Comparison of neutron organ and effective dose coefficients for PIMAL stylized phantom in bent postures in standard irradiation geometries

Neutron dose coefficients for standard irradiation geometries have been reported in International Commission on Radiological Protection (ICRP) Publication 116 for the ICRP Publication 110 adult reference phantoms. In the present work, organ and effective dose coefficients have been calculated for a receptor in both upright and articulated (bent) postures representing more realistic working postures exposed to a mono-energetic neutron radiation field. This work builds upon prior work by Dewji and co-workers comparing upright and bent postures for exposure to mono-energetic photon fields. Simulations were conducted using the Oak Ridge National Laboratory’s articulated stylized adult phantom, “Phantom wIth Moving Arms and Legs” (PIMAL) software package, and the Monte Carlo N-Particle (MCNP) version 6.1.1 radiation transport code. Organ doses were compared for the upright and bent (45° and 90°) phantom postures for neutron energies ranging from 1 × 10^??9 to 20 MeV for the ICRP Publication 116 external exposure geometries—antero-posterior (AP), postero-anterior (PA), and left and right lateral (LLAT, RLAT). Using both male and female phantoms, effective dose coefficients were computed using ICRP Publication 103 methodology. The resulting coefficients for articulated phantoms were compared to those of the upright phantom. Computed organ and effective dose coefficients are discussed as a function of neutron energy, phantom posture, and source irradiation geometry. For example, it is shown here that for the AP and PA irradiation geometries, the differences in the organ coefficients between the upright and bent posture become more pronounced with increasing bending angle. In the AP geometry, the brain dose coefficients are expectedly higher in the bent postures than in the upright posture, while all other organs have lower dose coefficients, with the thyroid showing the greatest difference. Overall, the effective dose estimated for the upright phantom is more conservative than that for the articulated phantom, which may have ramifications in the estimation or reconstruction of radiation doses.     

Feasibility of reducing differences in estimated doses in nuclear medicine between a patient-specific and a reference phantom

The feasibility of reducing the differences between patient-specific internal doses and doses estimated using reference phantoms was evaluated. Relatively simple adjustments to a polygon-surface ICRP adult male reference phantom were applied to fit selected individual dimensions using the software Rhinoceros®4.0. We tested this approach on two patient-specific phantoms: the biggest and the smallest phantoms from the Helmholtz Zentrum München library. These phantoms have unrelated anatomy and large differences in body-mass-index. Three models approximating each patient’s anatomy were considered: the voxel and the polygon-surface ICRP adult male reference phantoms and the adjusted polygon-surface reference phantom. The Specific Absorbed Fractions (SAFs) for internal photon and electron sources were calculated with the Monte Carlo code EGSnrc. Employing the time-integrated activity coefficients of a radiopharmaceutical (S)-4-(3-¹⁸F-fluoropropyl)-l-glutamic acid and the calculated SAFs, organ absorbed-dose coefficients were computed following the formalism promulgated by the Committee on Medical Internal Radiation Dose. We compared the absorbed-dose coefficients between each patient-specific phantom and other models considered with emphasis on the cross-fire component. The corresponding differences for most organs were notably lower for the adjusted reference models compared to the case when reference models were employed. Overall, the proposed approach provided reliable dose estimates for both tested patient-specific models despite the pronounced differences in their anatomy. To capture the full range of inter-individual anatomic variability more patient-specific phantoms are required. The results of this test study suggest a feasibility of estimating patient-specific doses within a relative uncertainty of 25% or less using adjusted reference models, when only simple phantom scaling is applied.     

