Absorbed fractions in a voxel-based phantom calculated with the MCNP-4B code

Simple variance reduction in Monte Carlo calculations of specific absorbed fractions: Russian roulette and splitting at the source organ

PURPOSE

To investigate the capabilities of several variance reduction techniques in the calculation of specific absorbed fractions in cases where the source and the target organs are far away and/or the target organs have a small volume.

METHODS

The specific absorbed fractions have been calculated by using the Monte Carlo code PENELOPE and by assuming the thyroid gland as the source organ and the testicles, the urinary bladder, the uterus, and the ovaries as the target ones. A mathematical anthropomorphic phantom, similar to the MIRD-type phantoms, has been considered. Photons with initial energies of 50, 100 and 500 keV were emitted isotropically from the volume of the source organ. Simulations have been carried out by implementing the variance reduction techniques of splitting and Russian roulette at the source organ only and the interaction forcing at the target organs. The influence of the implementation details of those techniques have been investigated and optimal parameters have been determined. All simulations were run with a CPU time of 1.5 · 10⁵ s.

RESULTS

Specific absorbed fractions with relative uncertainties well below 10% have been obtained in most cases, agreeing with those used as reference. The best value for the factor defining the application of the Russian roulette technique was r = 0.3. The best value for the splitting number was between s = 3 and s = 10, depending on the specific energies and target organs.

CONCLUSIONS

The proposed strategy provides an effective method for computing specific absorbed fractions for the most unfavorable situations, with a computing effort that is considerably reduced with respect to other methodologies.

Stylized versus voxel phantoms: a juxtaposition of organ depth distributions

For external irradiation, the variability in organ dose estimation found between computational phantom generations arises particularly from the differences in organ positioning. This work represents the first effort to quantify the differences in organ depth below the body surface between a stylized and voxel phantom series. Herein, the revised Oak Ridge National Laboratory stylized phantom series and the University of Florida/National Cancer Institute voxel phantom series were compared. Both series include whole-body models of the newborn; the 1-, 5-, 10-, and 15-year-old; and the adult human. Organ depths from eight different directions applicable to external irradiation geometries were computed: antero-posterior, postero-anterior, left and right lateral, rotational, isotropic, cranial and caudal directions. Organ depths in the stylized phantoms were computed using a ray-tracing technique available through Monte Carlo radiation transport simulations in MCNP6. Organ depths in the voxel phantom were found using phantom matrix manipulation. Resultant organ depths for both series were plotted as distributions; available are twenty-four organs and two bone tissue distributions for each of six phantom ages and in each of the eight directional geometries. Quantitative data descriptors (e.g. mean and median depths) were also tabulated. For demonstration purposes, a literature review of relevant stylized versus voxel comparison works was performed to explore where the quantification of organ depth differences can provide further insight or evidence to study conclusions. The entire dataset of organ depth distributions and their data descriptors can be found in online supplementary files.

Development and Validation of RAPID: A Patient-Specific Monte Carlo Three-Dimensional Internal Dosimetry Platform

This work describes the development and validation of a patient-specific Monte Carlo internal dosimetry platform called RAPID (Radiopharmaceutical Assessment Platform for Internal Dosimetry). RAPID utilizes serial PET/CT or SPECT/CT images to calculate voxelized three-dimensional (3D) internal dose distributions with the Monte Carlo code Geant4. RAPID's dosimetry calculations were benchmarked against previously published S-values and specific absorbed fractions (SAFs) calculated for monoenergetic photon and electron sources within the Zubal phantom and for S-values calculated for a variety of radionuclides within spherical tumor phantoms with sizes ranging from 1 to 1000 g. The majority of the S-values and SAFs calculated in the Zubal Phantom were within 5% of the previously published values with the exception of a few 10 keV photon SAFs that agreed within 10%, and one value within 16%. The S-values calculated in the spherical tumor phantoms agreed within 2% for ¹⁷⁷Lu, ¹³¹I, ¹²⁵I, ¹⁸F, and ⁶⁴Cu, within 3.5% for ²¹¹At and ²¹³Bi, within 6.5% for ¹⁵³Sm, ¹¹¹In, ⁸⁹Zr, and ²²³Ra, and within 9% for ⁹⁰Y, ⁶⁸Ga, and ¹²⁴I. In conclusion, RAPID is capable of calculating accurate internal dosimetry at the voxel-level for a wide variety of radionuclides and could be a useful tool for calculating patient-specific 3D dose distributions.

A comparison between GATE4 results and MCNP4B published data for internal radiation dosimetry

Aim: GATE, has been designed as upper layer of the GEANT4 toolkit for nuclear medicine application including internal dosimetry. However, its results have not been fully compared to the well-developed codes and anthropomorphic voxel phantoms have never been used with GATE/GEANT for internal dosimetry. The aim of present study was to compare the internal dose calculated by GATE/GEANT with the MCNP4B published data. Methods: The Zubal phantom was used to model a typical adult male. Activity was assumed uniformly distributed in liver, kidneys, lungs, spleen, pancreas and adrenals. GATE/ GEANT Monte Carlo package was used for estimation of doses in the phantom. Simulations were performed for photon energy of 0.01–1 MeV and mono-energetic electrons of 935 keV. Specific absorbed fractions for photons and S-factors for electrons were calculated. Results: On average, GATE/GEANT produces higher photon SAF (Specific Absorbed Fraction) values (+2.7%) for self-absorption and lower values (-2.9%) for cross-absorption. The difference was higher for paired organs particularly lungs. Moreover the photon SAF values for lungs as source organ at the energy of 200 and 500 keV was considerably higher with MCNP4B compared to GATE. Conclusion: Despite of differences between the GATE4 and MCNP4B, the results can be considered ensuring. This may be considered as validation of GATE/GEANT as a proprietary code in nuclear medicine for radionuclide dosimetry applications.

