EGSnrc Code Research Articles

Thermoplastic polymers as water substitutes

Background. New applications of 3D printing have recently appeared in the fields of radiotherapy and radiology, but the knowledge of many radiological characteristics of the compounds involved is still limited. Therefore, studies are needed to improve our understanding about the transport and interaction of ionizing radiation in these materials. Purpose. The purpose of this study is to perform an analysis of the most important radiation interaction parameters in thermoplastic materials used in Fused Deposition Modeling 3D printing. Additionally, we propose improvements to bring their characteristics closer to those of water and use them as water substitutes in applications such as radiodiagnosis, external radiotherapy, and brachytherapy. Methods. We have calculated different magnitudes as mass linear attenuation, mass energy absorption coefficients, as well as stopping power and electronic density of several thermoplastic materials along with various compounds that have been used as water substitutes and in a new proposed blend. To perform these computations, we have used the XCOM and ESTAR databases from NIST and the EGSnrc code for Montecarlo simulations. Results. From the representation of the calculated interaction parameters, we have been able to establish relationships between their properties and the proportion of certain chemical elements. In addition, studying these same characteristics in different commercial solutions used as substitutes for water phantoms allows us to extrapolate improvements for these polymers. Conclusion. The radiological characteristics of the analyzed thermoplastic materials can be improved by adding some chemical elements with atomic numbers higher than oxygen and by using polyethylene in new blends.

Exposure computational models with voxel phantoms coupled to EGSnrc Monte Carlo code

In computational dosimetry of ionizing radiation, the energy deposited in radiosensitive organs and tissues is evaluated when an anthropomorphic simulator (phantom) is irradiated using Exposure Computational Models (ECMs). An ECM is a virtual scene with a phantom positioned mathematically relative to a radioactive source. The initial state includes information like the type of primary particle, its energy, starting point coordinates, and direction. Subsequently, robust Monte Carlo (MC) codes are used to simulate the particle's mean free path, interaction with the medium's atoms, and energy deposition. These are common steps for simulations involving photons and/or primary electrons. The GDN (Research Group on Numerical Dosimetry and the Research Group on Computational Dosimetry and Embedded Systems) has published ECMs with voxel phantoms irradiated by photons using the MC code EGSnrc. This work has led to specific computational tools development for various numerical dosimetry stages, including input file preparation, ECM execution, and result analysis. Since 2004, the GDN developed in-house applications like FANTOMAS, CALDose_X, DIP, and MonteCarlo. Certain previously used phantoms are reintroduced to provide historical context in the ECMs' production timeline, emphasizing additive modifications inherent in systematic theme studies. The dosimetric evaluations used the binary version of the MASH (Male Adult mesh) phantom, converted to the SID (Dosimetric Information System) text file type. This format has been used by the group since 2021 to couple a voxel phantom to the EGSnrc user code. The ECM included an environmental dosimetry problem simulation. Most of these tools are accessible on the GDN page (http://dosimetrianumerica.org).

Electron modes simulation of a Siemens-Primus-Linac at 10, 12, and 14 MeV with various field sizes

The electron mode of a Siemens Primus linear accelerator (SPLA) is simulated using Monte Carlo (MC) simulation, BEAMnrc, with various field sizes 10 × 10, 15 × 15, and 20 × 20 cm2 at different e-beam energies of 10, 12, 14 MeV. To verify the simulation outcomes, measured data will be compared to them. A SPLA's electron beam is simulated using the BEAMnrc based on the EGSNRC code. At the end of each field size simulation the BEAMnrc will be used to acquire the phase-space (PhSp) file. Moreover, the PhSp file is utilized by DOSXYZnrc and BEAMDP to estimate the energy distribution and percentage-depth-dose (PDD) data for various field sizes. The results show minor PDD variations for the field sizes 10 × 10, 15 × 15, and 20 × 20 cm2. However, for all energies, the most considerable discrepancies in RMAX, R90, R80, and R50 between computation and experimental outcomes are less than 2 mm. Also, all field sizes were given surface dosages ranging from 2.6% to 8%. On the other hand, the variations in surface dosages quickly faded in the buildup area, reaching 0% within the first 3 cm and having no discernible impact on clinical outcomes. The MC simulation with BEAMnrc code for e-beam energies of 10, 12, and 14 MeV was shown to be accurate in replicating the SPLA's head with different field sizes. Also, the mean dosage is the acceptable limit in the buildup area for all energies and all field sizes.

