Electron Gamma Shower Version Research Articles

Evaluation of the electron transport algorithm in magnetic field in EGS5 Monte Carlo code.

To evaluate the accuracy of electron transport in the magnetic field of Electron Gamma Shower version 5 (EGS5) by using the special Fano cavity test. To simulate electron transport in the magnetic field, the trajectory of the electron was reconstructed with a short step length to restrict fractional energy loss, and the maximum user step length (mxustep) was set at 0.01cm or 0.001cm. For the special Fano cavity test, three-layer slab Fano test geometry was used, and uniform and isotropic per unit mass mono-energetic electrons with 0.01, 0.1, 1.0, and 10MeV were permitted from the central axis of geometry in 0.35T and 1.5T. Furthermore, the magnetic field strength was scaled based on the mass density of the material. The relative difference between the calculated dose to gap and the theoretical value was evaluated. Furthermore, the special Fano cavity test was also performed using EGSnrc with the electron-enhanced electric and magnetic field macros under the same conditions, and the results were compared with those of EGS5. Deviations in 0.35T were within 0.3% regardless of the parameter settings. In 1.5T, stable results within 0.3% were obtained using 0.001cm as the mxustep, except for one at 10MeV. Further, the accuracy of EGSnrc was within 0.2%, except for 10MeV for a 0.2-cm gap in 1.5T. EGS5 with the appropriate parameter settings enable electron transport in magnetic fields similar with the accuracy of EGSnrc.

Experimental study of humidity effect on charge measurement of reference ionization chambers in clinical high-energy photon beams.

In the practice code of dosimetry, humidity effect is assumed to be constant as far as the measurements are performed in the relative humidity (RH) range of (20-80)%; thus, the humidity effect can be ignored with a dose uncertainty of 0.15%. This assumption is based on the previous experimental results by Niatel and Guiho. Rogers and Ross calculated the stopping power ratio of humid air and dry air for high-energy electron beams by using a Monte Carlo code. They demonstrate that the W value, the mean energy required to create an ion pair in air, is independent of the beam quality when the air is dry, and that the traditional humidity correction can be used also for high-energy photon and electron beams; however, this was only a computational study. In the present study, we measured the humidity correction of Farmer-type ionization chambers in high-energy photon beams and determined the W values of humid air using the calculated energy deposition of humid air with a Monte Carlo code. Furthermore, we proposed an analytical expression to determine a practical humidity correction for an ionization chamber as a function of absolute humidity. Experiments were carried out using a clinical linear accelerator (linac, Elekta Precise) at the National Metrology Institute of Japan (NMIJ). A shield box was constructed downstream of the linac and connected to an air processor, which maintained the temperature around 22°C and controlled the humidity in the range of (10-70)% inside the box. We prepared two Farmer-type ionization chambers: PTW 30013 and Exradin A19. Each ionization chamber was placed inside the box and irradiated with 6-, 10-, and 15-MV high-energy photon beams from the clinical linac. The energy deposition to the humid air inside the ionization chamber was calculated using the Electron Gamma Shower Version 5 (EGS5) code system. Stabilization for the humidity of the ionization chamber was completed within 3h. The polarity and ion recombination corrections did not show any change in the humidity range studied. The measured humidity correction and the evaluated W values of humid air in high-energy photon beams were in good agreement with those by Rogers in TG-21 and by Niatel in the range of RH (10-70)%. Humidity correction of ionization chambers in high-energy photon beams from the clinical linac was determined experimentally. Using the analytical expression for the energy depositions by EGS5, the analytical expression for the W values was also derived.

Implementing displacement damage calculations for electrons and gamma rays in the Particle and Heavy-Ion Transport code System

Abstract In this study, the Monte Carlo displacement damage calculation method in the Particle and Heavy-Ion Transport code System (PHITS) was improved to calculate displacements per atom (DPA) values due to irradiation by electrons (or positrons) and gamma rays. For the damage due to electrons and gamma rays, PHITS simulates electromagnetic cascades using the Electron Gamma Shower version 5 (EGS5) algorithm and calculates DPA values using the recoil energies and the McKinley–Feshbach cross section. A comparison of DPA values calculated by PHITS and the Monte Carlo assisted Classical Method (MCCM) reveals that they were in good agreement for gamma-ray irradiations of silicon and iron at energies that were less than 10 MeV. Above 10 MeV, PHITS can calculate DPA values not only for electrons but also for charged particles produced by photonuclear reactions. In DPA depth distributions under electron and gamma-ray irradiations, build-up effects can be observed near the target’s surface. For irradiation of 90-cm-thick carbon by protons with energies of more than 30 GeV, the ratio of the secondary electron DPA values to the total DPA values is more than 10% and increases with an increase in incident energy. In summary, PHITS can calculate DPA values for all particles and materials over a wide energy range between 1 keV and 1 TeV for electrons, gamma rays, and charged particles and between 10−5 eV and 1 TeV for neutrons.