Specific Absorbed Fractions of Internal Photon and Electron Emitters in a Human Voxel-based Phantom: A Monte Carlo Study

The specific absorbed fraction (SAF) of energy is an essential element of internal dose assessment. Here reported a set of SAFs calculated for selected organs of a human voxel-based phantom. The Monte Carlo transport code GATE version 6.1 was used to simulate monoenergetic photons and electrons with energies ranging from 10 keV to 2 MeV. The particles were emitted from three source organs: kidneys, liver, and spleen. SAFs were calculated for three target regions in the body (kidneys, liver, and spleen) and compared with the results obtained using the MCNP4B and GATE/GEANT4 Monte Carlo codes. For most photon energies, the self-irradiation is higher, and the cross-irradiation is lower in the GATE results compared to the MCNP4B. The results show generally good agreement for photons and high-energy electrons with discrepancies within − 2% ±3%. Nevertheless, significant differences were found for cross-irradiation of photons of lower energy and electrons of higher energy due to statistical uncertainties larger than 10%. The comparisons of the SAF values for the human voxel phantom do not show significant differences, and the results also demonstrated the usefulness and applicability of GATE Monte Carlo package for voxel level dose calculations in nonuniform media. The present SAFs calculation for the Zubal voxel phantom is validated by the intercomparison of the results obtained by other Monte Carlo codes.

A review of the use and potential of the GATE Monte Carlo simulation code for radiation therapy and dosimetry applications

In this paper, the authors' review the applicability of the open-source GATE Monte Carlo simulation platform based on the GEANT4 toolkit for radiation therapy and dosimetry applications. The many applications of GATE for state-of-the-art radiotherapy simulations are described including external beam radiotherapy, brachytherapy, intraoperative radiotherapy, hadrontherapy, molecular radiotherapy, and in vivo dose monitoring. Investigations that have been performed using GEANT4 only are also mentioned to illustrate the potential of GATE. The very practical feature of GATE making it easy to model both a treatment and an imaging acquisition within the same framework is emphasized. The computational times associated with several applications are provided to illustrate the practical feasibility of the simulations using current computing facilities.

Evaluation of specific absorbed fractions from internal photon sources in the ICRP Reference Male Phantom

In order to estimate the dose caused by internal radiation, it is necessary to know the specific absorbed fraction (SAF) values; through this work these values have been calculated using the Adult Male Reference Computational Phantom (RCP-AM) from the Publication 110 of the International Commission Radiologic Protection and the Monte Carlo transport code MCNPX. These values were calculated for a combination of 980 pairs of source and target organs, for a total of 12 energies. The results were validated and compared with the results reported by other authors: Hadid et al. (RCP-AM), Petoussi-Henss and Zankl (Golem) and the Oak Ridge National Laboratory (ORNL) stylised model reported by Cristy and Eckerman. Mostly, the SAF values calculated with the RCP-AM do not present significant differences in relation to its previous model Golem. When comparing the SAF values of RCP-AM with that of the ORNL stylised model, huge differences were found. These differences can be explained by the shape of the organs and their relative positions, which are more realistic in the voxelised phantoms.

Specific absorbed fractions in thyroid diagnostics and treatment: Monte Carlo calculation with PENELOPE

In nuclear medicine, diagnostic and therapy procedures in which a certain radiopharmaceutical is administered to a patient are performed. An important point is the determination of the dose absorbed by the important organs of the patient due to these procedures. This dose depends on the particular radionuclide used and the so-called specific absorbed fractions. In this work, by means of Monte Carlo (MC) simulation, the specific absorbed fractions in case the thyroid gland acts as a source organ and for photon energies between 30 keV and 2 MeV have been determined. The computer code PENELOPE has been used as well as the adult male mathematical phantom provided with the distribution of this code. Three different simulation types were carried out. In one of them, only photon transport was considered. In the other two, electron transport was included, doing a detailed and a mixed simulation. In general, the fractions were estimated with uncertainties <9 %, for the mixed and detailed simulations, and <3 %, for the simulation in which only photons are included. For some target organs and, especially for energies <100 keV, the uncertainties found were larger. The results obtained here have been compared with those obtained by other authors using other MC codes. A good agreement has been found in 80 % of the cases.