Validation of HDR brachytherapy doses in the treatment of keloid scars using the egs_brachy Monte Carlo application

Objective. Radiotherapy is a well-known alternative in the treatment of keloid scars to reduce the recurrence of scars. The purpose of this study was to investigate the feasibility and accuracy of dose delivered from a high-dose-rate (HDR) afterloaders in keloid scar brachytherapy using Monte Carlo (MC) simulations and measurements. Approach. Treatment doses and central axis dose profiles were measured using radiophotoluminescence dosimeters and radiochromic films, respectively, with two HDR afterloaders, both using an Ir-192 source, in a phantom made of solid water and polycarbonate sheets. The nominal treatment dose calculated by the AAPM Task Group No. 43 (TG-43) dose model was set to 8.5 Gy at a distance of 0.5 cm laterally from the middle of the source line located in a plastic applicator simulating a 15 cm long surgically removed scar treatment with 30 equally spaced (0.5 cm) source positions. The dose profiles were measured at three different distances from the applicator and the absolute doses at four points at different distances. MC simulations were performed using the egs_brachy, which is based on EGSnrc code system. Main results. The measured and simulated dose profiles match well, especially at 10.0 mm (difference <1%) and 15.0 mm depths (difference <4%), and with a small dose difference at 5.0 mm depth (difference <4%). Point dose measurements agreed well in the dose maximum area (difference <7%) with the simulated dose profiles, although the largest difference near the edge of the profile was <30%. The dose differences between the TG-43 dose model and the MC simulation were small (differences <4%). Significance. Simulated and measured dose levels at a depth of 0.5 cm showed that the nominal treatment dose can be achieved with the utilized setup. The measurement results of the absolute dose agree well with the corresponding simulation results.

Development of anthropomorphic computational phantoms at the UFPE

To evaluate the amount of energy deposited in radiosensitive organs and tissues of the human body, when an anthropomorphic phantom is irradiated, researchers in numerical dosimetry use the so-called exposure computational models (ECMs). One can imagine an ECM as a virtual scene composed of a phantom in a mathematically defined position in relation to a radioactive source. The source in these ECMs produces the initial state of the simulation: the position, direction, and energy with which each particle enters the phantom are essential variables. For subsequent states of a particle history, robust Monte Carlo (MC) codes are used. For the subsequent states of a particle's history, robust Monte Carlo (MC) codes are used, which simulate the average free path that the particle performs without interacting, its interaction with the atoms in the medium and the amount of energy deposited per interaction. MC codes also evaluate normalization quantities, so the results are printed in text files in the form of conversion coefficients between the absorbed dose and the selected normalization quantity. From the 2000s, the authors have published ECMs where a voxel phantom is irradiated by photons in the environment of the MC code EGSnrc (EGS = Electron Gamma Shower; nrc = National Research Council Canada). The production of articles, dissertations and theses required the use of specific computational tools, such as the FANTOMAS, DIP (Digital Image Processing) and Monte Carlo applications, for the various steps of numerical dosimetry, which ranges from the preparation of input files to the execution from the ECM to the organization and graphical and numerical analysis of the results. This article reviews computational phantoms for dosimetry mainly those produced in DEN-UFPE dissertations and thesis.

Validation of an EGSnrc-based Monte Carlo model for a complex 2D-array for technical QA measurements of a linear accelerator.

