Juying Zhang

A boundary-representation method for designing whole-body radiation dosimetry models: pregnant females at the ends of three gestational periods--RPI-P3, -P6 and -P9.

Fetuses are extremely radiosensitive and the protection of pregnant females against ionizing radiation is of particular interest in many health and medical physics applications. Existing models of pregnant females relied on simplified anatomical shapes or partial-body images of low resolutions. This paper reviews two general types of solid geometry modeling: constructive solid geometry (CSG) and boundary representation (BREP). It presents in detail a project to adopt the BREP modeling approach to systematically design whole-body radiation dosimetry models: a pregnant female and her fetus at the ends of three gestational periods of 3, 6 and 9 months. Based on previously published CT images of a 7-month pregnant female, the VIP-Man model and mesh organ models, this new set of pregnant female models was constructed using 3D surface modeling technologies instead of voxels. The organ masses were adjusted to agree with the reference data provided by the International Commission on Radiological Protection (ICRP) and previously published papers within 0.5%. The models were then voxelized for the purpose of performing dose calculations in identically implemented EGS4 and MCNPX Monte Carlo codes. The agreements of the fetal doses obtained from these two codes for this set of models were found to be within 2% for the majority of the external photon irradiation geometries of AP, PA, LAT, ROT and ISO at various energies. It is concluded that the so-called RPI-P3, RPI-P6 and RPI-P9 models have been reliably defined for Monte Carlo calculations. The paper also discusses the needs for future research and the possibility for the BREP method to become a major tool in the anatomical modeling for radiation dosimetry.

RPI-AM and RPI-AF, a pair of mesh-based, size-adjustable adult male and female computational phantoms using ICRP-89 parameters and their calculations for organ doses from monoenergetic photon beams

This paper describes the development of a pair of adult male and adult female computational phantoms that are compatible with anatomical parameters for the 50th percentile population as specified by the International Commission on Radiological Protection (ICRP). The phantoms were designed entirely using polygonal mesh surfaces--a Boundary REPresentation (BREP) geometry that affords the ability to efficiently deform the shape and size of individual organs, as well as the body posture. A set of surface mesh models, from Anatomium 3D P1 V2.0, including 140 organs (out of 500 available) was adopted to supply the basic anatomical representation at the organ level. The organ masses were carefully adjusted to agree within 0.5% relative error with the reference values provided in the ICRP Publication 89. The finalized phantoms have been designated the RPI adult male (RPI-AM) and adult female (RPI-AF) phantoms. For the purposes of organ dose calculations using the MCNPX Monte Carlo code, these phantoms were subsequently converted to voxel formats. Monoenergetic photons between 10 keV and 10 MeV in six standard external photon source geometries were considered in this study: four parallel beams (anterior-posterior, posterior-anterior, left lateral and right lateral), one rotational and one isotropic. The results are tabulated as fluence-to-organ-absorbed-dose conversion coefficients and fluence-to-effective-dose conversion coefficients and compared against those derived from the ICRP computational phantoms, REX and REGINA. A general agreement was found for the effective dose from these two sets of phantoms for photon energies greater than about 300 keV. However, for low-energy photons and certain individual organs, the absorbed doses exhibit profound differences due to specific anatomical features. For example, the position of the arms affects the dose to the lung by more than 20% below 300 keV in the lateral source directions, and the vertical position of the testes affects the dose by more than 80% below 150 keV in the PA source direction. The deformability and adjustability of organs and posture in the RPI adult phantoms may prove useful not only for average workers or patients for radiation protection purposes, but also in studies involving anatomical and posture variability that is important in future radiation protection dosimetry.