Comparison of photon organ and effective dose coefficients for PIMAL stylized phantom in bent positions in standard irradiation geometries

Computational phantoms with articulated arms and legs have been constructed to enable the estimation of radiation dose in different postures. Through a graphical user interface, the Phantom wIth Moving Arms and Legs (PIMAL) version 4.1.0 software can be employed to articulate the posture of a phantom and generate a corresponding input deck for the Monte Carlo N-Particle (MCNP) radiation transport code. In this work, photon fluence-to-dose coefficients were computed using PIMAL to compare organ and effective doses for a stylized phantom in the standard upright position with those for phantoms in realistic work postures. The articulated phantoms represent working positions including fully and half bent torsos with extended arms for both the male and female reference adults. Dose coefficients are compared for both the upright and bent positions across monoenergetic photon energies: 0.05, 0.1, 0.5, 1.0, and 5.0 MeV. Additionally, the organ doses are compared across the International Commission on Radiological Protection’s standard external radiation exposure geometries: antero-posterior, postero-anterior, left and right lateral, and isotropic (AP, PA, LLAT, RLAT, and ISO). For the AP and PA irradiation geometries, differences in organ doses compared to the upright phantom become more profound with increasing bending angles and have doses largely overestimated for all organs except the brain in AP and bladder in PA. In LLAT and RLAT irradiation geometries, energy deposition for organs is more likely to be underestimated compared to the upright phantom, with no overall change despite increased bending angle. The ISO source geometry did not cause a significant difference in absorbed organ dose between the different phantoms, regardless of position. Organ and effective fluence-to-dose coefficients are tabulated. In the AP geometry, the effective dose at the 45° bent position is overestimated compared to the upright phantom below 1 MeV by as much as 27% and 82% in the 90° position. The effective dose in the 45° bent position was comparable to that in the 90° bent position for the LLAT and RLAT irradiation geometries. However, the upright phantom underestimates the effective dose to PIMAL in the LLAT and RLAT geometries by as much as 30% at 50 keV.     

Evaluation of conversion coefficients from measurable to risk quantities for external exposure over contaminated soil by use of physical human phantoms

Conversion coefficients from measurable quantities such as air kerma free-in-air or personal dose equivalent to effective dose were determined by phantom experiments. Heterogenic anthropomorphic phantoms representing children of one and five years age, and a Rando phantom representing an adult were exposed in the open field contaminated by different levels of radiocesium in the upper soil layer, in a forest site and inside a wooden house. LiF thermoluminescent (TL) detectors were used inside the phantoms for the estimation of organ doses and effective dose. Personal dosimeters similar to those used in radiation protection for individual dose measurements were placed onto the phantom surface (chest area). The ratios of dose values in separate organs to air kerma free-in-air varied from 0.69 to 1.15 for the children phantoms, and from 0.55 to 0.94 for the adult phantom, respectively, when irradiated in the open field. Body size (weight) was found to be the most important factor influencing the values of the conversion coefficients. The differences observed can reach approximately 40% when comparing conversion factors from air kerma free-in-air to effective dose for adults and newborns. For conversion coefficients from personal dose to effective dose, these differences can reach approximately 15%. The dependences of the various conversion coefficients on body mass were quantified by regression analysis. The results were compared with those calculated for a plane mono-energetic photon source having an energy of 700 keV and being located in the ground at a depth of 0.5 g cm⁻². Calculated and measured conversion coefficients from air kerma free-in-air to effective dose agreed within 12%.     

Effective dose rate coefficients for exposure to contaminated soil

The Oak Ridge National Laboratory Center for Radiation Protection Knowledge has undertaken calculations related to various environmental exposure scenarios. A previous paper reported the results for submersion in radioactive air and immersion in water using age-specific mathematical phantoms. This paper presents age-specific effective dose rate coefficients derived using stylized mathematical phantoms for exposure to contaminated soils. Dose rate coefficients for photon, electron, and positrons of discrete energies were calculated and folded with emissions of 1252 radionuclides addressed in ICRP Publication 107 to determine equivalent and effective dose rate coefficients. The MCNP6 radiation transport code was used for organ dose rate calculations for photons and the contribution of electrons to skin dose rate was derived using point-kernels. Bremsstrahlung and annihilation photons of positron emission were evaluated as discrete photons. The coefficients calculated in this work compare favorably to those reported in the US Federal Guidance Report 12 as well as by other authors who employed voxel phantoms for similar exposure scenarios.     

