EGSnrc Code Research Articles

Effects of internal and external scatter on the build-up characteristics of Monte Carlo calculated absorbed dose for electron irradiation

The effects of internal and external scatter on surface, build-up and depth dose characteristics simulated by Monte Carlo code EGSnrc for varying field size and SSD for a 10 MeV monoenergetic electron beam with and without an accelerator model are extensively studied in this paper. In particular, sub-millimetre surface PDD was investigated. The percentage depth doses affected significantly by the external scatter show a larger build-up dose. A forward shifted Dmax depth and a sharper fall-off region compared to PDDs with only internal scatter considered. The surface dose with both internal and external scatter shows a marked decrease at 110 cm SSD, and then slight further changes with the increasing SSD since few external scattered particles from accelerator model can reach the phantom for large SSDs. The sharp PDD increase for the 5 cm x 5 cm field compared to other fields seen when only internal scatter is considered is significantly less when external scatter is also present. The effect of external scatter on surface PDD is more pronounced for large fields than small fields (5 cm x 5 cm field).

SU-FF-T-248: Perturbation Factors for the Plane-Parallel NACP02 Ionisation Chamber in Electron Beams

Purpose: To calculate perturbation factors, pwall and pcav, for plane‐parallel ion chambers in electron beams, and to investigate their origin. Method and Materials: Current dosimetry protocols for clinical electron beams (AAPM, IAEA, IPEM) recommend the use of plane‐parallel ion chambers, such as the NACP‐02. These protocols describe how dose‐to‐water calibration factors, ND,w, are used to derive dose to water from ionization measurements in water or plastic phantoms. These factors implicitly include chamber perturbation factors for the non‐medium equivalence of the chamber walls and cavity material, pwall and pcav. The perturbation factors are currently assumed unity for well‐guarded plane‐parallel ion chambers. In this work we have used the Monte Carlo (MC) code EGSnrc to calculate perturbation factors in water, plastics and graphite (used in standard labs) for electron beams ranging from 4–20 MeV for the NACP‐02 ion chamber. The MC model was first validated against measurements of chamber response in a solid slab phantom where thin slabs of different materials (plastics, Al, Cu) could be brought in close contact with the ion chamber. In addition, stopping power ratios, medium‐to‐air, were calculated for clinical and calibrationelectron beams.Results: The agreement between calculated and measured ion chamber response in the slab phantoms was within 1%. Cavity perturbation factors, pcav, for the NACP‐02 chamber were found to be within 1% of unity within statistical uncertainty (0.2%). Wall perturbation factors, pwall, for the energy range 4–20 MeV were found to vary from (1.7±0.2)% to (0.2±0.2)% for the NACP‐02 chamber. Stopping powers, water‐to‐air, were found to differ by up to 0.5% from the currently recommended values (Burns et al 1996). Conclusion: Electron perturbation factors for the NACP‐02 plane‐parallel ion chamber were found to differ significantly from unity, which is the currently assumed value.

A Monte Carlo study of the energy dependence of Al2O3:C crystals for real-time in vivo dosimetry in mammography

In a previous experimental study, a novel method for in vivo dosimetry has been investigated, based on radioluminescence (RL) and optically stimulated luminescence (OSL). However, because of the large difference in atomic composition between the detector material and the breast tissue, relatively large energy dependence in low-energy X-ray beams can be expected. In the present work, the energy dependence of Al2O3:C crystals was modelled with the Monte Carlo code EGSnrc using three types of X-ray spectra. The results obtained (5.6-7.3%) agree with a previously determined experimental result (9%) within the combined standard uncertainty of the two methods. The influence of the size of the crystal on the energy dependence was investigated together with the effect of varying the thickness of the surrounding light-protective material. The results obtained indicate a minor effect owing to the thickness of the light-protective material, and a somewhat larger effect from reducing the diameter of the crystal. The outcome of this study can be used to improve the future design of the RL/OSL dosimetry system for use in mammography.

