EGS4 Monte Carlo Code Research Articles

Combined experimental and Monte Carlo verification of brachytherapy plans for vaginal applicators

Dose rates in a phantom around a shielded and an unshielded vaginal applicator containing Selectron low-dose-rate sources were determined by experiment and Monte Carlo simulation. Measurements were performed with thermoluminescent dosimeters in a white polystyrene phantom using an experimental protocol geared for precision. Calculations for the same set-up were done using a version of the EGS4 Monte Carlo code system modified for brachytherapy applications into which a new combinatorial geometry package developed by Bielajew was recently incorporated. Measured dose rates agree with Monte Carlo estimates to within 5% (1 SD) for the unshielded applicator, while highlighting some experimental uncertainties for the shielded applicator. Monte Carlo calculations were also done to determine a value for the effective transmission of the shield required for clinical treatment planning, and to estimate the dose rate in water at points in axial and sagittal planes transecting the shielded applicator. Comparison with dose rates generated by the planning system indicates that agreement is better than 5% (1 SD) at most positions. The precision thermoluminescent dosimetry protocol and modified Monte Carlo code are effective complementary tools for brachytherapy applicator dosimetry.

Application of the EGS4 Monte Carlo Code to a Study of Multilayer Gamma-Ray Exposure Buildup Factors of up to 40 mfp

Multilayer γ-ray exposure buildup factors of up to 40 mfp were calculated using an electron-photon cascade Monte Carlo code, EGS4, as a point isotropic source. A kind of splitting technique was used in the EGS4 calculations in order to obtain reasonable results at very deep penetration problems, such as 40 mfp.The double-layer γ-ray exposure buildup factors were calculated for combinations of water, iron and lead for 0.1, 0.3, 0.6, 1.0, 3.0, 6.0 and 10MeV γ-rays. The thickness of the first medium was set to 1, 5, 10 or 20 mfp. As typical triple-layered shields, the calculations were performed for 4 different configurations of water, iron and lead.The behavior of multilayer buildup factors for deep penetration problems was studied using the obtained results. Buildup factors after the bounday show the tendency to vary largely depending on the material combination and the source energy of γ-rays.

Calculation of microplanar beam dose profiles in a tissue/lung/tissue phantom

Recent advances in synchrotron generated x-ray beams with a high fluence rate permit investigation of the application of an array of closely spaced, parallel or converging microplanar beams in radiotherapy. The proposed technique takes advantage of the hypothesized repair mechanism of capillary cells between alternate microbeam zones, which regenerates the lethally irradiated endothelial cells. The lateral and depth doses of 100 keV microplanar beams are investigated for different beam dimensions and spacings in a tissue, lung and tissue/lung/tissue phantom. The EGS4 Monte Carlo code is used to calculate dose profiles at different depths and bundles of beams (up to cm square cross section). The maximum dose on the beam axis (peak) and the minimum interbeam dose (valley) are compared at different depths, bundles, heights, widths and beam spacings.

Analysis of 4 πγ ionization chamber response using EGS4 Monte Carlo code

The response of three types of 4 πγ re-entrant well pressure ionization chambers filled with argon or nitrogen were calculated by means of the EGS4 Monte Carlo code as a function of photon energy and compared with experimental results. Excellent agreement was found between calculations and experimental results. This code was also used for the evaluation of corrections required in the change of source conditions such as the displacement of source position. In the medium and high energy region, the chamber response increased when the source was moved to off-center position from center whereas the response decreased in low energy region. This phenomena can be explained by a change ofgeometry and the absorption of photons in an inner cylindrical absorber tube of the chamber.

One-sided imaging of large, dense objects using 511-keV photons from induced-pair production

This work investigates the use of annihilation photons from gamma-ray-induced electron-positron pair production to inspect objects when only one side is accessible. The Z/sup 2/-dependence of the pair-production cross section and the high penetration of 511-keV photons allow this method to differentiate high-Z materials in low-Z matrices. The experimental results for the dependence of the back-streaming photon yield on Z shows a factor of 20 increase for elements over the range 4<Z<82. Results for point-by-point images obtained in line scans of representative geometries are also shown. Comparisons of experimental data and simulation studies based on the EGS4 Monte Carlo code agree to within 5%.

