCs of ICRP Publication 116 which are based on the ICRP VRCPs to investigate impact of the new mesh-type reference phantoms on the DC values. We found that the MRCPs generally provide DCs of organ/tissue doses and effective doses similar to those from the VRCPs for penetrating radiations (uncharged particles), whereas significant DC differences were observed for weakly penetrating radiations (charged particles) mainly due to the improved representation of the detailed anatomical structures in the MRCPs over the VRCPs.

Posture-dependent dose coefficients of mesh-type ICRP reference computational phantoms for photon external exposures.

Recently, the International Commission on Radiological Protection (ICRP) developed new mesh-type reference computational phantoms (MRCPs) that provide high deformability compared with the current voxel-type reference computational phantoms of ICRP Publication 110. Taking advantage of this deformability, in the present study, the MRCPs were deformed to five non-standing postures (i.e. walking, sitting, bending, kneeling, and squatting) by developing and using a systematic posture-change method based on the as-rigid-as-possible (ARAP) shape-deformation algorithm and motion-capture technology. The non-standing MRCPs were then implemented in the Geant4 Monte Carlo code to calculate a comprehensive dataset of dose coefficients (DCs) for photon external exposures. These include the dose coefficients for 29 individual organs/tissues and the dose coefficients for effective doses from 0.01 MeV to 10 GeV in the antero-posterior (AP), postero-anterior (PA), left-lateral (LLAT), right-lateral (RLAT), rotational (ROT), and isotropic (ISO) geometries. To investigate the dosimetric impact of posture, the DCs of the non-standing MRCPs were compared with those of the original MRCPs (in the standing posture). The results showed that organ/tissue doses are significantly influenced by posture, with arm position mostly influencing dose to organs/tissues in the torso region and leg position influencing dose in the pelvic region. For most cases, the gonads showed notably large differences, ranging from a few tens of percentage points to several orders of magnitude, depending on posture and irradiation geometry. The effective doses showed much smaller differences than the organ/tissue doses, but they were nonetheless significant: for example, the kneeling MRCPs in the AP geometry showed lower values at energies <10 MeV by up to 30% and greater values at higher energies by up to 40%. The presented results indicate that not only different irradiation geometries, but also different postures might be necessary in DC calculations for reliable dose estimates for radiological protection purposes.

Mesh-type reference Korean phantoms (MRKPs) for adult male and female for use in radiation protection dosimetry.

In the present study, to overcome the dosimetric limitations of the previous voxel-type reference Korean computational phantoms due to their limited voxel resolutions (i.e. on the order of millimeters) and the nature of voxel geometry, a pair of new reference Korean phantoms, called mesh-type reference Korean phantoms (MRKPs), were developed for the adult male and female in a high-quality/fidelity mesh format. The developed phantoms include all target and source regions required for effective dose calculation, even micrometer-scale target and source regions of the respiratory and alimentary tract organs, skin, urinary bladder, and eye lens. The developed phantoms, which are in either the polygon-mesh (PM) format or the tetrahedral-mesh (TM) format as necessary, can be directly used in several general-purpose Monte Carlo codes (e.g. Geant4, MCNP6, and PHITS) without voxelization. In order to understand the dosimetric impact of the new phantoms, the dose coefficients (=fluence-to-effective dose conversion coefficients) were calculated for photons and electrons with energies ranging from 10 keV to 10 GeV for the anterior-posterior (AP) irradiation geometry and compared with those of the previous voxel-type reference Korean phantoms. The results demonstrate that the effective dose coefficients of the MRKPs were generally similar to those of the previous voxel-type reference phantoms for photons; however, for electrons, significant differences were observed at energies lower than 1 MeV that were mainly due to the explicit definition of the 50 µm-thick radiosensitive target layer in the skin of the new mesh phantoms.

Percentile-specific computational phantoms constructed from ICRP mesh-type reference computational phantoms (MRCPs).

Recently, the Task Group 103 of the International Commission on Radiological Protection (ICRP) has developed new mesh-type reference computational phantoms (MRCPs) for adult male and female. When compared to the current voxel-type reference computational phantoms in ICRP Publication 110, the MRCPs have several advantages, including deformability which makes it possible to create phantoms in different body sizes or postures. In the present study, the MRCPs were deformed to produce a set of percentile-specific phantoms representing the 10th, 50th and 90th percentiles of standing height and body weight in Caucasian population. For this, anthropometric parameters for the percentile-specific phantoms were first derived from the anthropometric software and survey data. Then, the MRCPs were modified to match the derived anthropometric parameters. For this, first, the MRCPs were scaled in the axial direction to match the head height, torso length, and leg length. Then, the head, torso, and legs were scaled in the transversal directions to match the lean body mass for the percentile-specific phantoms. Finally, the scaled phantoms were manually adjusted to match the body weight and the remaining anthropometric parameters (upper arm, waist, buttock, thigh, and calf circumferences and sagittal abdominal diameter). The constructed percentile-specific phantoms and the MRCPs were implemented into the Geant4 Monte Carlo code to calculate organ doses for a cesium-137 contaminated floor. The results showed that organ doses of the 50th percentile (both standing height and body weight) phantoms are very close to those of the MRCPs. There were noticeable differences in organ doses, however, for the 10th and 90th percentile phantoms when compared with those of the MRCPs. The results of the present study confirm the general intuition that a small person receives higher doses than a large person when exposed to a static radiation field, and organs closer to the source receive higher doses.