Validation of the MC-GPU Monte Carlo code against the PENELOPE/penEasy code system and benchmarking against experimental conditions for typical radiation qualities and setups in interventional radiology and cardiology

IntroductionInterventional procedures are associated with potentially high radiation doses to the skin. The 2013/59/EURATOM Directive establishes that the equipment used for interventional radiology must have a device or a feature informing the practitioner of relevant parameters for assessing patient dose at the end of the procedure. Monte Carlo codes of radiation transport are considered to be one of the most reliable tools available to assess doses. However, they are usually too time consuming for use in clinical practice. This work presents the validation of the fast Monte Carlo code MC-GPU for application in interventional radiology.MethodologiesMC-GPU calculations were compared against the well-validated Monte Carlo simulation code PENELOPE/penEasy by simulating the organ dose distribution in a voxelized anthropomorphic phantom. In a second phase, the code was compared against thermoluminescent measurements performed on slab phantoms, both in a calibration laboratory and at a hospital.ResultsThe results obtained from the two simulation codes show very good agreement, differences in the output were within 1%, whereas the calculation time on the MC-GPU was 2500 times shorter. Comparison with measurements is of the order of 10%, within the associated uncertainty.ConclusionsIt has been verified that MC-GPU provides good estimates of the dose when compared to PENELOPE program. It is also shown that it presents very good performance when assessing organ doses in very short times, less than one minute, in real clinical set-ups. Future steps would be to simulate complex procedures with several projections.     

External exposure of deciduous tooth enamel to photons: dose conversion coefficients for standard radiation fields

Dose conversion coefficients for teeth of children were computed for external photon sources by means of Monte Carlo methods using a modified MIRD-type mathematical phantom of a 5-year-old child. The tooth region is separated into eight smaller regions that represent incisors, canines, first and second molars. Each of these sub-regions is separated into enamel and dentin parts. Dose conversion coefficients were computed as ratio of absorbed dose in the enamel and air kerma. They are given for unidirectional (AP, PA, RLAT, LLAT), rotational (ROT) and isotropic (ISO) photon sources in the energy range from 10 keV to 10 MeV. All computations were performed with the MCNP4 code including coupled electron-photon transport. The computed coefficients demonstrate a significant non-linearity versus photon energy, which is more pronounced than that observed for adult phantoms. Due to this non-linearity, use of the EPR-measured doses in human teeth requires information on the incident photon fluence spectra. The data presented can be used for assessment of public exposure.     

基于体素模型的中子外照射小鼠器官剂量分布

获得外照射条件下动物的器官剂量,为生物效应评价、剂量-效应关系研究提供准确的剂量信息和依据,是近年来辐射生物效应研究的热点.本文基于建立的一个质量为26.9g的小鼠体素模型（体素精度为0.2mm×0.2mm×0.2mm,体素数量为9424000）,利用蒙特卡罗方法计算了5种照射几何条件（左侧向、右侧向、腹背向、背腹向和各向同性）下,中子能量为10-9～20MeV共37个能量点的小鼠器官剂量转换系数,基于此套数据,在照射条件已知情况下,可以获得小鼠的器官剂量;分析讨论了照射几何条件、中子能量及器官的位置对小鼠器官剂量的影响.     