Study of the impact of artificial articulations on the dose distribution under medical irradiation

Perturbations due to the presence of high density heterogeneities in the body are not correctly taken into account in the Treatment Planning Systems currently available for external radiotherapy. For this reason, the accuracy of the dose distribution calculations has to be improved by using Monte Carlo simulations. In a previous study, we established a theoretical model by using the Monte Carlo code EGSnrc [I. Kawrakow, D.W.O. Rogers, The EGSnrc code system: MC simulation of electron and photon transport. Technical Report PIRS-701, NRCC, Ottawa, Canada, 2000] in order to obtain the dose distributions around simple heterogeneities. These simulations were then validated by experimental results obtained with thermoluminescent dosemeters and an ionisation chamber. The influence of samples composed of hip prostheses materials (titanium alloy and steel) and a substitute of bone were notably studied. A more complex model was then developed with the Monte Carlo code BEAMnrc [D.W.O. Rogers, C.M. MA, G.X. Ding, B. Walters, D. Sheikh-Bagheri, G.G. Zhang, BEAMnrc Users Manual. NRC Report PPIRS 509(a) rev F, 2001] in order to take into account the hip prosthesis geometry. The simulation results were compared to experimental measurements performed in a water phantom, in the case of a standard treatment of a pelvic cancer for one of the beams passing through the implant. These results have shown the great influence of the prostheses on the dose distribution.

Treatment parameters for beta and gamma devices in peripheral endovascular brachytherapy

To determine dosimetric parameters, such as radial and longitudinal dose profiles, for beta and gamma devices in peripheral endovascular brachytherapy. An (192)Ir high-dose rate stepping source, a (90)Sr source train, and a (32)P-coated radiation balloon were investigated. The treatment-planning software PLATO, Monte Carlo code EGSnrc, and GafChromic film dosimetry were used to analyze the dose distribution of these devices. For a 5-mm-diameter vessel, the ratio between the dose at 2 mm depth and the dose at the lumen surface was 1.8, 3.4, and 16.2 for the (192)Ir, (90)Sr, and (32)P devices, respectively. The dose variation at the reference depth of 2 mm into the vessel wall was 7-18 Gy, for different analyzed dose prescriptions. The reference lumen dose was different by a factor >8. For all three devices, the reference isodose length was not <5 mm on the proximal and distal edge of the active source length. A complete set of dose parameters for beta and gamma sources has to be considered for appropriate treatment planning and performance, including reporting of reference depth dose, reference lumen dose, and reference isodose length.

Relationship between gamma radiation dose monitors and source location

Area radiation monitors inside nuclear power plants can be used for severe accident monitoring purposes. The work presented here simulates readings from area monitors located at different points in nuclear power plants and is part of a larger work that seeks to evaluate whether such data could be related to the type of nuclear emergency that produces an increase in monitor readings. The relationship between monitor outputs and workspace dimensions and radiation energy is analysed. This study has been performed by simulating photons of different energies being emitted from different compartments with monitors located at several places on the walls, using the EGSnrc code (Kawrakow and Rogers, 2000).

Design of Ion Chamber for Beam Loss Monitor of PEFP

A beam loss monitoring system is being designed for PEFP(Proton Engineering Frontier Project) by Korea Atomic Energy Research Institute(KAERI). The beam loss monitoring system is based on the ion chamber and an electrometer. An air-filled ion chamber was designed and fabricated as a prototype detector of beam loss monitor. The radiation response of the ion chamber was simulated by using EGSnrc code. The collection efficiency and stability of the ion chamber with respect to the different radii of inner electrodes is discussed. The proper choice of high voltage bias polarity for the ion chamber is also studied. The test of the ion chamber under high dose rate is under way at KAERI gamma irradiation facility at the exposure rate of 106R/h.

Comparison of measured and Monte Carlo calculated dose distributions in inhomogeneous phantoms in clinical electron beams

Calculations of dose distributions in heterogeneous phantoms in clinical electron beams, carried out using the fast voxel Monte Carlo (MC) system XVMC and the conventional MC code EGSnrc, were compared with measurements. Irradiations were performed using the 9 MeV and 15 MeV beams from a Varian Clinac-18 accelerator with a 10 × 10 cm2 applicator and an SSD of 100 cm. Depth doses were measured with thermoluminescent dosimetry techniques (TLD 700) in phantoms consisting of slabs of Solid Water™ (SW) and bone and slabs of SW and lung tissue-equivalent materials. Lateral profiles in water were measured using an electron diode at different depths behind one and two immersed aluminium rods. The accelerator was modelled using the EGS4/BEAM system and optimized phase-space files were used as input to the EGSnrc and the XVMC calculations. Also, for the XVMC, an experiment-based beam model was used. All measurements were corrected by the EGSnrc-calculated stopping power ratios. Overall, there is excellent agreement between the corrected experimental and the two MC dose distributions. Small remaining discrepancies may be due to the non-equivalence between physical and simulated tissue-equivalent materials and to detector fluence perturbation effect correction factors that were calculated for the 9 MeV beam at selected depths in the heterogeneous phantoms.