Multi-axis ionization chamber arrays can be used for quality assurance (QA) and measurement of linear accelerator (linac) specific data. In this work, the ability of the IC Profiler (Sun Nuclear Corp., Melbourne, Florida) detector array to measure the photon beam quality specifier %dd(10) and TPR20, 10 was investigated. To investigate the method for beam energy QA using a two-dimensional detector array, a Monte Carlo-based model of the detector array was developed and validated. A Monte Carlo-based model of the IC Profiler detector array with Quad Wedge accessories was developed in detail from drawings provided by the manufacturer using the egs++ class library from the EGSnrc code system. Monte Carlo simulations were used to calculate the absorbed dose in the 251 ionization chambers of the IC profiler in the 6 MV Elekta Precise radiation field. To validate the results from the Monte Caro simulations, measurements were performed on clinical 6 MV linacs. To vary the photon beam quality of the Elekta 6 MV linac, the current of the bending magnet was varied. Furthermore, the area ratio was calculated from IC Profiler measurements with the Quad Wedgeaccessories. Measurements as well as Monte Carlo simulations confirmed the linear relationship between the area ratio and the investigated photon beam quality specifier %dd(10) and TPR20, 10 for the investigated radiation source. Furthermore, the Monte Carlo-simulated data were within the 95% confidence interval of the linear fit to the measured data. This enabled the Monte Carlo-based IC Profiler model to be used for further investigations. The values were calculated for various electron beam sizes and the angle of incidence on the target of thelinac. A Monte-Carlo-based model of the detector array was developed, which could successfully reproduce measurements, demonstrating that even very complex geometries can be modeled in EGSnrc. Moreover, the study showed that the validated Monte Carlo model has the potential to investigate variations in beam parameters and their effects on AR ratios and %dd(10) that may not be investigated experimentally. While these findings may help users gain a deeper understanding of the QA method, the Monte Carlo model enables other complex investigations, such as the simulation of measurements in the presence of magnetic fields, or the simulation of measurements on novel treatment delivery techniques anddevices.

Assessment of the detection efficiency calibration of high-resolution gamma-ray spectrometers by EGSnrc and MCNP6.2 Monte Carlo codes

Determination of the activity content of radionuclides with high accuracy and precision with gamma-ray spectrometry is strongly based on the detection efficiency calibration of the detector. The detector efficiency calibration is determined with standard reference sources and/or Monte Carlo codes. The current study provides methodology that can be used to generate efficiency curve with simulations instead of experiments. As a result, the need for additional experiments can be reduced. The implementation of a specific script written on EGSnrc Monte Carlo code was investigated to calculate the detection efficiency with high-level accuracy for high-purity germanium (HPGe) spectrometry. For this purpose, two HPGe detectors (ORTEC and PGT) used routinely in the activity measurement of gamma emitting radionuclides were modelled with EGSnrc code to obtain detection efficiencies for 40–2000 keV energy range. The detector models were benchmarked against MCNP6.2 Monte Carlo code with a hypothetical source geometry. The code-to-code comparison between EGSnrc and MCNP6.2 codes indicted that the codes can be used to generate a reliable detection efficiency curve. The benchmark results showed that median arithmetic deviations of the efficiencies of ORTEC and PGT detectors are found as 0.88% and 1.69%, respectively. The experimental studies were performed with 22Na, 57Co, 60Co, 137Cs, and 241Am standard disc sources for the ORTEC detector and with in-house prepared 152Eu sandwich-type source for the PGT detector. The sources were counted in the energy range of 59.5–1408 keV. The efficiency values from simulations were used in the calculation of the activities of the aforementioned gamma sources with a mathematical formula and resulting measured activity values were compared with reference activity values given in the certificates. In addition, the applicability of spreadsheet approach that is usually used to determine the combined standard uncertainty for alpha-particle spectrometry was investigated for gamma-ray spectrometry. The performance of the recommended procedure was scored in terms of relative bias, z-score, u-test, trueness, and precision. The activity results obtained with simulated efficiencies showed good agreement with certified values.

Investigation of Monte Carlo simulations of the electron transport in external magnetic fields using Fano cavity test