Generating and using patient-specific whole-body models for organ dose estimates in CT with increased accuracy: Feasibility and validation

The estimation of patient dose using Monte Carlo (MC) simulations based on the available patient CT images is limited to the length of the scan. Software tools for dose estimation based on standard computational phantoms overcome this problem; however, they are limited with respect to taking individual patient anatomy into account. The purpose of this study was to generate whole-body patient models in order to take scattered radiation and over-scanning effects into account. Thorax examinations were performed on three physical anthropomorphic phantoms at tube voltages of 80 kV and 120 kV; absorbed dose was measured using thermoluminescence dosimeters (TLD). Whole-body voxel models were built as a combination of the acquired CT images appended by data taken from widely used anthropomorphic voxel phantoms. MC simulations were performed both for the CT image volumes alone and for the whole-body models. Measured and calculated dose distributions were compared for each TLD chip position; additionally, organ doses were determined.MC simulations based only on CT data underestimated dose by 8%–15% on average depending on patient size with highest underestimation values of 37% for the adult phantom at the caudal border of the image volume. The use of whole-body models substantially reduced these errors; measured and simulated results consistently agreed to better than 10%.This study demonstrates that combined whole-body models can provide three-dimensional dose distributions with improved accuracy. Using the presented concept should be of high interest for research studies which demand high accuracy, e.g. for dose optimization efforts.     

Dose coefficients of percentile-specific computational phantoms for photon external exposures

The use of dose coefficients (DCs) based on the reference phantoms recommended by the International Commission on Radiological Protection (ICRP) with a fixed body size may produce errors to the estimated organ/tissue doses to be used, for example, for epidemiologic studies depending on the body size of cohort members. A set of percentile-specific computational phantoms that represent 10th, 50th, and 90th percentile standing heights and body masses in adult male and female Caucasian populations were recently developed by modifying the mesh-type ICRP reference computational phantoms (MRCPs). In the present study, these percentile-specific phantoms were used to calculate a comprehensive dataset of body-size-dependent DCs for photon external exposures by performing Monte Carlo dose calculations with the Geant4 code. The dataset includes the DCs of absorbed doses for 29 individual organs/tissues from 0.01 to 104 MeV photon energy, in the antero-posterior, postero-anterior, right lateral, left lateral, rotational, and isotropic geometries. The body-size-dependent DCs were compared with the DCs of the MRCPs in the reference body size, showing that the DCs of the MRCPs are generally similar to those of the 50th percentile standing height and body mass phantoms over the entire photon energy region except for low energies (≤ 0.03 MeV); the differences are mostly less than 10%. In contrast, there are significant differences in the DCs between the MRCPs and the 10th and 90th percentile standing height and body mass phantoms (i.e., H10M10 and H90M90). At energies of less than about 10 MeV, the MRCPs tended to under- and over-estimate the organ/tissue doses of the H10M10 and H90M90 phantoms, respectively. This tendency was revised at higher energies. The DCs of the percentile-specific phantoms were also compared with the previously published values of another phantom sets with similar body sizes, showing significant differences particularly at energies below about 0.1 MeV, which is mainly due to the different locations and depths of organs/tissues between the different phantom libraries. The DCs established in the present study should be useful to improve the dosimetric accuracy in the reconstructions of organ/tissue doses for individuals in risk assessment for epidemiologic investigations taking body sizes into account.     

The construction of computer tomographic phantoms and their application in radiology and radiation protection

Summary In order to assess human organ doses for risk estimates under natural and man made radiation exposure conditions, human phantoms have to be used. As an improvement to the mathematical anthropomorphic phantoms, a new family of phantoms is proposed, constructed from computer tomographic (CT) data. A technique is developed which allows any physical phantom to be converted into computer files to be used for several applications. The new human phantoms present advantages towards the location and shape of the organs, in particular the hard bone and bone marrow. The CT phantoms were used to construct three dimensional images of high resolution; some examples are given and their potential is discussed. The use of CT phantoms is also demonstrated to assess accurately the proportion of bone marrow in the skeleton. Finally, the use of CT phantoms for Monte Carlo (MC) calculations of doses resulting from various photon exposures in radiology and radiation protection is discussed.Dedicated to Prof. W. Jacobi on the occasion of his 60th birthday     

Comparison of specific absorbed fractions (SAF) in the Brazilian adult male and the reference man phantoms.