Computer aided design of a polarised source for in vivo X-ray flourescence analysis

A system has been developed which uses Monte Carlo computer simulations to aid the design and optimisatioin of a polarised source for in vivo X-ray fluorescence (XRF) analysis of heavy metals. The system is based on a version of the Monte Carlo code EGS4 which includes polarised photon interactions, running on a personal computer. The code was used to construct a model of a clinical polarised XRF system (based on a 300 kV therapy X-ray source) under development at Swansea for the measurement of Pt-based chemotheraphy drugs in head and neck tumours. Several simulations were performed to investigate the variation of XRF measurement sensitivity with the material composition and geometry of the system components. Pt XRF spectra generated by the model were found to be in good agreement with experimental data. The optimum operating voltage for the system, predicted by the simulations and confirmed by experiment, was approx. 200 kV. The accuracy of the results obtained to data indicates that this technique will greatly facilitate future design and optimisation studies of a wide variety of XRF systems.

A Monte Carlo study of verification imaging in high dose rate brachytherapy.

We have been evaluating the practicality of monitoring the position of an 192Ir source during high dose rate (HDR) brachytherapy treatments using x-ray fluoroscopy. The EGS4 Monte Carlo code has been used to simulate the interactions of 192Ir photons with the patient and the CsI phosphor of an x-ray image intensifier to predict what signals will be generated by these 192Ir photons. The calculations show that it is the 192Ir photons scattered within the patient that are mainly responsible for generating the spurious signals in the x-ray image intensifier that degrade image quality. The scattered 192Ir photons are distributed in the energy range (15-200 keV), which is markedly lower than the average energy of the primaries (360 keV), and therefore interact more efficiently with the CsI phosphor of the x-ray image intensifier. Experimental measurements support these observations, demonstrating that spurious signals produced by the 192Ir source become appreciably larger when the 192Ir source is located within a scattering object rather than air. For a 10 cm airgap, the signal-to-noise ratio (SNR) can decrease by factors ranging between 3 and 10 (no antiscatter grid), depending on the position of a 7 Ci 192Ir source inside a 30 cm thick water phantom. In typical clinical situations, a focused grid (Pb, 12:1, 40 lines/cm) can increase the SNR by about a factor of 2. Furthermore, the SNR rapidly increases with increasing airgap, such that a 20 cm airgap can be as effective as a 12:1 air interspaced grid in eliminating the spurious signals. Our results suggest that use of a high-current x-ray fluoroscopy technique, a large airgap, and a well-designed anti-scatter grid can make the fluoroscopic monitoring of source position in HDR brachytherapy feasible. This, in turn, can improve the quality assurance of such treatments.

Influence of Catheter Materials and Tissue Composition on Low Dose Rate Iridium-192 Seed Implant Dosimetry

Purpose: To quantify the perturbing influence of catheter materials and tissue composition on the dose distribution in water around a low dose rate, steel encapsulated Ir-192 seed. Methods and Materials: The EGS4 Monte Carlo code was used to estimate point doses in a bounded phantom within a cylinder with a radius of 3 cm surrounding the source. Simulations were performed in water and soft tissue media for an unadorned seed, and then in soft tissue for a nylon-ribbon-encased seed inside steel and nylon catheters. The latter simulations were repeated including inactive seeds on either side of the source at spacings of 1.0 and 0.5 cm. Sufficient numbers of photon histories were run to reduce the relative statistical uncertainty of most dose estimates to <0.1%. Contributions from primary and scattered photons were scored separately. Results: For the unadorned source, doses in soft tissue were uniformly ∼1% lower than in water. Introduction of either catheter perturbed the dose in soft tissue by <0.4% along the transverse source axis, r. In the long axis direction, d, dose in soft tissue for the steel catheter relative to the unadorned source fell with increasing distance and decreasing radius, r, and trended oppositely for the nylon catheter. Neighboring seeds did not affect the dose distribution except at r = 0.2 cm for seeds spaced at 0.5 cm, where dose fell ∼8% in the interval d = 4–7 cm. For several Paris system implant geometries, the presence of catheters altered basal doses in a systematic manner. Conclusion: Perturbations of the dose distribution for LDR Ir-192 seeds in water introduced by treatment catheters and tissue composition are small but systematic. If desired, they can be accounted for in clinical volumes of interest by simple adjustment of dose calculation parameters.