Internal photon and electron dosimetry of the newborn patient--a hybrid computational phantom study

Estimates of radiation absorbed dose to organs of the nuclear medicine patient are a requirement for administered activity optimization and for stochastic risk assessment. Pediatric patients, and in particular the newborn child, represent that portion of the patient population where such optimization studies are most crucial owing to the enhanced tissue radiosensitivities and longer life expectancies of this patient subpopulation. In cases where whole-body CT imaging is not available, phantom-based calculations of radionuclide S values--absorbed dose to a target tissue per nuclear transformation in a source tissue--are required for dose and risk evaluation. In this study, a comprehensive model of electron and photon dosimetry of the reference newborn child is presented based on a high-resolution hybrid-voxel phantom from the University of Florida (UF) patient model series. Values of photon specific absorbed fraction (SAF) were assembled for both the reference male and female newborn using the radiation transport code MCNPX v2.6. Values of electron SAF were assembled in a unique and time-efficient manner whereby the collisional and radiative components of organ dose--for both self- and cross-dose terms--were computed separately. Dose to the newborn skeletal tissues were assessed via fluence-to-dose response functions reported for the first time in this study. Values of photon and electron SAFs were used to assemble a complete set of S values for some 16 radionuclides commonly associated with molecular imaging of the newborn. These values were then compared to those available in the OLINDA/EXM software. S value ratios for organ self-dose ranged from 0.46 to 1.42, while similar ratios for organ cross-dose varied from a low of 0.04 to a high of 3.49. These large discrepancies are due in large part to the simplistic organ modeling in the stylized newborn model used in the OLINDA/EXM software. A comprehensive model of internal dosimetry is presented in this study for the newborn nuclear medicine patient based upon the UF hybrid computational phantom. Photon dose response functions, photon and electron SAFs, and tables of radionuclide S values for the newborn child--both male and female--are given in a series of four electronic annexes available at stacks.iop.org/pmb/57/1433/mmedia. These values can be applied to optimization studies of image quality and stochastic risk for this most vulnerable class of pediatric patients.

Comparison of internal dosimetry factors for three classes of adult computational phantoms with emphasis on I-131 in the thyroid

The S values for 11 major target organs for I-131 in the thyroid were compared for three classes of adult computational human phantoms: stylized, voxel and hybrid phantoms. In addition, we compared specific absorbed fractions (SAFs) with the thyroid as a source region over a broader photon energy range than the x- and gamma-rays of I-131. The S and SAF values were calculated for the International Commission on Radiological Protection (ICRP) reference voxel phantoms and the University of Florida (UF) hybrid phantoms by using the Monte Carlo transport method, while the S and SAF values for the Oak Ridge National Laboratory (ORNL) stylized phantoms were obtained from earlier publications. Phantoms in our calculations were for adults of both genders. The 11 target organs and tissues that were selected for the comparison of S values are brain, breast, stomach wall, small intestine wall, colon wall, heart wall, pancreas, salivary glands, thyroid, lungs and active marrow for I-131 and thyroid as a source region. The comparisons showed, in general, an underestimation of S values reported for the stylized phantoms compared to the values based on the ICRP voxel and UF hybrid phantoms and relatively good agreement between the S values obtained for the ICRP and UF phantoms. Substantial differences were observed for some organs between the three types of phantoms. For example, the small intestine wall of ICRP male phantom and heart wall of ICRP female phantom showed up to eightfold and fourfold greater S values, respectively, compared to the reported values for the ORNL phantoms. UF male and female phantoms also showed significant differences compared to the ORNL phantom, 4.0-fold greater for the small intestine wall and 3.3-fold greater for the heart wall. In our method, we directly calculated the S values without using the SAFs as commonly done. Hence, we sought to confirm the differences observed in our S values by comparing the SAFs among the phantoms with the thyroid as a source region for selected target organs--small intestine wall, lungs, pancreas and breast--as well as illustrate differences in energy deposition across the energy range (12 photon energies from 0.01 to 4 MeV). Differences were found in the SAFs between phantoms in a similar manner as the differences observed in S values but with larger differences at lower photon energies. To investigate the differences observed in the S and SAF values, the chord length distributions (CLDs) were computed for the selected source--target pairs and compared across the phantoms. As demonstrated by the CLDs, we found that the differences between phantoms in those factors used in internal dosimetry were governed to a significant degree by inter-organ distances which are a function of organ shape as well as organ location.

Effect of voxel size and computation method on Tc-99m MAA SPECT/CT-based dose estimation for Y-90 microsphere therapy

The use of selective internal radiation therapy for treatment of hepatocellular carcinoma and liver metastases using Y-90 labeled microspheres has become an effective and widely used treatment regimen. However, dosimetric evaluations of this treatment are still primitive as uniform distribution models based only on injected activity are often used. This investigation attempts to quantify the effectiveness of several sophisticated patient-specific techniques which utilize the source distribution of Tc-99m MAA simulation studies to perform voxelized dosimetric computations. Among these techniques are complete Monte-Carlo radiation transport computation in patient-specific CT-based voxel phantoms, local energy deposition in patient specific phantoms and kernel transport techniques in water. Each technique was evaluated using three different phantom voxel dimensions and SPECT reconstruction matrix sizes. Dose evaluation results using all methods were compared to the exact solution, obtained using fully 3-D Monte-Carlo simulations with source distribution based not on SPECT data, but on the injected activity and exact boundaries of the anthropomorphic phantom used in the study. The results of this study show that at large voxel sizes and using SPECT reconstructions with a small matrix size (64 x 64), Monte-Carlo and local deposition methods are nearly equivalent. However, using a large SPECT reconstruction matrix (256 x 256) the local deposition method is significantly more accurate than full 3-D Monte-Carlo transport, and with a negligible computational burden.