Simulating nuclear particle transport in stochastic media using Perlin noise functions

Many nuclear particle transport problems require the simulation of stochastic media – that is, media whose composition varies randomly as a function of position. Examples include biological tissues, mineral slurries and crushed mineral ores. Such materials are usually represented in particle transport codes as being homogeneous with an appropriate average composition. This approximation can introduce significant errors. A new method is described for simulating the transport of neutral particles in such media using a three-dimensional Perlin noise function [Comput. Graph. 19 (3) (1985) 287]. The technique allows many properties of the stochastic medium to be specified, including the mean scale of the inhomogeneities, their shape and surface roughness and the proportions of the different media making up the stochastic medium. A key property of the new method is that the tracking speed is independent of the number density of the inhomogeneities. The tracking algorithm has been implemented into the EGSnrc [The EGSnrc code system: Monte Carlo simulation of electron and photon transport, NRCC Report PIRS-701, 2000] Monte Carlo code. An example application is described, modelling X-ray fluorescence from a particulate mineral sample. In contrast to the homogeneous approximation, the new algorithm accurately reproduces the observed particle size dependence seen experimentally. The increase in computational time is fairly modest, even for very small particle sizes.

Interface perturbation effects in high-energy electron beams

Near interfaces between two different media exposed to high-energy electron beams substantial dose and fluence perturbations due to backscatter can be observed. In this work, dose and fluence perturbations were studied for 4–19 MeV electron beams at backscatter interfaces of polystyrene, graphite, water, aluminium and lead. Measurements of relative dose using an NPL-designed thin-window plane-parallel ion chamber and a Markus ion chamber were performed to determine the effect of different interface materials and thicknesses. Results of Monte Carlo simulations with the EGSnrc code, including models of the ion chambers, were found to be in excellent agreement with the measurements. The well-documented increasing dose perturbation with increasing effective atomic number of the backscatter material and decreasing electron beam energy was confirmed. Simulations in a simplified slab geometry showed that, despite the decrease of average electron energy with depth in water, the dose perturbations decrease with increasing depth of the interface in water for all the materials in the study. This was ascribed to the change of the electron angular distribution with depth which has a different effect in water and in the presence of a high-Z interface. Electron fluence perturbations near a lead/water interface were found to cause small differences in unrestricted mass collision stopping power ratios, water to air. Effects of bremsstrahlung photons, characteristic photons and positrons from the backscattering material were found to be insignificant for electron interface dosimetry. When comparing simulations using EGSnrc and the older version of the same code, EGS4, underestimations of the dose perturbation effects of up to 7% were found when using the latter code to simulate 4 MeV electrons irradiating a lead/water interface. It is concluded that EGSnrc is a highly suitable tool for electron interface dosimetry studies.

Determination and application of the reference isodose length (RIL) for commercial endovascular brachytherapy devices

Background and purpose: During the last years endovascular brachytherapy has been established as a new field in radiotherapy. In a recent recommendation of the EndoVAscular, Groupe Européen de Curiethérapie, European Society for Therapeutic Radiation Oncology, Working Group the reference isodose length (RIL) has been introduced as a central parameter for treatment planning. It is defined as the vessel length at the reference depth enclosed by the 90% isodose. The dose is normalized to 100% at the reference depth (i.e. lumen radius plus 1 mm into the vessel wall) at the central plane. In order to avoid a geographic miss the clinical target length has to be encompassed by the RIL. Material and methods: RILs are determined by Monte Carlo calculations and GafChromic film dosimetry for three endovascular brachytherapy devices currently in clinical use ( 192Ir seed ribbon, 32P wire source, 90Sr seed train). For all measurements, phantoms and devices the sensitive layer of GafChromic film is located in a plane at 2±0.1 mm parallel to the axis of the source delivery catheter. The EGSnrc code system is applied to calculate the dose profile at 2 and 3 mm distance from the source axis. Results: For the ten seed 192Ir source calculated RIL at 2 mm radial distance is 30.2 mm whereas the measured RIL is 33.5 mm. In case of a 20 mm 32P wire source with two steps the calculated RIL of 36.6 mm shows excellent agreement compared with the measured value of 36.2 mm. The calculated RIL of a 40 mm 90Sr seed train was 35.8 mm compared to a measured value of 34.8 mm. Conclusion: As a compromise between calculated and measured RIL values at 2 and 3 mm radial distances we propose to use a RIL of 30 mm for the 192Ir ten seed ribbon, 36 mm for the 32P wire source with two steps and 35 mm for the 40 mm 90Sr seed train. These parameter values can be used to define the maximum intervention length for endovascular sources.