The mathematical phantom of the Brazilian man was developed because many anatomical differences exist between South Americans, Europeans and North Americans. The objective of this work was to compare specific absorbed fractions (SAF) obtained for a model of the Brazilian adult male with those for the reference adult calculated by Snyder et al. in 1974 and to evaluate the importance of these new values in calculating radiation doses in diagnosis and therapy. The length and mass of the total body for the Brazilian man phantom were obtained from tables provided by the Brazilian government (IBGE) in which the masses of organs were measured atautopsy. Monte Carlo methods (using the ALGAM-97 computer code) were applied to calculate SAF for internal organs and the total body. The mathematical phantom designed by Snyder et al. represents very closely Reference Man, as defined in ICRP publication 23. SAF for the whole body were not more than 15% different between the two phantoms. The differences between both models are more significant for individual organs. When the source organ is the lung and red marrow is the target, for initial photon energy of 10 keV, the results obtained indicate that marrow receives 64% more dose in Brazilian model than in the Reference Man model. Eighty tables were made for 97 distinct organs (target-source) and the comparison made between the, Brazilian man and Reference man.     

A method for calculating the dose absorbed by terrestrial animals to assess the environmental impact of radioactive contamination

A method for calculating the exposures of terrestrial animals in areas contaminated with radionuclides using a point source dose function is presented. To take into account scattered γ-radiation, the Berger formula for dose buildup factor in an infinite air medium has been parameterized. In the dosimetric model proposed, an animal phantom is presented as a parallelepiped to estimate external exposures and as a tissue-quivalent sphere to estimate internal doses. Using analytical expressions, dose rate conversion coefficients for external and internal exposures of animals have been estimated for individual radionuclides. For energies of γ-rays above 50 keV, the results are in good agreement with those estimated by the Monte Carlo method for ellipsoidal phantoms of animals.     

Effective dose conversion coefficients for radionuclides exponentially distributed in the ground

In order to provide fundamental data required for dose evaluation due to environmental exposures, effective dose conversion coefficients, that is, the effective dose rate per unit activity per unit area, were calculated for a number of potentially important radionuclides, assuming an exponential distribution in ground, over a wide range of relaxation depths. The conversion coefficients were calculated for adults and a new-born baby on the basis of dosimetric methods that the authors and related researchers have previously developed, using Monte Carlo simulations and anthropomorphic computational phantoms. The differences in effective dose conversion coefficients due to body size between the adult and baby phantoms were found to lie within 50?%, for most cases; however, for some low energies, differences could amount to a factor of 3. The effective dose per unit source intensity per area was found to decrease by a factor of 2–5, for increasing relaxation depths from 0 to 5?g/cm², above a source energy of 50?keV. It is also shown that implementation of the calculated coefficients into the computation of the tissue weighting factors and the adult reference computational phantoms of ICRP Publication 103 does not significantly influence the effective dose conversion coefficients of the environment. Consequently, the coefficients shown in this paper could be applied for the evaluation of effective doses, as defined according to both recommendations of ICRP Publications 103 and 60.     

Development of Monte Carlo based real-time treatment planning system with fast calculation algorithm for boron neutron capture therapy

PurposeWe simulated the effect of patient displacement on organ doses in boron neutron capture therapy (BNCT). In addition, we developed a faster calculation algorithm (NCT high-speed) to simulate irradiation more efficiently.MethodsWe simulated dose evaluation for the standard irradiation position (reference position) using a head phantom. Cases were assumed where the patient body is shifted in lateral directions compared to the reference position, as well as in the direction away from the irradiation aperture.For three groups of neutron (thermal, epithermal, and fast), flux distribution using NCT high-speed with a voxelized homogeneous phantom was calculated. The three groups of neutron fluxes were calculated for the same conditions with Monte Carlo code. These calculated results were compared.ResultsIn the evaluations of body movements, there were no significant differences even with shifting up to 9 mm in the lateral directions. However, the dose decreased by about 10% with shifts of 9 mm in a direction away from the irradiation aperture.When comparing both calculations in the phantom surface up to 3 cm, the maximum differences between the fluxes calculated by NCT high-speed with those calculated by Monte Carlo code for thermal neutrons and epithermal neutrons were 10% and 18%, respectively. The time required for NCT high-speed code was about 1/10th compared to Monte Carlo calculation.ConclusionsIn the evaluation, the longitudinal displacement has a considerable effect on the organ doses.We also achieved faster calculation of depth distribution of thermal neutron flux using NCT high-speed calculation code.     