Effect of patient morphology on dosimetric calculations for internal irradiation as assessed by comparisons of Monte Carlo versus conventional methodologies

Dosimetric calculations are performed with an increasing frequency before or after treatment in targeted radionuclide therapy, as well as for radiation protection purposes in diagnostic nuclear medicine. According to the MIRD committee formalism, the mean absorbed dose to a target is given by the product of the cumulated activity and a dose-conversion factor, known as the S factor. Standard S factors have been published for mathematic phantoms and for unit-density spheres. The accuracy of the results from the use of these S factors is questionable, because patient morphology can vary significantly. The aim of this work was to investigate differences between patient-specific dosimetric results obtained using Monte Carlo methodology and results obtained using S factors calculated on standard models.

METHODS

The CT images of 9 patients, who ranged in size, were used. Patient-specific S factors for 131I were calculated with the MCNPX2.5.0 Monte Carlo code using a tool for personalized internal dose assessment, OEDIPE; standard S factors from OLINDA/EXM were compared against the patient-specific S factors. Furthermore, realistic biodistributions and cumulated activities for normal organs and tumors were used, and mean organ- and tumor-absorbed doses calculated with OEDIPE and OLINDA/EXM were compared.

RESULTS

The ratio of the standard and the patient-specific S factors were between 0.49 and 1.84 for a target distant from the source for 4 organs and 2 tumors studied as source and targets. For the case of self-irradiation, the equivalent ratio ranged between 0.45 and 2.47 and between 1.00 and 1.06 when mass correction was applied. Differences in mean absorbed doses were as high as 140% when realistic cumulated activity values were used. These values decreased to less than 26% in all cases studied when mass correction was applied to the self-irradiation given by OLINDA/EXM.

CONCLUSION

Standard S factors can yield mean absorbed doses for normal organs or tumors with a reasonable accuracy (26% for the cases studied) as compared with absorbed doses calculated with Monte Carlo, provided that they have been corrected for mass.

Three-dimensional imaging-based radiobiological dosimetry

Targeted radionuclide therapy holds promise as a new treatment for cancer. Advances in imaging are making it possible for researchers to evaluate the spatial distribution of radioactivity in tumors and normal organs over time. Matched anatomical imaging, such as combined single-photon emission computed tomography/computed tomography and positron emission tomography/computed tomography, has also made it possible to obtain tissue density information in conjunction with the radioactivity distribution. Coupled with sophisticated iterative reconstruction algorithms, these advances have made it possible to perform highly patient-specific dosimetry that also incorporates radiobiological modeling. Such sophisticated dosimetry techniques are still in the research investigation phase. Given the attendant logistical and financial costs, a demonstrated improvement in patient care will be a prerequisite for the adoption of such highly-patient specific internal dosimetry methods.

Method for Fast CT/SPECT-Based 3D Monte Carlo Absorbed Dose Computations in Internal Emitter Therapy

The DPM (Dose Planning Method) Monte Carlo electron and photon transport program, designed for fast computation of radiation absorbed dose in external beam radiotherapy, has been adapted to the calculation of absorbed dose in patient-specific internal emitter therapy. Because both its photon and electron transport mechanics algorithms have been optimized for fast computation in 3D voxelized geometries (in particular, those derived from CT scans), DPM is perfectly suited for performing patient-specific absorbed dose calculations in internal emitter therapy. In the updated version of DPM developed for the current work, the necessary inputs are a patient CT image, a registered SPECT image, and any number of registered masks defining regions of interest. DPM has been benchmarked for internal emitter therapy applications by comparing computed absorption fractions for a variety of organs using a Zubal phantom with reference results from the Medical Internal Radionuclide Dose (MIRD) Committee standards. In addition, the β decay source algorithm and the photon tracking algorithm of DPM have been further benchmarked by comparison to experimental data. This paper presents a description of the program, the results of the benchmark studies, and some sample computations using patient data from radioimmunotherapy studies using (131)I.

Treating voxel geometries in radiation protection dosimetry with a patched version of the Monte Carlo codes MCNP and MCNPX

The question of Monte Carlo simulation of radiation transport in voxel geometries is addressed. Patched versions of the MCNP and MCNPX codes are developed aimed at transporting radiation both in the standard geometry mode and in the voxel geometry treatment. The patched code reads an unformatted FORTRAN file derived from DICOM format data and uses special subroutines to handle voxel-to-voxel radiation transport. The various phases of the development of the methodology are discussed together with the new input options. Examples are given of employment of the code in internal and external dosimetry and comparisons with results from other groups are reported.

Specific absorbed fraction for Korean adult voxel phantom from internal photon source

Absorbed fraction (AF) and specific absorbed fraction (SAF) are crucial values for the calculation of radionuclide S-values and consequently for internal dose estimates. The formalism of the Medical Internal Radiation Dose (MIRD) committee of the Society of Nuclear Medicine (SNM) has been utilised as a standard in the calculation of individual organ doses for biologically distributed radionuclides and for different types of radiation. Although those quantities are highly sensitive to individual anatomical difference, the SAF dataset calculated by Caucasian-based stylised phantoms have been applied to Korean population until now. This study was intended to calculate the SAFs by using realistic Korean voxel phantom and Monte Carlo transport technique for the first time and compare the results with those of the existing Caucasian-based data and the Korean stylised phantom published recently. The up-to-date realistic Korean voxel phantom, KTMAN-2, which was developed from computed tomography (CT) images of an average Korean adult male, was employed for Monte Carlo calculation using EGSnrc user-code, developed for the purpose of this study. The SAFs for 32 target organs and tissues from the photon source, uniformly deposited in a total of 37 source organs and tissues, were calculated from KTMAN-2. The results were compared with those for an adult phantom of Oak Ridge National Laboratory (ORNL) and Korean adult stylised phantom. Two major reasons of discrepancy were analysed: (1) racial difference between the Korean and the Caucasian and (2) anatomical difference between stylised and voxel phantoms. When the source organ was identical to the target organ, difference in SAF caused by the difference in target-organ mass between the Korean and the Caucasian phantoms was mainly observed. When the source and target organs were not identical, significant difference in SAF was observed which was mainly attributed to the difference in inter-organ distance and organ shape between voxel and stylised phantoms.