Perfusion CT of the Brain and Liver and of Lung Tumors: Use of Monte Carlo Simulation for Patient Dose Estimation for Examinations With a Cone-Beam 320-MDCT Scanner.

The purpose of this study was to estimate the patient dose from perfusion CT examinations of the brain, lung tumors, and the liver on a cone-beam 320-MDCT scanner using a Monte Carlo simulation and the recommendations of the International Commission on Radiological Protection (ICRP). A Monte Carlo simulation based on the Electron Gamma Shower Version 4 package code was used to calculate organ doses and the effective dose in the reference computational phantoms for an adult man and adult woman as published by the ICRP. Three perfusion CT acquisition protocols--brain, lung tumor, and liver perfusion--were evaluated. Additionally, dose assessments were performed for the skin and for the eye lens. Conversion factors were obtained to estimate effective doses and organ doses from the volume CT dose index and dose-length product. The sex-averaged effective doses were approximately 4 mSv for perfusion CT of the brain and were between 23 and 26 mSv for the perfusion CT body protocols. The eye lens dose from the brain perfusion CT examination was approximately 153 mGy. The sex-averaged peak entrance skin dose (ESD) was 255 mGy for the brain perfusion CT studies, 157 mGy for the lung tumor perfusion CT studies, and 172 mGy for the liver perfusion CT studies. The perfusion CT protocols for imaging the brain, lung tumors, and the liver performed on a 320-MDCT scanner yielded patient doses that are safely below the threshold doses for deterministic effects. The eye lens dose, peak ESD, and effective doses can be estimated for other clinical perfusion CT examinations from the conversion factors that were derived in this study.

Monte Carlo simulation of irradiation and killing in three-dimensional cell populations with lognormal cellular uptake of radioactivity

Purpose: The biological response of tissue exposed to radiations emitted by internal radioactivity is often correlated with the mean absorbed dose to a tissue element. However, experimental studies show that even when the mean absorbed dose to the tissue element is constant, the response of the cell population within the tissue element can vary significantly depending on the distribution of radioactivity at the cellular and multicellular levels. The present work develops theoretical models to simulate these observations.Materials and methods: Two theoretical models were created to simulate experimental three-dimensional cell culture models with homogeneous and inhomogeneous tissue environments. The cells were assigned activities according to lognormal distributions of an alpha particle emitter or a monoenergetic electron emitter. Absorbed doses to the cell nuclei were assessed with point-kernel geometric-factor and Electron Gamma Shower version nrc (EGSnrc) Monte Carlo radiation transport simulations, respectively. The self- and cross-dose to individual cell nuclei were calculated and a Monte Carlo method was used to determine their fate. Survival curves were produced after tallying the live and dead cells.Results: Both percent cells labeled and breadth of lognormal distribution affected the dose distribution at the cellular level, which in turn, influenced the shape of the cell survival curves.Conclusions: Multicellular Monte Carlo dosimetry-models offer improved capacity to predict response to radiopharmaceuticals compared to approaches based on mean absorbed dose to the tissue.

Development of a geometry‐based respiratory motion– simulating patient model for radiation treatment dosimetry

Temporal and spatial anatomic changes caused by respiration during radiation treatment delivery can lead to discrepancies between prescribed and actual radiation doses. The present paper documents a study to construct a respiratory‐motion‐simulating, four‐dimensional (4D) anatomic and dosimetry model for the study of the dosimetric effects of organ motion for various radiation treatment plans and delivery strategies. The non‐uniform rational B‐splines (NURBS) method has already been used to reconstruct a three‐dimensional (3D) VIP‐Man (“visible photographic man”) model that can reflect the deformation of organs during respiration by using time‐dependent equations to manipulate surface control points. The EGS4 (Electron Gamma Shower, version 4) Monte Carlo code is then used to apply the 4D model to dose simulation. We simulated two radiation therapy delivery scenarios: gating treatment and 4D image‐guided treatment. For each delivery scenario, we developed one conformal plan and one intensity‐modulated radiation therapy plan. A lesion in the left lung was modeled to investigate the effect of respiratory motion on radiation dose distributions. Based on target dose–volume histograms, the importance of using accurate gating to improve the dose distribution is demonstrated. The results also suggest that, during 4D image‐guided treatment delivery, monitoring of the patient's breathing pattern is critical. This study demonstrates the potential of using a “standard” motion‐simulating patient model for 4D treatment planning and motion management.PACS numbers: 87.53.Bn, 87.53.Kn, 87.53.Tf, 87.53.Wz, 87.57.Gg, 89.80.+h