PurposeMonte Carlo simulations are crucial for calculating magnetic field correction factors kB for the dosimetry in external magnetic fields. As in Monte Carlo codes the charged particle transport is performed in straight condensed history (CH) steps, the curved trajectories of these particles in the presence of external magnetic fields can only be approximated. In this study, the charged particle transport in presence of a strong magnetic field B→ was investigated using the Fano cavity test. The test was performed in an ionization chamber and a diode detector, showing how the step size restrictions must be adjusted to perform a consistent charged particle transport within all geometrical regions. MethodsMonte Carlo simulations of the charged particle transport in a magnetic field of 1.5 T were performed using the EGSnrc code system including an additional EMF-macro for the transport of charged particle in electro-magnetic fields. Detailed models of an ionization chamber and a diode detector were placed in a water phantom and irradiated with a so called Fano source, which is a monoenergetic, isotropic electron source, where the number of emitted particles is proportional to the local density. ResultsThe results of the Fano cavity test strongly depend on the energy of charged particles and the density within the given geometry. By adjusting the maximal length of the charged particle steps, it was possible to calculate the deposited dose in the investigated regions with high accuracy (<0.1%). The Fano cavity test was performed in all regions of the detailed detector models. Using the default value for the step size in the external magnetic field, the maximal deviation between Monte Carlo based and analytical dose value in the sensitive volume of the ion chamber and diode detector was 8% and 0.1%, respectively. ConclusionsThe Fano cavity test is a crucial validation method for the modeled detectors and the transport algorithms when performing Monte Carlo simulations in a strong external magnetic field. Special care should be given, when calculating dose in volumes of low density. This study has shown that the Fano cavity test is a useful method to adapt particle transport parameters for a given simulation geometry.

Monte Carlo optimization and experimental validation of a prototype ionization chamber for accurate magnetic resonance image guided radiation therapy (MRgRT) daily output constancy measurements in solid phantoms.

To optimize the design, develop and test a prototype ionization chamber for accurate daily output constancy measurements in solid phantoms in clinical magnetic resonance-guided radiation therapy (MRgRT) radiotherapy beams. Up to 4% variations in response using commercial ionization chambers have been previously reported; the prototype ionization chamber developed here aims to minimize thesevariations. Monte Carlo simulations with the EGSnrc code system are used to optimize an ionization chamber design by increasing the thickness of a brass (high-density, nonferromagnetic, easy-to-machine) wall until results consistent with no air gap are produced for simulations with a 1.5 T and 0.35T magnetic field, with a 0.2mm air gap and varying the placement of the chamber model within the air gap. Based on the results of these simulations, prototype ionization chambers are manufactured and tested in conventional linac beams and in a 7MV Elekta Unity MR-linac. The chambers are rotated about their axes, both parallel and perpendicular to the 1.5T magnetic field, through 360º in a plastic phantom with measurements made at each cardinal angle. This reveals any variation in chamber response by varying the thickness of the air gap between the chamber and thephantom. Monte Carlo simulations demonstrate that the optimal thickness of the chamber wall to mitigate the effect of an asymmetric air gap between the chamber and the plastic phantom is 1.1mm of brass. With this thickness, the differences between simulations with and without an air gap and with asymmetric placement of the chamber within the air gap are less than 0.2%. A prototype chamber constructed with a 1.1mm brass wall thickness exhibits less than 0.3% variation in response when rotated about its axis in the plastic phantom in a beam from an MR-linac, independent of whether its axis is parallel or perpendicular to the magneticfield. The optimized ionization chamber design and validated prototype for accurate MR-linac daily output constancy measurements allows utilization of conventional phantoms and procedures in MRgRT systems. This can minimize disruption to clinical workflow for MR-linac quality assurancemeasurements.

Enhancement of the EGSnrc code egs_chamber for fast fluence calculations of charged particles

PurposeSimulation of absorbed dose deposition in a detector is one of the key tasks of Monte Carlo (MC) dosimetry methodology. Recent publications (Hartmann and Zink, 2018; Hartmann and Zink, 2019; Hartmann et al., 2021) have shown that knowledge of the charged particle fluence differential in energy contributing to absorbed dose is useful to provide enhanced insight on how response depends on detector properties. While some EGSnrc MC codes provide output of charged particle spectra, they are often restricted in setup options or limited in calculation efficiency. For detector simulations, a promising approach is to upgrade the EGSnrc code egs_chamber which so far does not offer charged particle calculations. MethodsSince the user code cavity offers charged particle fluence calculation, the underlying algorithm was embedded in egs_chamber. The modified code was tested against two EGSnrc applications and DOSXYZnrc which was modified accordingly by one of the authors. Furthermore, the gain in efficiency achieved by photon cross section enhancement was determined quantitatively. ResultsElectron and positron fluence spectra and restricted cema calculated by egs_chamber agreed well with the compared applications thus demonstrating the feasibility of the new code. Additionally, variance reduction techniques are now applicable also for fluence calculations. Depending on the simulation setup, considerable gains in efficiency were obtained by photon cross section enhancement. ConclusionThe enhanced egs_chamber code represents a valuable tool to investigate the response of detectors with respect to absorbed dose and fluence distribution and the perturbation caused by the detector in a reasonable computation time. By using intermediate phase space scoring, egs_chamber offers parallel calculation of charged particle fluence spectra for different detector configurations in one single run.