Variation in lunar neutron dose estimates

The radiation environment on the Moon includes albedo neutrons produced by primary particles interacting with the lunar surface. In this work, HZETRN2010 is used to calculate the albedo neutron contribution to effective dose as a function of shielding thickness for four different space radiation environments and to determine to what extent various factors affect such estimates. First, albedo neutron spectra computed with HZETRN2010 are compared to Monte Carlo results in various radiation environments. Next, the impact of lunar regolith composition on the albedo neutron spectrum is examined, and the variation on effective dose caused by neutron fluence-to-effective dose conversion coefficients is studied. A methodology for computing effective dose in detailed human phantoms using HZETRN2010 is also discussed and compared. Finally, the combined variation caused by environmental models, shielding materials, shielding thickness, regolith composition and conversion coefficients on the albedo neutron contribution to effective dose is determined. It is shown that a single percentage number for characterizing the albedo neutron contribution to effective dose can be misleading. In general, the albedo neutron contribution to effective dose is found to vary between 1-32%, with the environmental model, shielding material and shielding thickness being the driving factors that determine the exact contribution. It is also shown that polyethylene or other hydrogen-rich materials may be used to mitigate the albedo neutron exposure.     

Using matrix summation method for three dimensional dose calculation in brachytherapy

AimThe purpose of this study is to calculate radiation dose around a brachytherapy source in a water phantom for different seed locations or rotation the sources by the matrix summation method.BackgroundMonte Carlo based codes like MCNP are widely used for performing radiation transport calculations and dose evaluation in brachytherapy. But for complicated situations, like using more than one source, moving or rotating the source, the routine Monte Carlo method for dose calculation needs a long time running.Materials and methodsThe MCNPX code has been used to calculate radiation dose around a ¹⁹²Ir brachytherapy source and saved in a 3D matrix. Then, we used this matrix to evaluate the absorbed dose in any point due to some sources or a source which shifted or rotated in some places by the matrix summation method.ResultsThree dimensional (3D) dose results and isodose curves were presented for ¹⁹²Ir source in a water cube phantom shifted for 10 steps and rotated for 45 and 90° based on the matrix summation method. Also, we applied this method for some arrays of sources.ConclusionThe matrix summation method can be used for 3D dose calculations for any brachytherapy source which has moved or rotated. This simple method is very fast compared to routine Monte Carlo based methods. In addition, it can be applied for dose optimization study.     

Occupational exposures during abdominal fluoroscopically guided interventional procedures for different patient sizes — A Monte Carlo approach

In this study we evaluated the occupational exposures during an abdominal fluoroscopically guided interventional radiology procedure. We investigated the relation between the Body Mass Index (BMI), of the patient, and the conversion coefficient values (CC) for a set of dosimetric quantities, used to assess the exposure risks of medical radiation workers. The study was performed using a set of male and female virtual anthropomorphic phantoms, of different body weights and sizes. In addition to these phantoms, a female and a male phantom, named FASH3 and MASH3 (reference virtual anthropomorphic phantoms), were also used to represent the medical radiation workers. The CC values, obtained as a function of the dose area product, were calculated for 87 exposure scenarios. In each exposure scenario, three phantoms, implemented in the MCNPX 2.7.0 code, were simultaneously used. These phantoms were utilized to represent a patient and medical radiation workers. The results showed that increasing the BMI of the patient, adjusted for each patient protocol, the CC values for medical radiation workers decrease. It is important to note that these results were obtained with fixed exposure parameters.