Age-dependent organ and effective dose coefficients for external photons: a comparison of stylized and voxel-based paediatric phantoms

This present study investigates the anatomical realism of conventional stylized models of children by comparing organ dose conversion coefficients for the ORNL paediatric phantom series with those determined in the UF (University of Florida) voxel paediatric phantoms. The latter includes whole-body models of a 9 month male, 4 year female, 8 year female, 11 year male and a 14 year male. Of these phantoms, the 1 year, 5 year and 10 year ORNL phantoms, and 9 month male, 4 year female and 11 year male UF voxel phantoms were selected for side-by-side comparisons under idealized external photon irradiation. Organ absorbed dose per unit air kerma (Gy/Gy) for various radiosensitive organs and tissues were calculated for monoenergetic photons over the energy range of 15 keV to 10 MeV and for six irradiation geometries: anterior-posterior (AP), posterior-anterior (PA), right lateral (RLAT), left lateral (LLAT), rotational (ROT) and isotropic (ISO). Differences in organ dose conversion coefficients for the gonads, bone marrow, colon, lung and stomach, to which prominent tissue weighting factors are assigned, were depicted and analysed. Two major causes of observed differences were suggested: differences in organ shape and position and the differences in tissue shielding by overlying tissue regions within the phantoms. Significant discrepancies caused by anatomical differences between the two types of phantoms are also reported for several organs, and in particular, the thyroid and urinary bladder. The results of this study suggest that the paediatric series of ORNL phantoms also have less realistic internal organ and body anatomy and that dose conversion coefficients from these stylized phantoms should be re-evaluated using paediatric voxel phantoms.

Validation of a personalized dosimetric evaluation tool (Oedipe) for targeted radiotherapy based on the Monte Carlo MCNPX code

Dosimetric studies are necessary for all patients treated with targeted radiotherapy. In order to attain the precision required, we have developed Oedipe, a dosimetric tool based on the MCNPX Monte Carlo code. The anatomy of each patient is considered in the form of a voxel-based geometry created using computed tomography (CT) images or magnetic resonance imaging (MRI). Oedipe enables dosimetry studies to be carried out at the voxel scale. Validation of the results obtained by comparison with existing methods is complex because there are multiple sources of variation: calculation methods (different Monte Carlo codes, point kernel), patient representations (model or specific) and geometry definitions (mathematical or voxel-based). In this paper, we validate Oedipe by taking each of these parameters into account independently. Monte Carlo methodology requires long calculation times, particularly in the case of voxel-based geometries, and this is one of the limits of personalized dosimetric methods. However, our results show that the use of voxel-based geometry as opposed to a mathematically defined geometry decreases the calculation time two-fold, due to an optimization of the MCNPX2.5e code. It is therefore possible to envisage the use of Oedipe for personalized dosimetry in the clinical context of targeted radiotherapy.

Comparison between effective doses for voxel-based and stylized exposure models from photon and electron irradiation

For the last two decades, the organ and tissue equivalent dose as well as effective dose conversion coefficients recommended by the International Commission on Radiological Protection (ICRP) have been determined with exposure models based on stylized MIRD5-type phantoms representing the human body with its radiosensitive organs and tissues according to the ICRP Reference Man released in Publication No. 23, on Monte Carlo codes sometimes simulating rather simplified radiation physics and on tissue compositions from different sources. Meanwhile the International Commission on Radiation Units and Measurements (ICRU) has published reference data for human tissue compositions in Publication No. 44, and the ICRP has released a new report on anatomical reference data in Publication No. 89. As a consequence many of the components of the traditional stylized exposure models used to determine the effective dose in the past have to be replaced: Monte Carlo codes, human phantoms and tissue compositions. This paper presents results of comprehensive investigations on the dosimetric consequences to be expected from the replacement of the traditional stylized exposure models by the voxel-based exposure models. Calculations have been performed with the EGS4 Monte Carlo code for external and internal exposures to photons and electrons with the stylized, gender-specific MIRD5-type phantoms ADAM and EVA on the one hand and with the recently developed tomographic or voxel-based phantoms MAX and FAX on the other hand for a variety of exposure conditions. Ratios of effective doses for the voxel-based and the stylized exposure models will be presented for external and internal exposures to photons and electrons as a function of the energy and the geometry of the radiation field. The data indicate that for the exposure conditions considered in these investigations the effective dose may change between +60% and -50% after the replacement of the traditional exposure models by the voxel-based exposure models.