Evaluation of the EGSnrc Monte Carlo code for interface dosimetry near high-Z media exposed to kilovolt and 60Co photons

High atomic number (Z) heterogeneities in tissue exposed to photons with energies of up to about 1 MeV can cause significant dose perturbations in their immediate vicinity. The recently released Monte Carlo (MC) code EGSnrc (Kawrakow 2000a Med. Phys. 27 485–98) was used to investigate the dose perturbation of high-Z heterogeneities in tissue in kilovolt (kV) and 60Co photon beams. Simulations were performed of measurements with a dedicated thin-window parallel-plate ion chamber near a high-Z interface in a 60Co photon beam (Nilsson et al 1992 Med. Phys. 19 1413–21). Good agreement was obtained between simulations and measurements for a detailed set of experiments in which the thickness of the ion chamber window, the thickness of the air gap between ion chamber and heterogeneity, the depth of the ion chamber in polystyrene and the material of the interface was varied. The EGSnrc code offers several improvements in the electron and photon production and transport algorithms over the older EGS4/PRESTA code (Nelson et al 1985 Stanford Linear Accelerator Center Report SL AC-265, Bielajew and Rogers 1987 Nucl. Instrum. Methods Phys. Res. B 18 165–81). The influence of the new EGSnrc features was investigated for simulations of a planar slab of a high-Z medium embedded in water and exposed to kV or 60Co photons. It was found that using the new electron transport algorithm in EGSnrc, including relativistic spin effects in elastic scattering, significantly affects the calculation of dose distribution near high-Z interfaces. The simulations were found to be independent of the maximum fractional electron energy loss per step (ESTEPE), which was often a cause for concern in older EGS4 simulations. Concerning the new features of the photon transport algorithm sampling of the photoelectron angular distribution was found to have a significant effect, whereas the effect of binding energies in Compton scatter was found to be negligible. A slight dose artefact very close to high-Z interfaces exposed to kilovolt x-rays was discovered when atomic relaxation processes following excitation were omitted.

Monte Carlo study of correction factors for Spencer-Attix cavity theory at photon energies at or above 100 keV.

To develop a primary standard for 192Ir sources, the basic science on which this standard is based, i.e., Spencer-Attix cavity theory, must be established. In the present study Monte Carlo techniques are used to investigate the accuracy of this cavity theory for photons in the energy range from 20 to 1300 keV, since it is usually not applied at energies below that of 137Cs. Ma and Nahum [Phys. Med. Biol. 36, 413-428 (1991)] found that in low-energy photon beams the contribution from electrons caused by photons interacting in the cavity is substantial. For the average energy of the 192Ir spectrum they found a departure from Bragg-Gray conditions of up to 3% caused by photon interactions in the cavity. When Monte Carlo is used to calculate the response of a graphite ion chamber to an encapsulated 192Ir source it is found that it differs by less than 0.3% from the value predicted by Spencer-Attix cavity theory. Based on these Monte Carlo calculations, for cavities in graphite it is concluded that the Spencer-Attix cavity theory with delta = 10 keV is applicable within 0.5% for photon energies at 300 keV or above despite the breakdown of the assumption that there is no interaction of photons within the cavity. This means that it is possible to use a graphite ion chamber and Spencer-Attix cavity theory to calibrate an 192Ir source. It is also found that the use of delta related to the mean chord length instead of delta = 10 keV improves the agreement with Spencer-Attix cavity theory at 60Co from 0.2% to within 0.1% of unity. This is at the level of accuracy of which the Monte Carlo code EGSnrc calculates ion chamber responses. In addition, it is shown that the effects of other materials, e.g., insulators and holders, have a substantial effect on the ion chamber response and should be included in the correction factors for a primary standard of air kerma.