Study of scattered photons from the collimator system of Leksell Gamma Knife using the EGS4 Monte Carlo Code

In the algorithm of Leksell GAMMAPLAN (the treatment planning software of Leksell Gamma Knife), scattered photons from the collimator system are presumed to have negligible effects on the Gamma Knife dosimetry. In this study, we used the EGS4 Monte Carlo (MC) technique to study the scattered photons coming out of the single beam channel of Leksell Gamma Knife. The PRESTA (Parameter Reduced Electron-Step Transport Algorithm) version of the EGS4 (Electron Gamma Shower version 4) MC computer code was employed. We simulated the single beam channel of Leksell Gamma Knife with the full geometry. Primary photons were sampled from within the 60Co source and radiated isotropically in a solid angle of 4pi. The percentages of scattered photons within all photons reaching the phantom space using different collimators were calculated with an average value of 15%. However, this significant amount of scattered photons contributes negligible effects to single beam dose profiles for different collimators. Output spectra were calculated for the four different collimators. To increase the efficiency of simulation by decreasing the semiaperture angle of the beam channel or the solid angle of the initial directions of primary photons will underestimate the scattered component of the photon fluence. The generated backscattered photons from within the 60Co source and the beam channel also contribute to the output spectra.

511keV消滅放射線および<sup>99m</sup>Tcガンマ線に対する診断用X線防護衣および防護用鉛ガラス衝立の遮蔽能力評価

Various pharmaceutical companies in Japan are making radioactive drugs available for positron emission tomography (PET) in hospitals without a cyclotron. With the distribution of these drugs to hospitals, medical check-ups and examinations using PET are expected to increase. However, the safety guidelines for radiation in the new deployment of PET have not been adequately improved. Therefore, we measured the shielding effect of a clinical X-ray protector and lead glass against annihilation radiation and gamma rays of (99m)Tc. We then calculated the shielding effect of a 0.25 mm lead protector, 1 mm lead, and lead glass using the EGS4 (Electron Gamma Shower Version 4) code. The shielding effects of 22-mm lead glass against annihilation radiation and gamma rays of (99m)Tc were approximately 31.5% and 93.3%, respectively. The clinical X-ray protector against annihilation radiation approximately doubled the skin-absorbed dose.

Diagnostic x-ray dosimetry using Monte Carlo simulation

An Electron Gamma Shower version 4 (EGS4) based user code was developed to simulate the absorbed dose in humans during routine diagnostic radiological procedures. Measurements of absorbed dose using thermoluminescent dosimeters (TLDs) were compared directly with EGS4 simulations of absorbed dose in homogeneous, heterogeneous and anthropomorphic phantoms. Realistic voxel-based models characterizing the geometry of the phantoms were used as input to the EGS4 code. The voxel geometry of the anthropomorphic Rando phantom was derived from a CT scan of Rando. The 100 kVp diagnostic energy x-ray spectra of the apparatus used to irradiate the phantoms were measured, and provided as input to the EGS4 code. The TLDs were placed at evenly spaced points symmetrically about the central beam axis, which was perpendicular to the cathode–anode x-ray axis at a number of depths. The TLD measurements in the homogeneous and heterogenous phantoms were on average within 7% of the values calculated by EGS4. Estimates of effective dose with errors less than 10% required fewer numbers of photon histories (1 × 107) than required for the calculation of dose profiles (1 × 109). The EGS4 code was able to satisfactorily predict and thereby provide an instrument for reducing patient and staff effective dose imparted during radiological investigations.

A practical technique for verification of three-dimensional conformal dose distributions in stereotactic radiosurgery.

The trend toward conformal techniques in stereotactic radiosurgery necessitates an accurate and practical method for verification of irregular three-dimensional dose distributions. This work presents the design and evaluation of a phantom system facilitating the measurement of conformal dose distributions using one or more arrays of up to 20 radiographic films separated by 3.2 mm-thick tissue-equivalent spacers. Using Electron Gamma Shower version 4 (EGS4) Monte Carlo simulation, we show that for 6 MV radiosurgical photon beams this arrangement preserves tissue-equivalence to within 1%. The phantom provides 0.25 mm in-plane spatial resolution and multiple sets of films may be used to resample the dose volume in orthogonal planes. Dedicated software has been developed to automate the process of ordering and orienting of scanned film images, conversion of scanned pixel value to dose, resampling of one or more sets of film images and subsequent export of images in DICOM format for coregistration of planned and measured dose volumes. Calculated and measured isodose surfaces for a simple, circular-beam treatment agree to within 1.5 mm throughout the dose volume. For conformal radiosurgical applications, the measured and planned dose distributions agree to within the uncertainty of the manufacture of irregularly shaped collimators. The sensitivity of this technique to minor spatial inaccuracies in beam shaping is also demonstrated.