ANALISA DISTRIBUSI DOSIS PADA FANTOM HOMOGEN DAN INHOMOGEN DENGAN SIMULASI MONTE CARLO

Monte Carlo simulation with electron gamma shower (EGSnrc) code can produce 3-dimensional dose distribution data. The percent depth dose curve (PDD), dose profile, and isodose curve can be extracted through this 3-dimensional data. In this study, a photon source with an energy of 2 MeV is placed at the source to surface distance (SSD) from the phantom surface by adjusting the collimator aperture which is used to control the size of the exposure field. The SSD distance was varied at a distance of 50 cm, 70 cm, 80 cm, 90 cm, and 100 cm. The size of the exposure area is also varied by 2 × 2 cm 2 , 5 × 5 cm 2 , 7 × 7 cm 2 , and 10 × 10 cm 2 . The dose distribution analysis was carried out on homogeneous phantoms containing water and inhomogeneous phantoms containing tissue/bone/lung/bone/tissue material. The PDD curve and dose profile represent dose changes with depth and x or y direction, respectively. The PDD curve on a homogeneous phantom shows an increase in the value on the surface of the phantom to the depth with the maximum dose. This curve then decreases gradually with increasing depth. Inhomogeneous ghosts show spikes in the border region of the two media with different densities. This is due to the contribution of the backscattered electrons generated by the bone media and into the tissue media. This phenomenon is also seen in the isodose curve for inhomogeneous phantoms.

Abstract P3-19-10: Subcutaneous layer dosimetry of the breast and chest wall at clinical beam energies without bolus: A Monte Carlo and analytical anisotropic algorithm (AAA) calculation study