A Monte Carlo study of lung counting efficiency for female workers of different breast sizes using deformable phantoms

There are currently no physical phantoms available for calibrating in vivo counting devices that represent women with different breast sizes because such phantoms are difficult, time consuming and expensive to fabricate. In this work, a feasible alternative involving computational phantoms was explored. A series of new female voxel phantoms with different breast sizes were developed and ported into a Monte Carlo radiation transport code for performing virtual lung counting efficiency calibrations. The phantoms are based on the RPI adult female phantom, a boundary representation (BREP) model. They were created with novel deformation techniques and then voxelized for the Monte Carlo simulations. Eight models have been selected with cup sizes ranging from AA to G according to brassiere industry standards. Monte Carlo simulations of a lung counting system were performed with these phantoms to study the effect of breast size on lung counting efficiencies, which are needed to determine the activity of a radionuclide deposited in the lung and hence to estimate the resulting dose to the worker. Contamination scenarios involving three different radionuclides, namely Am-241, Cs-137 and Co-60, were considered. The results show that detector efficiencies considerably decrease with increasing breast size, especially for low energy photon emitting radionuclides. When the counting efficiencies of models with cup size AA were compared to those with cup size G, a difference of up to 50% was observed. The detector efficiencies for each radionuclide can be approximated by curve fitting in the total breast mass (polynomial of second order) or the cup size (power).

An investigation of voxel geometries for MCNP-based radiation dose calculations.

Voxelized geometry such as those obtained from medical images is increasingly used in Monte Carlo calculations of absorbed doses. One useful application of calculated absorbed dose is the determination of fluence-to-dose conversion factors for different organs. However, confusion still exists about how such a geometry is defined and how the energy deposition is best computed, especially involving a popular code, MCNP5. This study investigated two different types of geometries in the MCNP5 code, cell and lattice definitions. A 10 cm × 10 cm × 10 cm test phantom, which contained an embedded 2 cm × 2 cm × 2 cm target at its center, was considered. A planar source emitting parallel photons was also considered in the study. The results revealed that MCNP5 does not calculate total target volume for multi-voxel geometries. Therefore, tallies which involve total target volume must be divided by the user by the total number of voxels to obtain a correct dose result. Also, using planar source areas greater than the phantom size results in the same fluence-to-dose conversion factor.

MO-E-AUD B-05: Development of Whole-Body Phantoms Representing An Average Adult Male and Female Using Surface-Geometry Methods

Purpose: To apply a series of Whole‐Body Phantoms Representing An Average Adult Male and Female Using Surface‐Geometry Methods to the study of external radiation dosimetry.Method and Materials: Boundary reprentation was used to deform the original organs automatically into two sets of standard RPI Adult Male/Female phantoms with volume/mass matched with those of the ICRP. To finally define the phantom geometries in Monte Carlo codes for dose calculations, we developed a software to convert the finished surface phantoms into the voxel phantoms at any desired voxel size. The voxelization used the parity count method together with the method of ray stabbing on polygon surface. The corresponding Monte Carlo input file was derived automatically by our program “Phantom Processor”. Average absorbed doses to organs were obtained by MCNPX.Results: The volume/mass data of the standard RPI Adult Male/Female phantoms match with those of the ICRP. After mesh voxelization, the volume/mass data of the voxel phantoms have the relative error less than 0.5%. The voxel resolutions of the Male/Female are 3.2 mm and 3.0 mm respectively. The average absorbed doses of internal organs were calculated using the 6 external neutron irradiation geometries. All results were normalized by the unit source fluence in accordance with the standard usage in radiation protection dosimetry for reporting fluence to absorbed dose conversion coefficients. Typically, 107 histories were simulated and the uncertainties were better than about 1% for most of the target organs.Conclusion: A series of RPI Adult Male/Female phantoms have been developed. Using our software we have developed additional registration and deformation algorithms that allow a mesh‐based phantom to “morph” into a different individual. This series of phantoms were voxelized and implanted into MCNPX. The results suggest that Monte Carlo calculations can be performed for various internal and external exposures to ionizing radiation.