Chord‐based versus voxel‐based methods of electron transport in the skeletal tissues

Anatomic models needed for internal dose assessment have traditionally been developed using mathematical surface equations to define organ boundaries, shapes, and their positions within the body. Many researchers, however, are now advocating the use of tomographic models created from segmented patient computed tomography (CT) or magnetic resonance (MR) scans. In the skeleton, however, the tissue structures of the bone trabeculae, marrow cavities, and endosteal layer are exceedingly small and of complex shape, and thus do not lend themselves easily to either stylistic representations or in-vivo CT imaging. Historically, the problem of modeling the skeletal tissues has been addressed through the development of chord-based methods of radiation particle transport, as given by studies at the University of Leeds (Leeds, U.K.) using a 44-year male subject. We have proposed an alternative approach to skeletal dosimetry in which excised sections of marrow-intact cadaver spongiosa are imaged directly via microCT scanning. The cadaver selected for initial investigation of this technique was a 66-year male subject of nominal body mass index (22.7 kg m(-2)). The objectives of the present study were to compare chord-based versus voxel-based methods of skeletal dosimetry using data from the UF 66-year male subject. Good agreement between chord-based and voxel-based transport was noted for marrow irradiation by either bone surface or bone volume sources up to 500-1000 keV (depending upon the skeletal site). In contrast, chord-based models of electron transport yielded consistently lower values of the self-absorbed fraction to marrow tissues than seen under voxel-based transport at energies above 100 keV, a feature directly attributed to the inability of chord-based models to account for nonlinear electron trajectories. Significant differences were also noted in the dosimetry of the endosteal layer (for all source tissues), with chord-based transport predicting a higher fraction of energy deposition than given by voxel-based transport (average factor of about 1.6). The study supports future use of voxel-based skeletal models which (1) permit nonlinear electron trajectories across the skeletal tissues, (2) do not rely on mathematical algorithms for treating the endosteal tissue layer, and (3) do not implicitly assume independence of marrow and bone trajectories as is the case for chord-based skeletal models.

OEDIPE: A Personalized Dosimetric Tool Associating Voxel-Based Models with MCNPX

AIM

A new tool, named OEDIPE (a French acronym that stands for "Tool for Personalized Internal Dose Assessment") was developed to carry out personalized internal dosimetry calculations for nuclear medicine (for both therapeutic and diagnostic procedures) and for radiation safety (in the case of internal contamination). It was developed under the PV-Wave visual data analysis system by the Institute of Radioprotection and Nuclear Safety (IRSN) in collaboration with the French Institute of Health and Medical Research (INSERM). This software creates anthropomorphic voxel-based phantoms from computed tomography (CT) and magnetic resonance imaging (MRI) patient images through the use of a friendly graphical user interface (GUI). Several tools have been built-in to allow for image segmentation. Source data, including VOI localization and cumulated activities, are assessed by single photon emission computed tomography (SPECT) images, and the source may be specified in any number of organs either as a point source or a homogeneously distributed source. It is also possible to choose the dosimetric parameters required for the study (mean organ dose or a dose distribution). Phantom, source, and dosimetric parameters are automatically written into a file. That file is then processed by the Monte Carlo code MCNPX (LANL) to perform the actual dose calculation.

RESULTS

OEDIPE can compute either the absorbed dose in each organ (in a few minutes), or the absorbed dose in each voxel of the phantom (i.e. the spatial dose distribution at a tissue level) in a few hours or more. OEDIPE automatically reads the MCNPX output file and processes results to give a list of absorbed doses in each organ or a plot of isodose curves superimposed onto the phantom. Because of the long calculation times required to compute an absorbed dose within an entire whole-body phantom at a spatial resolution of a few millimeters, modifications were made to reduce computational times to reasonable values. To illustrate this tool, results of a dosimetric study of technetium-99m labeling of a bone-scanning agent are presented.

CONCLUSION

OEDIPE is a tool that can be used for patient-specific dosimetry--for example, in targeted radiotherapy--by taking into account the individual patient anatomy, including tumors.

S-values calculated from a tomographic head/brain model for brain imaging

A tomographic head/brain model was developed from the Visible Human images and used to calculate S-values for brain imaging procedures. This model contains 15 segmented sub-regions including caudate nucleus, cerebellum, cerebral cortex, cerebral white matter, corpus callosum, eyes, lateral ventricles, lenses, lentiform nucleus, optic chiasma, optic nerve, pons and middle cerebellar peduncle, skull CSF, thalamus and thyroid. S-values for C-11, O-15, F-18, Tc-99m and I-123 have been calculated using this model and a Monte Carlo code, EGS4. Comparison of the calculated S-values with those calculated from the MIRD (1999) stylized head/brain model shows significant differences. In many cases, the stylized head/brain model resulted in smaller S-values (as much as 88%), suggesting that the doses to a specific patient similar to the Visible Man could have been underestimated using the existing clinical dosimetry.

Estimation of Patient Dose from Radiopharmaceuticals Using Voxel Models

The aim of this study was to demonstrate the advantages of patient dosimetry using voxel models and to present sets of dose estimates for patients of different gender and size. These models offer greater realism with respect to organ shape and topology than the well-established Medical Internal Radiation Dose (MIRD)-type mathematical models. At the National Research Centre for Environment and Health (GSF), specific absorbed fractions have been previously calculated for 4 male and 3 female voxel models, representing different age and stature, for a wide range of source organs. For this study, estimates both for established and new radiopharmaceuticals were performed using biokinetic data from International Commission on Radiological Protection (ICRP). The above calculations allowed for comparison to the MIRD technique in relation to the resulting absorbed organ and effective doses. Furthermore, data sets representing a range of voxel phantoms were investigated. It was found that dose differences among the voxel models can amount up to a factor of 3.