Abstract PURPOSE: Breast skin thickness is 3 mm (1). The subcutaneous layer (SL) lies immediately beneath the skin and is at risk for local recurrence after breast cancer surgery and adjuvant radiotherapy. As the SL lies within the buildup region for megavoltage radiation treatment, placement of bolus for patients receiving chest wall (CW) radiotherapy (RT) is routine at many centers. The dermal toxicity of bolus is well known: in 12 studies of CW RT, the pooled risk of Grade 3 acute toxicity is 9.6% with bolus and 1.2% without bolus (2). Meanwhile, bolus is rarely used after breast-conserving surgery (BCS) RT for patients with similar cancers. This study examines variation in tangential RT dose coverage of the SL in the intact breast and bolus-free CW with clinically relevant photon beam energies using Monte Carlo (MC) calculations and a commercial treatment planning system (TPS). METHODS: Thirty CT datasets from patients without skin involvement were identified. There were two groups of patients: 15 treated with BCS RT and 15 treated with CW RT. In each group, 5-patient subgroups had tangent-beam RT planned without bolus with 6, 10, and 15 MV photons, respectively, using the Analytical Anisotropic Algorithm (AAA) algorithm (v.13.6.23) in Eclipse v.15.6 (Varian Medical Systems Inc., Palo Alto, CA). On each CT, the SL was segmented as a high-resolution shell from 3 to 5 mm below the body contour in the Eclipse TPS v.15.6 (Varian Medical Systems Inc., Palo Alto, CA). A 1x1x1 mm MC dose simulation was performed using EGSnrc code (BEAMnrc/DOSXYZnrc). The MC dose distributions were imported back into the TPS for comparison with AAA calculations. The V95% and V90% for the SL were calculated for each case and the mean V95% and V90% were reported for each subgroup. A t-test was used with a two-sided alpha = 0.05 for statistical analysis. RESULTS: The mean separation increased with use of higher energies for both BCS and CW RT. The MC-calculated mean SL V90% and V95% were higher for CW RT than for BCS RT at each energy. The V90% coverage was 91.5% for CW and 74.4 % for BCS at 6 MV (p&amp;lt;0.001), 89.3% for CW and 61.3% for BCS (p&amp;lt;0.001) at 10 MV and 87.1% for CW and 60.9% for BCS (p&amp;lt;0.001) at 15 MV (Table 1). For SL V95% the CW coverage was higher than the BCS coverage for every energy. For SL V90% at 6 MV, the AAA and MC calculations agreed within 2.5%, with the MC being slightly higher. The agreement between AAA and MC decreased for higher energies with MC reporting higher SL V90% coverage by up to 16.3%. The higher MC-calculated dose to the SL is consistent with the literature (3). CONCLUSION: MC and AAA SL dose calculations agreed well for 6 MV, but AAA underestimated the dose for 10 and 15 MV. For 6-15 MV photons, the MC-calculated dosimetric coverage of the SL is higher for CW RT than BCS RT. Since radiation oncologists are satisfied with the SL coverage by BCS RT, bolus is not needed for CW RT, because, without bolus, CW RT delivers a higher SL dose than BCS RT. REFERENCES:. 1.Pope TL Jr, et al.J Can Assoc Radiol. 1984 Dec;35(4):365-8. PMID: 6526847. 2.Dahn HM, et al. Crit Rev Oncol Hematol. 2021 Jun 5;163:10339. PMID: 34102286. 3.Panettieri V, et al., Radiother Oncol 2009; 93: 94-101 Table 1.Monte Carlo calculated mean V90% and V95% for Breast and Chest Wall for each energyV95%V95%V95%V90%V90%V90%Energy(MV)Chest Wall(Mean)Breast. (Mean)p-valueChest Wall(Mean)Breast. (Mean)p-value661.9%35.1%&amp;lt;0.00191.5%74.4%&amp;lt;0.0011065.2%39.7%&amp;lt;0.00189.3%61.3%&amp;lt;0.0011561.7%38.9%0.01287.1%60.9%&amp;lt;0.001 Citation Format: Dylan Narinesingh, Alan Nichol, Alanah Bergman, Tony Popescu. Subcutaneous layer dosimetry of the breast and chest wall at clinical beam energies without bolus: A Monte Carlo and analytical anisotropic algorithm (AAA) calculation study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-19-10.

Monte Carlo Study on Dose Distributions Around 192Ir, 169Yb, and 125I Brachytherapy Sources Using EGSnrc-based egs_brachy User-code.

As per the recommendations of the American Association of Physicists in Medicine Task Group 43, Monte Carlo (MC) investigators should reproduce previously published dose distributions whenever new features of the code are explored. The purpose of the present study is to benchmark the TG-43 dosimetric parameters calculated using the new MC user-code egs_brachy of EGSnrc code system for three different radionuclides 192Ir, 169Yb, and 125I which represent high-, intermediate-, and low-energy sources, respectively. Brachytherapy sources investigated in this study are high-dose rate (HDR) 192Ir VariSource (Model VS2000), 169Yb HDR (Model 4140), and 125I -low-dose-rate (LDR) (Model OcuProsta). The TG-43 dosimetric parameters such as air-kerma strength, S k, dose rate constant, Λ, radial dose function, g(r) and anisotropy function, F(r,θ) and two-dimensional (2D) absorbed dose rate data (along-away table) are calculated in a cylindrical water phantom of mass density 0.998 g/cm3 using the MC code egs_brachy. Dimensions of phantom considered for 192Ir VS2000 and 169Yb sources are 80 cm diameter ×80 cm height, whereas for 125I OcuProsta source, 30 cm diameter ×30 cm height cylindrical water phantom is considered for MC calculations. The dosimetric parameters calculated using egs_brachy are compared against the values published in the literature. The calculated values of dose rate constants from this study agree with the published values within statistical uncertainties for all investigated sources. Good agreement is found between the egs_brachy calculated radial dose functions, g(r), anisotropy functions, and 2D dose rate data with the published values (within 2%) for the same phantom dimensions. For 192Ir VS2000 source, difference of about 28% is observed in g(r) value at 18 cm from the source which is due to differences in the phantom dimensions. The study validates TG-43 dose parameters calculated using egs_brachy for 192Ir, 169Yb, and 125I brachytherapy sources with the values published in the literature.