WE‐E‐BRD‐08: Next‐Generation Deformable Patient Modeling for Monte Carlo Assessment of Organ Doses

Purpose: Whole‐body patient models of various sizes and postures are needed for the assessment of organdoses in CTimaging, internal nuclear medicine and external‐beam radiation treatment procedures. This paper discusses a deformable mesh‐based modeling method to create patient‐specific phantoms that are morphed by changing to 5th‐ to 95th‐percentiles of body height and weight, as well as internal organ volume and masse. Method and Materials: The mesh‐based reference adult male and female phantoms were deformed by mainly two different percentile data: 1) the whole‐body size percentile data which were defined by the anthropometric parameters such as height and weight from the National Health and Nutrition Examination Survey (NHANES). 2) individual internal organ percentile data which were derived by the cumulative pattern analysis based on the International Commission on Radiological Protection (ICRP) 23 and 89 references. These mesh‐based percentile phantoms were converted into the voxel‐based phantoms. The final step is to link the voxel phantom with correct tissue density and elemental composition, so that radiation transport through the human‐body phantom was modeled correctly in a Monte Carlo code. Results: The whole‐body size percentile models have been created by the NHANES anthropometric data and the details of organ percentiles derived from ICRP references. The deformability of the RPI reference adult phantoms has been shown through the demonstration of percentiles‐ and postures‐specific adult models. Conclusion: A next generation deformable patient modeling method has been demonstrated. With the mesh deformation algorithms, the individual organs are able to be deformed to match the volumes and masses with desired organ percentiles. The flexible modeling allows patients to be represented in various sizes and postures for the purpose of Monte Carlodose calculations. This study also identified the need for further research to develop method to run Monte Carlo calculations in mesh geometry directly.

TU‐C‐304A‐03: Stylized MIRD Phantoms Should Be Replaced by Anatomically Realistic Phantoms: Discrepancies In Red Bone Marrow Doses From CT Scans

Purpose: To test the hypothesis that the stylized MIRD phantoms would cause significant error in the estimated red bone marrow (RBM) dose from CT scans in comparison with anatomically realistic phantoms. Method and Materials: The MC model of the CT scanner include the source geometry, movement, source energy spectrum, bow‐tie filter, as well and the beam shape. MCNPX 2.5.0 was used to simulate the RBM dose from various CT scanning procedures. To calculate the absorbed dose to the RBM as a function of photon fluence in the spongiosa and the photon energy, an F4 tally together with a set of DE/DF cards in MCNPX were used to score the photon fluence in MCNPX. The stylized MIRD phantom and the anatomically realistic RPI Adult Male and Adult Female phantoms were implemented in the MCNPX to determine organ doses using the same dose algorithm. Results: For all the cases studied, the RBM doses calculated using RPI adult phantoms were gearter than those obtained from MIRD‐ORNL phantoms. For the chest CT scan, the RBM dose ratio (RPI‐AM to MIRD‐ORNL) is about 1.50 (1.48–1.51), and RBM dose ratio of female phantoms is about 1.28 (1.24–1.32). For the abdominal‐pelvis CT scan, the RBM dose ratios are 1.30 (1.28–1.31) and 1.32 (1.28–1.38) for male and female phantom, respectively. These differences are mainly from the anatomical differences in the phantoms. Conclusion: As the RBM is not uniformly distributed in the human body, the homogeneous bone mixtures definition by MIRD phantoms underestimated the dose by as much as 50% in certain cases. This test concludes that the simplified MIRD phantoms used in existing CT dose software should and can be replaced by realistic phantoms. This is an opportunity to improve the anatomical realism and therefore the associated dose and risk assessments for patients who undergo CT examinations.

SU-GG-BRC-04: Electronic Versus HDR Ir-192 Brachytherapy: Organ Dose Comparisons for Breast Cancer Using a Monte Carlo Patient Phantom