Neutron-fluence-to-dose conversion coefficients in an anthropomorphic phantom

A set of fluence-to-effective-dose conversion coefficients has been calculated for neutrons with energies <20 MeV using a high-resolution anthropomorphic phantom (Zubal model) and the MCNPX code. The calculation used 13 monodirectional monoenergetic neutron beams in the energy range 10(-9) to 20 MeV, under three different source irradiation configurations: anterior-posterior, posterior-anterior and left lateral. Dose calculations were performed for 18 selected organs of the body, for which the International Commission on Radiological Protection and the International Commission on Radiological Units and Measurements have set tissue weighting factors for the determination of the effective dose. Another set of neutron-fluence-to-effective-dose conversion coefficients was also calculated with the proposed modification wR from ICRP Publication 92. From comparison between the dose results calculated and the data reported for the MIRD and VIPMAN models, it can be concluded that, although some discrepancies exist between the Zubal model and the two other models, there is good agreement in the left lateral irradiation geometry.

Dosimetric comparison of Monte Carlo codes (EGS4, MCNP, MCNPX) considering external and internal exposures of the Zubal phantom to electron and photon sources

This paper aims at comparing dosimetric assessments performed with three Monte Carlo codes: EGS4, MCNP4c2 and MCNPX2.5e, using a realistic voxel phantom, namely the Zubal phantom, in two configurations of exposure. The first one deals with an external irradiation corresponding to the example of a radiological accident. The results are obtained using the EGS4 and the MCNP4c2 codes and expressed in terms of the mean absorbed dose (in Gy per source particle) for brain, lungs, liver and spleen. The second one deals with an internal exposure corresponding to the treatment of a medullary thyroid cancer by 131I-labelled radiopharmaceutical. The results are obtained by EGS4 and MCNPX2.5e and compared in terms of S-values (expressed in mGy per kBq and per hour) for liver, kidney, whole body and thyroid. The results of these two studies are presented and differences between the codes are analysed and discussed.

Current developments at IRSN on computational tools dedicated to assessing doses for both internal and external exposure

The paper presents the OEDIPE (French acronym that stands for tool for personalised internal dose assessment) and SESAME (for simulation of external source accident with medical images) computational tools, dedicated to internal and external dose assessment, respectively, and currently being developed at the Institute for Radiological Protection and Nuclear Safety. The originality of OEDIPE and SESAME, by using voxel phantoms in association with Monte Carlo codes, lies in their ability to construct personalised voxel phantoms from medical images and automatically generate the Monte Carlo input file and visualise the expected results. OEDIPE simulates in vivo measurements to improve their calibration, and calculates the dose distribution taking both internal contamination and internal radiotherapy cases into account. SESAME enables radiological overexposure doses to be reconstructed, as also victim, source and accident environment modelling. The paper presents the principles on which these tools function and an overview of specificities and results linked to their fields of application.

A stylized computational model of the head for the reference Japanese male

Computational models of human anatomy, along with Monte Carlo radiation transport simulations, have been used by Snyder et al. [MIRD Pamphlet No. 5, revised (The Society of Nuclear Medicine, New York, 1978)], Cristy and Eckerman [ORNL/TM-8381/VI, Oak Ridge National Laboratory, Oak Ridge, TN (1987)] and Zubal et al. [Med. Phys. 21, 299-302 (1994)] to estimate internal organ doses from internal and external radiation sources. These were created using physiological data from Caucasoid subjects but not from other races. There is a need for research to determine whether the obvious differences from the Caucasoid anatomy make these models unsuitable for estimating the absorbed dose in other races such as the Mongoloid. We used the cranial region of the adult Japanese male to represent the Mongoloid race. This region contains organs that are highly sensitive to radiation. The cranial region of a physical phantom produced by KYOTO KAGAKU Co., LTD. using numerical data from a Japanese Reference Man [Tanaka, Nippon Acta. Radiol. 48, 509-513 (1988)] was used to supply the data for the geometry of a stylized computational model. Our computational model was constructed with equations rather than voxel-based, in order to deal with as small a number of parameters as possible in the computer simulation experiment. The accuracy of our computational model was checked by comparing simulated experimental results obtained with MCNP4C with actual doses measured with thermoluminescence dosimeters (TLDs) inside the physical phantom from which our computational model was constructed. The TLDs, whose margin of error is less than +/-10%, were arranged at six positions. Co-60 was used as the radiation source. The irradiated dose was 2 Gy in terms of air kerma. In the computer simulation experiments, we used our computational model and Cristy's computational model, whose component data are those of the tissue substitute materials and of the human body as published in ICRU Report 46. The observed absorbed dose values (Gy) at all six points were calculated as the percentage difference between MCNP4C simulation and the TLDs. In our computational model, the average values of all the percentage differences were 6.0+/-4.0% (tissue substitute materials) and 7.6+/-6.6% (ICRU Report 46), respectively. In Cristy's model, the corresponding values were 20.4+/-3.8% (tissue substitute materials) and 21.0+/-4.1% (ICRU Report 46), respectively. Considering the margin of error in the radiation sensitivity of the TLDs, this study validates our computational model as a test object for radiation dosimetry studies.