Application of the EGS4 Monte Carlo Code to a Study of Multilayer Gamma-Ray Exposure Buildup Factors of up to 40 mfp.

Multilayer γ-ray exposure buildup factors of up to 40 mfp were calculated using an electron-photon cascade Monte Carlo code, EGS4, as a point isotropic source. A kind of splitting technique was used in the EGS4 calculations in order to obtain reasonable results at very deep penetration problems, such as 40 mfp.The double-layer γ-ray exposure buildup factors were calculated for combinations of water, iron and lead for 0.1, 0.3, 0.6, 1.0, 3.0, 6.0 and 10MeV γ-rays. The thickness of the first medium was set to 1, 5, 10 or 20 mfp. As typical triple-layered shields, the calculations were performed for 4 different configurations of water, iron and lead.The behavior of multilayer buildup factors for deep penetration problems was studied using the obtained results. Buildup factors after the bounday show the tendency to vary largely depending on the material combination and the source energy of γ-rays.

Response of NE213 liquid scintillation detectors to high-energy photons (7 MeV < Eγ < 20 MeV)

Two NE213 liquid scintillation detectors of 206 and 412 cm 3 in volume were calibrated in high-energy photon fields ( E γ > 7 MeV) at the PTB accelerator facility. The measured pulse height spectra are in excellent agreement with the response functions simulated with the EGS4 Monte Carlo code. The generally assumed linear dependence of the electron light output on the energy deposited does not hold for electron energies above 1.6 MeV. The nonlinear electron light output function was therefore included in the calculation of the response functions. The production of high-energy electrons and positrons in the target assembly and in the air was also taken into consideration.

Correction factors for parallel-plate chambers used in plastic phantoms in electron dosimetry

In electron beam dosimetry using parallel-plate chambers solid phantoms are sometimes necessary. To obtain the dose to water from the ionization obtained in the solid phantom, fluence correction factors and perturbation factors have to be applied. In this study fluence factors in a perturbation free geometry have been determined experimentally for common phantom materials. Wall perturbation factors for simulated Attix, NACP, and Roos chambers have also been determined for the same materials. Comparative Monte Carlo calculations have been performed using the EGS4 Monte Carlo code. Comparison with data in newly published protocols such as IAEA and IPEMB shows an agreement with the results obtained in this paper to within 1%, demonstrating that the data published in these protocols may be used with reasonable accuracy if recommended phantoms are used. The results also show that if unsuitable phantom materials are used, the wall perturbation factors may differ for different chambers and for different phantom materials by more than 3% and perturbation factors have to be considered in order to obtain a high accuracy in the dose determination.

Monte Carlo calculated dose distribution for endovascular HDR brachytherapy with Ir-192

