Chansoo Choi

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 developers 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.

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.

Continuously Deforming 4D Voxel Phantom for Realistic Representation of Respiratory Motion in Monte Carlo Dose Calculation

We propose a new type of computational phantom, the “4D voxel phantom,” for realistic modeling of continuous respiratory motion in Monte Carlo dose calculation. In this phantom, continuous respiratory motion is realized by linear interpolation of the deformation vector fields (DVFs) between the neighboring original phases in the 4D CT data of a patient and by subsequent application of the DVFs to the phase images or to the reference image to produce multiple inter-phase images between the neighboring original phase images. A 4D voxel phantom is a combination of high-temporal-resolution voxel phantoms and on-the-fly dose registration to the reference phase image. In the course of particle transport simulation, the dose or deposited energy is directly registered to the reference phase image on-the-fly (i.e., after each event) using a DVF for dose registration. In the present study, we investigated two methods - DRP (DIR [deformable image registration] with respect to Reference Phase image) and DNP (DIR with respect to Neighboring original Phase image) - for production of multiple inter-phase images or high-temporal-resolution voxel phantoms. Utilizing these two methods, two 4D voxel phantoms each with 100 phases were produced from the original 10-phase images of the 4D CT data of a real patient in order to compare the two methods and to test the feasibility of the 4D voxel phantom methodology in general. We found that it is possible to produce a 4D voxel phantom very rapidly (i.e., <;40 min on a 4-core personal computer for a 100-phase phantom) in a fully automated process. The dose calculation results showed that the constructed 100-phase 4D voxel phantoms provide cumulative-dose distributions very similar to those of the conventional 10-phase approach for stationary proton-beam irradiation. The passing rates of the dose distributions of the 4D voxel phantoms were higher than 99.9% according to the 3% and 3 mm gamma criteria, which results validate the 4D voxel phantom methodology. The point-and dose-tracking analysis data showed that the DRP method, which uses the minimal number of DIR operations but uses inverse DVFs, provides significantly better results than those of the DNP method, which uses only DIR to generate the DVFs for inter-phase image generation and dose registration. The present study also showed that the computation time does not significantly increase when the number of phases in the 4D voxel phantom is increased for more realistic representation of continuous respiratory motion; the only significant increase is in the memory occupancy, which grows almost linearly with the number of phases.

Methods, techniques and recent results in Monte Carlo simulation validation for sensitive applications

Concepts, methods and tools relevant to assessing the reliability of Monte Carlo simulation in sensitive experimental applications are briefly reviewed. An overview of ongoing research in the domain of simulation validation and related activities, such as software quality evaluation and uncertainty quantification, is summarized.

Testable physics by design

The ability to test scientific software needs to be supported by adequate software design. Legacy software systems are often characterized by the difficulty to test parts of the software, mainly due to existing dependencies on other parts. Methods to improve the testability of physics software are discussed, along with open issues specific to physics software for Monte Carlo particle transport. The discussion is supported by examples drawn from the experience with validating Geant4 physics.