Korean adult male voxel model KORMAN segmented from magnetic resonance images

A voxel model of Korean adult male, KORMAN, was developed by processing whole-body magnetic resonance (MR) images of a healthy volunteer who represents an approximately average Korean in height and weight. Layer by layer the MR images were semi-automatically segmented and indexed using a graphic software and digitizer to construct data arrays consisting of 250 x 120 x 170 voxels of a size of 2 x 2 x 10 mm3. To assess the utility of the model, some illustrative dosimetric calculations were made to obtain organ absorbed doses and effective doses to the KORMAN placed in broad parallel photon fields with energies ranging from 0.05 to 10 MeV. The results were compared with those based on the medical internal radiation dose (MIRD)-type models given in ICRP74. The effective doses of ICRP74 were higher than those of KORMAN with percent differences ranging from 6% (LLAT, 10 MeV) to 30% (PA, 0.05 MeV). Significant differences of more than 40% were observed in organ absorbed doses for some organs including bone surface (AP), stomach (PA), and testes (LAT) for low photon energy. These are mainly caused by difference in trunk thickness between MIRD-type model and KORMAN, and differences in organ positions in the body.

Preliminary results of 3D dose calculations with MCNP-4B code from a SPECT image

Interface software was developed to generate the input file to run Monte Carlo MCNP-4B code from medical image in Interfile format version 3.3. The software was tested using a spherical phantom of tomography slides with known cumulated activity distribution in Interfile format generated with IMAGAMMA medical image processing system. The 3D dose calculation obtained with Monte Carlo MCNP-4B code was compared with the voxel S factor method. The results show a relative error between both methods less than 1 %.

MAX meets ADAM: a dosimetric comparison between a voxel-based and a mathematical model for external exposure to photons

The International Commission on Radiological Protection intends to revise the organ and tissue equivalent dose conversion coefficients published in various reports. For this purpose the mathematical human medical internal radiation dose (MIRD) phantoms, actually in use, have to be replaced by recently developed voxel-based phantoms. This study investigates the dosimetric consequences, especially with respect to the effective male dose, if not only a MIRD phantom is replaced by a voxel phantom, but also if the tissue compositions and the radiation transport codes are changed. This task will be resolved by systematically replacing in the mathematical ADAM/GSF exposure model, first the radiation transport code, then the tissue composition and finally the phantom anatomy, in order to arrive at the voxel-based MAX/EGS4 exposure model. The results show that the combined effect of these replacements can decrease the effective male dose by up to 25% for external exposures to photons for incident energies above 30 keV for different field geometries, mainly because of increased shielding by a heterogeneous skeleton and by the overlying adipose and muscle tissue, and also because of the positions internal organs have in a realistically designed human body compared to their positions in the mathematically constructed phantom.

A review of US anthropometric reference data (1971–2000) with comparisons to both stylized and tomographic anatomic models

Two classes of anatomic models currently exist for use in both radiation protection and radiation dose reconstruction: stylized mathematical models and tomographic voxel models. The former utilize 3D surface equations to represent internal organ structure and external body shape, while the latter are based on segmented CT or MR images of a single individual. While tomographic models are clearly more anthropomorphic than stylized models, a given model's characterization as being anthropometric is dependent upon the reference human to which the model is compared. In the present study, data on total body mass, standing/sitting heights and body mass index are collected and reviewed for the US population covering the time interval from 1971 to 2000. These same anthropometric parameters are then assembled for the ORNL series of stylized models, the GSF series of tomographic models (Golem, Helga, Donna, etc), the adult male Zubal tomographic model and the UF newborn tomographic model. The stylized ORNL models of the adult male and female are found to be fairly representative of present-day average US males and females, respectively, in terms of both standing and sitting heights for ages between 20 and 60-80 years. While the ORNL adult male model provides a reasonably close match to the total body mass of the average US 21-year-old male (within approximately 5%), present-day 40-year-old males have an average total body mass that is approximately 16% higher. For radiation protection purposes, the use of the larger 73.7 kg adult ORNL stylized hermaphrodite model provides a much closer representation of average present-day US females at ages ranging from 20 to 70 years. In terms of the adult tomographic models from the GSF series, only Donna (40-year-old F) closely matches her age-matched US counterpart in terms of average body mass. Regarding standing heights, the better matches to US age-correlated averages belong to Irene (32-year-old F) for the females and Golem (38-year-old M) for the males. Both Helga (27-year-old F) and Donna, however, provide good matches to average US sitting heights for adult females, while Golem and Otoko (male of unknown age) yield sitting heights that are slightly below US adult male averages. Finally, Helga is seen as the only GSF tomographic female model that yields a body mass index in line with her average US female counterpart at age 26. In terms of dose reconstruction activities, however, all current tomographic voxel models are valuable assets in attempting to cover the broad distribution of individual anthropometric parameters representative of the current US population. It is highly recommended that similar attempts to create a broad library of tomographic models be initiated in the United States and elsewhere to complement and extend the limited number of tomographic models presently available for these efforts.

All about MAX: a male adult voxel phantom for Monte Carlo calculations in radiation protection dosimetry

The MAX (Male Adult voXel) phantom has been developed from existing segmented images of a male adult body, in order to achieve a representation as close as possible to the anatomical properties of the reference adult male specified by the ICRP. The study describes the adjustments of the soft-tissue organ masses, a new dosimetric model for the skin, a new model for skeletal dosimetry and a computational exposure model based on coupling the MAX phantom with the EGS4 Monte Carlo code. Conversion coefficients between equivalent dose to the red bone marrow as well as effective MAX dose and air-kerma free in air for external photon irradiation from the front and from the back, respectively, are presented and compared with similar data from other human phantoms.