Monte Carlo-based dosimetric studies of a locally developed 170Tm LDR brachytherapy seed source

170Tm is being explored as a source for applications in brachytherapy. Although it has adequate physical properties, such as a short half-life (128.6 d), high specific activity and a mean photon energy of about 66 keV, it has a drawback of low photon yield (only about six photon emissions/100 beta emissions). The objective of this work is to study the dosimetric characteristics of a locally developed 170Tm brachytherapy seed source using the Monte Carlo-based EGSnrc code system. In this study, we calculate the dose rate constant, air-kerma strength, radial dose function, anisotropic function and 2D dose-rate distributions in water. Separate simulations are carried out by considering the photon (gamma and characteristic x-ray) and beta spectra of the source. For regions close to the source (surface of the source < r < 0.4 cm), the dose is solely due to direct dose deposition by beta particles. At larger distances (0.4 cm < r <10 cm), the dose is due to bremsstrahlung photons produced by beta particles and photon emissions. The calculated value of the dose rate constant is 1.217 ± 0.052 cGy h−1U−1. The value of S k per mCi is 0.029 ± 0.0009 U mCi−1. The contributions of the inherent photon emission and the bremsstrahlung photons to the total S k are 0.58 and 0.42, respectively.

Proposal for a new operational quantity to assess the effective dose in the event of external exposure to photons

Despite its shortcomings and inconsistencies such as using the kerma approximation, the ICRU tissue, which does not really exist and the radiation quality factor Q defined in water, the personal dose equivalent Hp defined in the 1980s remains until now the operational quantity used to assess the effective dose E for external exposure. To help overcome most of these shortcomings and inconsistencies a new operational quantity Hp(30)water to replace Hp(10) is proposed. Fluence-to Hp(30) water conversion coefficients are calculated for photon energies from 0.02 to 10 MeV using the Monte Carlo transport code EGSnrc. Calculations are performed in a water cube phantom using full transport of secondary charged particles. The comparison of the conversion coefficients of Hp(30) and E shows that Hp(30) water can represent a good envelope of E over the whole energy range considered.

Technical Note: Implications of using EGSnrc instead of EGS4 for extracting electron stopping powers from measured energy spectra.

NRC Report PIRS-0626 (https://doi.org/10.4224/40000364) describes how measured electron energy deposition spectra can be used to determine the electronic stopping power. The stopping power is obtained by comparing measured spectra with spectra calculated using Monte Carlo techniques. The stopping powers reported in PIRS-0626 were obtained using the EGS4 Monte Carlo code. Since then, the EGSnrc code has been released which has more accurate electron transport algorithms. We calculate the effect on the measured stopping powers of using EGSnrc instead of EGS4. The EGS4 spectra calculated in PIRS-0626 were based on primary electron histories. We first show that those spectra, calculated in 1997, are consistent with current EGS4 spectra calculated using histories. EGSnrc spectra are also calculated using histories and these high-precision spectra are compared to extract any energy difference. The energy differences between the spectra are used to estimate the effect on the measured electronic stopping powers. The energy differences depend on the absorber material, the absorber thickness and the beam energy. The improved electron elastic scattering cross section of EGSnrc accounts for only part of the difference between the two codes. The effect on the extracted stopping power is largest for the lowest electron energies and can be as large as 0.9%. The calculated spectra show differences for lower energies, with the EGSnrc spectra having a larger proportion of low-energy electrons. The differences introduced by using EGSnrc instead of EGS4 can affect the estimated stopping power by almost 1% in the worst case but generally the effect is much smaller. We report corrections that can be applied to all the stopping power data in PIRS-0626. An experiment to measure the average energy to create an ion pair in air, , using aluminum detectors will provide an interesting test of the aluminum stopping power data as reported in PIRS-0626 and revised by this work.