X Ray Spectrometry of Low Energy Photons for Determining Conversion Coefficients from Air Kerma, Ka, to Personal Dose Equivalent, Hp(10), for Radiation Qualities of the ISO Narrow Spectrum Series

The pulse height spectra of the radiation qualities N-10 to N-120 of the ISO narrow spectrum series (N) were measured using a commercial Ge spectrometer and a specially designed lead collimator. For the unfolding of these spectra, three response matrices of the Ge spectrometer for different maximum energies and energy resolutions were calculated by the EGS4 (Electron Gamma Shower version 4) code incorporating the PRESTA and LSCAT options. Then, the pulse height spectra were unfolded with two different algorithms to check the correctness. To verify the measuring and unfolding method, the first half value layers (HVLs) were determined using small ionisation chambers and the fluence spectra. The mean photon energies were calculated, also on the basis of the fluence spectra, and compared with published values. For each radiation quality N-10 to N-120 the conversion coefficients hpK(10;N,a) from air kerma Ka to personal dose equivalent at 10mm depth, Hp (10,a), were calculated for angles of incidence a of 15, 30, 45, 60(and 75(between the unidirectional photon field and the normal to the slab phantom surface. Because the fluence spectra were influenced by the air density, in particular the low energy spectra, the hpK(10;N,a) values were normalised to reference conditions. Differences of up to about 88% between the hpK(10;N,a) values presented in this work and those given in ISO/FDIS 4037-3 were found.

Improvement of the Sensitivity of CdTe Radiation Detectors in the High Energy Region

Cadmium telluride (CdTe) radiation detectors are usable at room temperature. Recently, very pure CdTe crystals have become available thanks to recent improvements in their manufacturing process, making CdTe radiation detectors more attractive for industrial applications. The basic performance of a CdTe detector was investigated to examine the feasibility of such radiation detectors in nuclear power plants. Peak energy resolution is obtained in the energy range of 60 keV with a resolution of 3.2 keV; further, for photons under 800 keV the resolution remains better than 10 keV. The sensitivity decreases regularly as photon energy increases. The sensitivity of the CdTe radiation detector was calculated by the Monte Carlo simulation code EGS4 (Electron-Gamma Shower version 4). The sensitivity of CdTe radiation detectors in the high energy region has been improved by means of a multi-layered structure. Consequently, the relative sensitivity becomes greater than the number of layers because of the effects of multiple scattering in the CdTe multi-layered detector.

Calculations of ion chamber displacement effect corrections for medium-energy X-ray dosimetry

The authors investigated the displacement effect corrections for ionization chambers calibrated in air in terms of air kerma and used at a depth in a phantom irradiated by medium-energy X-ray beams for tube potentials between 100 kV and 300 kV. The EGS4 (electron gamma shower version 4) Monte Carlo code system has been used to simulate the coupled transport of photons and electrons. The chamber displacement correction factor, pdis, has been calculated both using a simple photon attenuation and scattering method and by direct Monte Carlo calculation of the ratio of the water kerma at a depth in a phantom to the water kerma in a low-density (the same density as that of air) water cavity with its centre placed at the same depth in the phantom irradiated by medium-energy X-ray beams. Good agreement has been achieved between the pdis factors obtained using the simple method and those obtained by the direct Monte Carlo simulation. In contrast to the values recommended by the IAEA code of practice, which exceed unity by up to 10%, our pdis values for a stemless NE2571 chamber vary between 0.994 and 1.001 for the same energy range and for a stemless NE2561 chamber between 0.974 and 0.993. The beam quality correction factor and stem correction factor have been investigated in separate works. The ratios of the overall correction factors calculated in this work for an NE2571 chamber to that for an NE2561 chamber are consistent with the recent experimental results.