Background and purpose: For endovascular HDR brachytherapy, very thin sources are required and the dose is specified at a short distance to the source centre down to 1.5 mm. The source which is used in the Nucletron HDR Selectron stepping source afterloader is treated by most dose calculation algorithms in clinical use as a point source, although its dimensions are large compared to these dose specification distances. Furthermore, inaccuracies might be introduced because consecutive dwell positions show an overlap of sources if the step size is smaller than the active length of the source. Materials and methods: In order to quantify these inaccuracies, we used the EGS4 Monte Carlo code to generate the dose distribution with 0.5 mm spatial resolution for a single source. From this, translation and superposition were used to calculate dose distributions for multiple dwell positions. The results are compared with those of other Monte Carlo computations and of a commercial brachytherapy planning system. Results and conclusions: Our Monte Carlo calculations showed that secondary electrons have no relevant influence on the dose distribution and that errors up to 25% are made when using the point source approximation for irradiations with a single dwell position. However, when three or more dwell positions are used with equal dwell times, the total error becomes negligibly small because the errors from subsequent dwell positions compensate each other. At distances larger than 5 mm, there is a good match between the Monte Carlo data and those of point source algorithms for all clinical relevant cases.

Characteristics of clinical electron beams: Current and optimal

This thesis presents the results of two investigations into the characteristics of electron beams for application in radiation therapy. The first involves modeling the interactions between the applicator of a Siemens Mevatron KD2 linear accelerator, and the accelerator's electron beam. EGS4 Monte Carlo code is used to simulate narrow beams incident on sections of applicator trimming plate edges in order to investigate the characteristics of scattering off these edges. A model for scatter from irregular apertures is developed based on a superposition of scatter kernels from edge elements. This model is found to give central‐axis depth dose and profile data consistent with measured results. The model is also used to obtain detailed information of the characteristics of particles scattered from the electron applicator. In the second investigation, optimal electron beam properties are determined by inversion of desired dose distributions using a simulated annealing optimization method. This technique is used for several regular desired dose distributions, and various intuitive results are obtained regarding the effects electron energy and angular modulation have on resulting dose distributions. The technique is also used for several complex desired dose distributions, indicating the relative influence energy and angular modulation can have on dose conformation with electrons.

Beta particle attenuation characteristics of CaSO4.

The beta particle mass attenuation coefficient for CaSO4 in a Teflon matrix has been estimated by two methods: an experiment using collimated sources and a collimated gas flow proportional counter and simulations using the EGS4 Monte Carlo code. The attenuation coefficient has been cast in the general form of mu/rho = aE(-b), where E is the maximum beta particle energy and a and b are fitting parameters. The a and b parameters are given as a function of the atomic number of the material.

Neutron sources in the Varian Clinac 2100C/2300C medical accelerator calculated by the EGS4 code.

The photoneutron yields produced in different components of the medical accelerator heads evaluated in these studies (24-MV Clinac 2500 and a Clinac 2100C/2300C running in the 10-MV, 15-MV, 18-MV and 20-MV modes) were calculated by the EGS4 Monte Carlo code using a modified version of the Combinatorial Geometry of MORSE-CG. Actual component dimensions and materials (i.e., targets, collimators, flattening filters, jaws and shielding for specific accelerator heads) were used in the geometric simulations. Calculated relative neutron yields in different components of a 24-MV Clinac 2500 were compared with the published measured data, and were found to agree to within +/-30%. Total neutron yields produced in the Clinac 2100/2300, as a function of primary electron energy and field size, are presented. A simplified Clinac 2100/2300C geometry is presented to calculate neutron yields, which were compared with those calculated by using the fully-described geometry.

ＢａＦ２シンチレーション検出器を用いた環境光子エネルギー分布評価法の開発

A BaF2 scintillation spectrometer has been constructed for the determination of photon energy spectra in workplaces around nuclear facilities. Energy absorption spectra by the BaF2 detector were calculated with the EGS4 Monte Carlo code in the energy region from 0.1 to 100MeV and a response matrix of the spectrometer was obtained from the energy absorption spectra, of which the energy resolutions were modified to fit to the experimental results. With the irradiation experiments using neutron-capture gamma rays and those from radioactive sources, it became clear that photon energy spectra can be evaluated within an error of about 10% in the energy region 0.1MeV to a few tens of megaelectronvolts.