Purpose: To quantify and compare the dose delivered to multiple organs‐at‐risk (OARs) in a female patient undergoing Xoft Axxent electronic (KVB) and high‐dose rate Ir‐192 (IBB) intracavitary balloon brachytherapy for breast cancer.Materials and Methods: A previous study has indicated that the dose to OARs such as the lungs and heart play a critical role in treatment planning. The anatomy of a female patient was represented by an adult female computational phantom which consists of over 140 organs. A balloon was inserted into a lumpectomy cavity in the left breast of the virtual patient. The Monte Carlo N‐Particle eXtended (MCNPX) code was used to simulate photon transport through the patient for hypothetical KVB and IBB scenarios. MCNPXs F6 tally was used to calculate the absorbed dose in organs distant from the treatment site. Results: In general, the KVB organdoses were more than a factor of 2 smaller than those of IBB because the low‐energy x‐rays are less penetrating. The distribution of organdoses shows a profound pattern depending on the distance, location, and organ shape. The largest doses were observed for organs such as the left lung and heart which are closest to the radiation source. For KVB, the doses received by the left lung and heart wall were 9.0% and 5.5% of that received by the planning target volume. These values were 11.0% and 11.3% for the IBB scenario. Conclusions: This paper reports, for the first time, a systematic comparison of multiple organdoses received from KVB and IBB. KVB may have safety advantages because its dose rate falls off faster than for IBB. As a previous clinical study found the target dose to be similar for these two methods, information on how healthy organs are irradiated will help decide when each modality is appropriate.

SU-FF-T-428: Deformable Computational Breast Phantoms for Monte Carlo Based Calibrations of Detector Systems Used for Assessing Internal Radioactivity Burden in the Lungs

Purpose: To demonstrate the feasibility of deformable patient modeling for the virtual calibration of detectors used to measure inhaled radioactivity in a female patients lungs for internal dose assessment. Materials and Methods: We have developed the ability to deform a mesh‐based phantom that consists of 140 highly detailed organs or tissues. The phantom can be adjusted to match a desired patient. A software was developed to deform the breasts of this phantom to create new models representing female patients with different breast cup sizes (ranging from AA to G) and breast glandularities. The geometries of these phantoms and a Phoswhich detector system were defined in a Monte Carlo code for virtual in‐vivo lung counting simulations involving various photon emitting radionuclides. The counting efficiencies for each of the virtual patients were calculated and compared. Results: The counting efficiency was found to decrease with increasing breast size and mass because of greater attenuation in the patient. For low energy emitting radionuclides such as Am‐241, roughly a 50% drop in counting efficiency was observed for the model with G cup size as compared to the smaller breasted AA model. Higher breast glandularities resulted in lower counting efficiencies; however, this effect was small. For the E breast cup size model, the counting efficiency for low energy emitters decreased by roughly 2% as the glandularity increased from 7% to 40%. Conclusions: In order to obtain accurate internal dosimetry estimates for female patients, the in‐vivo measurements of the activity in the lungs should account for breast size. The effect of breast glandularity can be ignored because it is negligibly small compared to other sources of experimental uncertainty.

TU‐C‐304A‐02: The Impact of the New ICRP‐103 Recommendations On the Assessment of Effective Doses From CT Procedures

Purpose: To apply a pair of adult phantoms representing ICRP‐89 50th‐percentile adult males and females to the study of impact of the new ICRP‐103 recommendations on the assessment of Effective Dose from CT procedures. Method and Materials: a pair of mesh‐based computational phantoms, RPI Adult Male (RPI‐AM) and RPI Adult Female (RPI‐AF) that were recently developed to represent the ICRP‐89 50th‐percentile adult males and adult females. This pair of phantoms has the detailed bone structures, including the spongiosa which contains the red bone marrow. The detailed RBM distribution was adjusted according to ICRP Publication 70. The CT scanner model used in this study is an MDCT scanner which includes the source geometry and movement, the source energy spectrum, the bow‐tie filter as well and the beam shape. CT scan protocols including whole body scan were carefully modeled in this study, and tube potential of 120 kVp were considered. All simulations were performed using the Monte Carlo code, MCNPX 2.5.0. The three‐correction factor method was used to calculate the RBM dose. Effective Dose results were calculated following the algorithm from ICRP 103. Results: A new set of organ absorbed dose results has been presented using this pair of new developed reference adult phantoms from CT procedures, as well as the effective dose results. Also the new results of red bone marrow dose have been provided. The recently published ICRP 103 updated the radio‐sensitive organ list; also it improved the algorithm of effective dose calculations. Conclusion: Advanced red bone marrow dose calculation method has been used in this study due to the detailed bone structures of this pair of RPI‐AM and RPI‐AF phantoms. This new set of effective dose dataset based on the new ICRP‐103 recommendations could be used to provide latest information for clinical diagnostic dosimetry area.