Monte Carlo calculated stem effect corrections for NE2561 and NE2571 chambers in medium-energy X-ray beams

Presents the stem effect correction factors for the NE2561 and NE2571 ionization chambers calibrated in air and used at a depth in a phantom irradiated by medium-energy X-ray beams. The in air and in phantom stem effect correction factors were calculated as the ratios of the absorbed dose in the air cavity of an ionization chamber without and with a chamber stem. The 'global' stem correction factor was then calculated as the ratio of the in phantom correction factor to the in air correction factor. The calculations were carried out using the EGS4 (electron gamma shower version 4) Monte Carlo code system together with the use of a correlated sampling variance reduction technique. The results show that in general the chamber stem increases the chamber response in air but decreases it in phantom. For an NE2571 chamber the 'global' stem effect correction factor varies from 1.014+or-0.001 at 70 kV (2.9 mm Al, 0.1 mm Cu) to 1.005+or-0.001 at 300 kV (21.5 mm Al, 5.1 mm Cu) while for an NE2561 chamber it varies from 1.039+or-0.002 to 1.010+or-0.002 for the same kilovoltage and HVL values. In a 60Co beam the global stem correction is 1.001+or-0.001 for an NE2571 chamber and 1.003+or-0.001 for an NE2561 chamber. This is consistent with recent experimental results.

Implementation of a Monte Carlo code on a parallel computer system

This paper reports the implementation of the EGS4 (Electron Gamma Shower version 4) Monte Carlo code system on a multiple-transputer system, the Edinburgh Concurrent Supercomputer (ECS). Basic aspects of running Monte Carlo calculations on parallel computer are described including distribution of random number seeds to the individual processes. Based on an independent task farm topology, an existing EGS4 usercode, DOSIMETER, which can be used to calculate the absorbed dose in cylindrical dosimeters placed in cubic phantoms, has been re-written for running on the ECS. The parallelised code consists of three sequential programmes: 1. (1) a ‘control’ process to generate tasks; 2. (2) a number of ‘simulation’ processes to consume tasks and to generate results; and 3. (3) an ‘analysis’ process to collect and to analyse results. A linear increase in computing speed has been achieved with the number of ‘simulation’ transputers used on the ECS. The basic techniques used in this work are also suitable for the implementation of the Monte Carlo code on other parallel computer systems which the implementation of the Monte Carlo code on other parallel computer systems which have similar operational structures as the ECS.

Calculation of absorbed dose ratios using correlated Monte Carlo sampling.

A correlated sampling variance reduction technique has been implemented in the EGS4 (Electron Gamma Shower version 4) Monte Carlo code system in order to calculate correction factors for radiation dosimeters. These correction factors are calculated as the ratios of absorbed doses in various geometries irradiated by photon and electron beams. In this paper the efficiency and accuracy of the calculation of the absorbed dose ratios with correlated Monte Carlo sampling is discussed. Theoretical analysis shows that the calculation efficiency and accuracy increases with the degree of correlation between the calculated doses in the individual geometries. Calculations of the absorbed doses in thin slab geometries and in air-filled ionization chambers have been performed. The results show that the gain in computing efficiency is up to a thousand fold for the calculations of the effect of the central electrode material and size on the response of cylindrical ionization chambers. The calculation accuracy (systematic uncertainty) is significantly improved when good correlation exists between the calculated doses in similar thin-slab geometries.

Correction factors for Fricke dosimetry in high-energy electron beams

The paper presents Fricke-to-water dose conversion and wall correction factors for Fricke ferrous sulphate dosimetry in high-energy electron beams. The dose conversion factor has been calculated as the ratio of the mean dose in water to the mean dose in the Fricke solution with a water-walled vessel and the wall correction factor accounts for the change in the Fricke dose due to the presence of the non-water wall material. The EGS4 (electron gamma shower version 4) Monte Carlo code system has been employed in the work together with the application of a correlated sampling variance reduction technique. The results show that for a Fricke dosimeter of 1.534 cm diameter and 5.5 cm length the dose conversion factor is nearly constant at 1.004 (within 0.1%) for electron energies of 11-25 MeV if the dosimeter is placed at the depth of maximum dose but can vary by a few per cent if the dosimeter is placed on the descending portion of the depth-dose curve.

A Comparison of Gamma-Ray Buildup Factors for Low-ZMaterial and for Low Energies Using Discrete Ordinates and Point Monte Carlo Methods

This paper discusses the calculation of exposure and absorbed dose buildup factors for a photon point source in infinite beryllium in the low-energy range of 0.03 to 0.3 MeV, for penetration depth up to 40 mfp, using two discrete ordinates codes, PALLAS-PL, SP-Br, and ANISN. Comparisons of both result to values obtained by point Monte Carlo calculations using the electron gamma shower version 4 code show reasonable agreement for two types of sources: normally incident and point isotropic sources. The fitting parameters of a geometric-progression method formula are determined for the resulting buildup factor data. These fitting parameters are in good agreement with the basic data within 5% over a great variation in magnitude.