Inclusion of thin target and source regions in alimentary and respiratory tract systems of mesh-type ICRP adult reference phantoms.

It is not feasible to define very small or complex organs and tissues in the current voxel-type adult reference computational phantoms of the International Commission on Radiological Protection (ICRP), which limit dose coefficients for weakly penetrating radiations. To address the problem, the ICRP is converting the voxel-type reference phantoms into mesh-type phantoms. In the present study, as a part of the conversion project, the micrometer-thick target and source regions in the alimentary and respiratory tract systems as described in ICRP Publications 100 and 66 were included in the mesh-type ICRP reference adult male and female phantoms. In addition, realistic lung airway models were simulated to represent the bronchial (BB) and bronchiolar (bb) regions. The electron specific absorbed fraction (SAF) values for the alimentary and respiratory tract systems were then calculated and compared with the values calculated with the stylized models of ICRP Publications 100 and 66. The comparisons show generally good agreement for the oral cavity, oesophagus, and BB, whereas for the stomach, small intestine, large intestine, extrathoracic region, and bb, there are some differences (e.g. up to ~9 times in the large intestine). The difference is mainly due to anatomical difference in these organs between the realistic mesh-type phantoms and the simplified stylized models. The new alimentary and respiratory tract models in the mesh-type ICRP reference phantoms preserve the topology and dimensions of the voxel-type ICRP phantoms and provide more reliable SAF values than the simplified models adopted in previous ICRP Publications.

Development of skeletal system for mesh-type ICRP reference adult phantoms.

The reference adult computational phantoms of the international commission on radiological protection (ICRP) described in Publication 110 are voxel-type computational phantoms based on whole-body computed tomography (CT) images of adult male and female patients. The voxel resolutions of these phantoms are in the order of a few millimeters and smaller tissues such as the eye lens, the skin, and the walls of some organs cannot be properly defined in the phantoms, resulting in limitations in dose coefficient calculations for weakly penetrating radiations. In order to address the limitations of the ICRP-110 phantoms, an ICRP Task Group has been recently formulated and the voxel phantoms are now being converted to a high-quality mesh format. As a part of the conversion project, in the present study, the skeleton models, one of the most important and complex organs of the body, were constructed. The constructed skeleton models were then tested by calculating red bone marrow (RBM) and endosteum dose coefficients (DCs) for broad parallel beams of photons and electrons and comparing the calculated values with those of the original ICRP-110 phantoms. The results show that for the photon exposures, there is a generally good agreement in the DCs between the mesh-type phantoms and the original voxel-type ICRP-110 phantoms; that is, the dose discrepancies were less than 7% in all cases except for the 0.03 MeV cases, for which the maximum difference was 14%. On the other hand, for the electron exposures (⩽4 MeV), the DCs of the mesh-type phantoms deviate from those of the ICRP-110 phantoms by up to ~1600 times at 0.03 MeV, which is indeed due to the improvement of the skeletal anatomy of the developed skeleton mesh models.

Incorporation of detailed eye model into polygon-mesh versions of ICRP-110 reference phantoms.

The dose coefficients for the eye lens reported in ICRP 2010 Publication 116 were calculated using both a stylized model and the ICRP-110 reference phantoms, according to the type of radiation, energy, and irradiation geometry. To maintain consistency of lens dose assessment, in the present study we incorporated the ICRP-116 detailed eye model into the converted polygon-mesh (PM) version of the ICRP-110 reference phantoms. After the incorporation, the dose coefficients for the eye lens were calculated and compared with those of the ICRP-116 data. The results showed generally a good agreement between the newly calculated lens dose coefficients and the values of ICRP 2010 Publication 116. Significant differences were found for some irradiation cases due mainly to the use of different types of phantoms. Considering that the PM version of the ICRP-110 reference phantoms preserve the original topology of the ICRP-110 reference phantoms, it is believed that the PM version phantoms, along with the detailed eye model, provide more reliable and consistent dose coefficients for the eye lens.