Organ doses of the fetus from external environmental exposures

This article presents nuclide-specific organ dose rate coefficients for environmental external exposures due to soil contamination assumed as a planar source at a depth of 0.5 g cm−2 in the soil and submersion to contaminated air, for a pregnant female and its fetus at the 24th week of gestation. Furthermore, air kerma free-in-air coefficient rates are listed. The coefficients relate the organ equivalent dose rates (Sv s−1) to the activity concentration of environmental sources, in Bq m−2 or Bq m−3, allowing to time-integrate over a particular exposure period. The environmental radiation fields were simulated with the Monte Carlo radiation transport codes PHITS and YURI. Monoenergetic organ dose rate coefficients were calculated employing the Monte Carlo code EGSnrc simulating the photon transport in the voxel phantom of a pregnant female and fetus. Photons of initial energies of 0.015–10 MeV were considered including bremsstrahlung. By folding the monoenergetic dose coefficients with the nuclide decay data, nuclide-specific organ doses were obtained. The results of this work can be employed for estimating the doses from external exposures to pregnant women and their fetus, until more precise data are available which include coefficients obtained for phantoms at different stages of pregnancy.

Relationship between dose and stopping power values for electrons in skin and muscle tissues.

Absorbed dose and stopping power of the target material are two important parameters for determining the radiation effects. In this work, the relationship between these two parameters has been investigated for electron beams incident on skin and muscle tissue. Absorbed dose was obtained by using the EGSnrc code and the stopping power values were calculated by considering the velocity-depended effective charge and mean excitation values. To obtain the relationship between absorbed dose and stopping power values, these parameters were graphed together and simple fitting functions have been obtained. The results obtained show that these parameters are linearly correlated with each other.

Tungsten carbide and LMPA electron cutouts: comparison and validation using Monte Carlo modelling and measurement of dose

Electron applicator cutouts for radiation therapy electron beam shaping are typically cast from Low Melting Point Alloys (LMPA), such as cerrobend. In this work, we describe the Monte Carlo modelling of novel 3D printed cutouts based on tungsten carbide powder and the resulting dose profiles subject to Elekta Agility electron beams. Cerrobend cutouts were also modelled using the Monte Carlo code EGSnrc. Cerrobend and tungsten carbide cutouts were found to have the same dose profiles within model variance. Computed profiles and percentage depth dose (PDDs) curves of the Elekta Agility accelerator model using standard cutouts in water were found to agree with water tank measurements using gamma criteria of 2%/2mm. The Monte Carlo computed dose profiles of the tungsten carbide cutouts in a polystyrene phantom were also found to agree with liquid-filled ionization chamber array measurements using gamma criteria of 2%/2mm. We conclude that the tungsten carbide cutouts are clinically equivalent to LMPA cutouts.

An inter-comparison between accuracy of EGSnrc and MCNPX Monte Carlo codes in dosimetric characterization of intraoperative electron beam

BackgroundIonometric dosimetry in IOERT is a complicated process, due to the sophisticated beam setup and the necessity for dedicated protocols for ion chamber response correction. On the other hand, the Monte Carlo (MC) technique can easily overcome such limitations and be considered as an alternative dosimetry approach. This paper presents a comparative analysis of two widely used MC codes, EGSnrc and MCNPX, for intraoperative electron beam dosimetry. MethodThe head of LIAC12, a dedicated IOERT accelerator, was modeled by both mentioned MC codes. Then, the percentage depth dose (PDD) curves, transverse dose profiles (TDPs), and output factor (OF) values were accordingly calculated within the water phantom. To realize the accuracy of MC codes in dosimetric characterization of intraoperative electron beam, their results were finally compared with those measured by corresponding ionometric dosimetry for all forms of electron energy/applicator size. ResultsA good agreement was observed between the simulated and measured PDDs/TDPs for both considered MC codes, such that the calculated gamma index values were always lower than unity for both considered MC codes. Nevertheless, the lower gamma index values were found in the case of the EGSnrc code. The maximum difference between the measured and calculated OF was obtained as 2.3% and 3.1% for EGSnrc and MCNPX code, respectively. ConclusionsAlthough both studied MC codes showed compatible results with the measured ones, EGSnrc code has superior accuracy in this regard and can be considered as a more reliable toolkit in Monte Carlo-based commissioning of dedicated IOERT accelerators.