Phase space evolution distribution functions for high energy electron beams

The phase space evolution (PSE) model is a 3D electron beam dose calculation model for radiation oncology. The PSE model is based upon the transport of electrons with a specific energy and direction over short distances (typically 0.3 - 1 cm). The result of the transport of these electrons is described by an energy and direction distribution of the electrons, which is stored in a database. The database is used by the PSE model at the time of the actual electron transport simulation. A good agreement between dose distributions calculated by the PSE model and EGS4 Monte Carlo code for mono-energetic, mono-directional electron beams was found. The differences in point dose are within 1 - 2% of the maximum dose. These differences can be caused by errors in the database used, or by assumptions made in the PSE model. The aim of this paper is to get more insight into the possible errors introduced by the database. Results show that the data in the database are in good agreement with EGS4 calculated data. Also the influence of the database on a PSE calculated dose distribution has been investigated. The differences between a PSE calculated dose distribution and an EGS4 calculated dose distribution can be reduced to < 0.5% if the database is replaced by a database partly created by EGS4. This shows that small errors in the database have a distinct effect on the dose distribution, and that this dose distribution can be calculated accurately by the PSE model if the right database is used.

Effects of the Photon Cross Sections and Energy-Absorption Coefficients of Air to the Gamma-Ray Point Isotropic Exposure Buildup Factors

The effects of using different photon cross-section libraries and energy-absorption coefficients on the gamma-ray point isotropic exposure buildup factors up to 40 mean free path (mfp) were studied using the EGS4 Monte Carlo code for water, iron, and lead from the 0.1- to 10-MeV energy regions. Differences due to the cross sections used exist, but are small, <10%, except those for lead at 0.1 and 10 MeV. The differences in the case of lead increase along with an increase in the depth and are nearly 30% at 40 mfp depth. The effects of using different energy-absorption coefficients of air are <2%.

Measurement of /spl gamma/-ray dose in a thermal neutron field by using a /sup 3/He-filtered GM counter

A new type of /sup 3/He-filtered GM counter has been used to measure the dose of /spl gamma/-rays existing in a thermal neutron field. The detector assembly consists of a commercially available GM counter (8.8 mm diameter and 31.8 mm height) surrounded by a cylindrical aluminum vessel (220.5 mm diameter and 163 mm height) filled with /sup 3/He gas in the pressure range up to 6 atmospheres. The contribution of secondary /spl gamma/-rays produced by thermal neutron captures in the GM counter itself can be excluded by increasing the gas pressure of /sup 3/He, because the thermal neutron capture reaction of /sup 3/He(n,p)T results in the emission of charged particles of a proton and a triton. In order to determine the response of the GM counter, the deposited energy distribution of the counter to monoenergetic /spl gamma/-rays was calculated by the EGS4 Monte Carlo code in the /spl gamma/-ray energy range from 60 keV to 10 MeV and then the response was derived through the calibration of the counter at /spl gamma/-ray energies of 60 keV, 662 keV and 1.17 MeV +1.33 MeV. On the other hand, the /spl gamma/-ray energy distribution in the thermal neutron field was calculated by the MCNP Monte Carlo calculation, assuming the three dimensional experimental assembly of a graphite pile including the concrete floor in the experimental room. As a result, the /spl gamma/-ray fluence and the effective dose equivalent in the thermal neutron field have been determined by using the GM counter response, the /spl gamma/-ray energy distribution in the field and the conversion factor from fluence to effective dose equivalent.

A hybrid multiple-scattering theory for electron-transport Monte Carlo calculations

This paper describes a new elastic multiple-scattering theory and its incorporation into the EGS4 Monte Carlo code. The important features of the new theory are: 1) the angular distributions are stable physically and numerically from the no-scattering small electron pathlength limit to the multiple scattering regime where electron pathlengths can be thousands of mean-free-paths long, 2) in the small-pathlength limit, the angular distributions may be made to conform to an arbitrary cross section. The object of this paper is to describe the implementation and show results that demonstrate its stability by comparing with distributions generated from a single elastic-scattering model and by showing convergence of backscatter calculations.