BEAMnrc Model Research Articles

Analysis of characteristics and validation of 6 MV photon beam produced by Elekta Synergy linear accelerator using EGSnrc Monte Carlo code

The most important step of the Monte Carlo (MC) dose calculation is the correct modelling of the radiation device. This study was aimed to determine the initial electron beam characteristics of the BEAMnrc model of 6 MV photon beam of Elekta Synergy IGRT System linear accelerator and to validate the simulation. For this purpose, EGSnrc/BEAMnrc/DOSXYZnrc Monte Carlo code were used for an accurate model of a 6 MV Elekta Synergy linac head. To analyze MC model, the dose distributions were calculated by EGSnrc/DOSXYZnrc simulation in a homogeneous water phantom. Then the results were compared with the measurements performed in the water phantom. In order to determine the mean energy and Full Width at Half Maximum (FWHM) suitable for the device in MC model, the Percentage of Depth Dose (PDD) and profile curves were compared for 10 × 10 cm2 field size. All data were analyzed using MATLAB. As a result, the mean energy of the device with the MC model was found to be compatible with the 6.2 MeV spectrum and 1 mm FWHM. For validation of MC model, PDD and profile curves for 5 × 5, 10 × 10, 15 × 15, 20 × 20 and 30 × 30 cm2 field sizes with 6.2 MeV spectrum and 1 mm FWHM were compared with water phantom measurement results.

Monte Carlo and analytic modeling of an Elekta Infinity linac with Agility MLC: Investigating the significance of accurate model parameters for small radiation fields.

PurposeTo explain the deviation observed between measured and Monaco calculated dose profiles for a small field (i.e., alternating open‐closed MLC pattern). A Monte Carlo (MC) model of an Elekta Infinity linac with Agility MLC was created and validated against measurements. In addition, an analytic model which predicts the fluence at the isocenter plane was used to study the impact of multiple beam parameters on the accuracy of dose calculations for small fields.MethodsA detailed MC model of a 6 MV Elekta Infinity linac with Agility MLC was created in EGSnrc/BEAMnrc and validated against measurements. An analytic model using primary and secondary virtual photon sources was created and benchmarked against the MC simulations and the impact of multiple beam parameters on the accuracy of the model for a small field was investigated. Both models were used to explain discrepancies observed between measured/EGSnrc simulated and Monaco calculated dose profiles for alternating open‐closed MLC leaves.Results MC‐simulated dose profiles (PDDs, cross‐ and in‐line profiles, etc.) were found to be in very good agreements with measurements. The best fit for the leaf bank rotation was found to be 9 mrad to model the defocusing of Agility MLC. Moreover, a very good agreement was observed between results from the analytic model and MC simulations for a small field. Modifying the radial size of the incident electron beam in the BEAMnrc model improved the agreement between Monaco and EGSnrc calculated dose profiles by approximately 16% and 30% in the position of maxima and minima, respectively.ConclusionAccurate modeling of the full‐width‐half‐maximum (FWHM) of the primary photon source as well as the MLC leaf design (leaf bank rotation, etc.) is essential for accurate calculations of dose delivered by small radiation fields when using virtual source or MC models of the beam.

The comparison between 6 MV Primus LINAC simulation output using EGSnrc and commissioning data

AbstractIntroductionMonte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy. The purpose of this research is comparison between 6 MV Primus LINAC simulation output with commissioning data using EGSnrc and build a Monte Carlo geometry of 6 MV Primus LINAC as realistically as possible. The BEAMnrc and DOSXYZnrc (EGSnrc package) Monte Carlo model of the LINAC head was used as a benchmark.MethodsIn the first part, the BEAMnrc was used for the designing of the LINAC treatment head. In the second part, dose calculation and for the design of 3D dose file were produced by DOSXYZnrc. The simulated PDD and beam profile obtained were compared with that calculated using commissioning data. Good agreement was found between calculated PDD (1·1%) and beam profile using Monte Carlo simulation and commissioning data. After validation, TPR20,10, TMR and Spvalues were calculated in five different field.ResultsGood agreement was found between calculated values by using Monte Carlo simulation and commissioning data. Average differences for five field sizes in this approach is about 0·83% for Sp. for TPR20,10differences for field sizes 10×10 cm2is 0·29% and for TMR in five field sizes, the average value is ~1·6%.ConclusionIn conclusion, the BEAMnrc and DOSXYZnrc codes package have very good accuracy in calculating dose distribution for 6 MV photon beam and it can be considered as a promising method for patient dose calculations and also the Monte Carlo model of primus linear accelerator built in this study can be used as method to calculate the dose distribution for cancer patients.

Monte Carlo simulation of Varian Linac for 6 MV photon beam with BEAMnrc code

The purpose of this study is to investigate the effects of the initial electron beam parameters on the absorbed dose distribution calculated with EGSnrc Monte Carlo code, for 6MV photon beam. A proposed methodology for benchmarking the BEAMnrc model of Varian Linac has been used. Also, a new photon cross section data based on ENDF/B-VII release 8 evaluation has been employed. The parameters tested include mean energy, radial intensity distribution and angular spread of the initial electron beam. Mean energy and angular spread were tested for a square irradiation field 10 × 10cm2, whereas beam width of the electron beam was studied for 10 × 10cm2 at different depths and 30 × 30cm2 at depth of 10cm. The results obtained are compared with measurement data to select the optimal electron beam parameters. The differences between MC calculation and measurements data are analyzed using gamma index criteria which fixed within 1% −1mm accuracy.The obtained results indicated that the depth-dose and dose-profile curves were considerably influenced by the mean energy of the electron beam. The depth-dose curves were unaffected by the beam width of the electron beam, for both irradiation fields. On the contrary, lateral dose-profile curves were affected by the beam width of initial electron beam. Both dose-profile and depth-dose curves were unaffected to the angular spread of the electron beam. A deep depth of 10 × 10cm2 is very accurate to tune the beam width. Mean energy and beam width must be tuned precisely, to get the MC does distribution with acceptable accuracy.

Abstract ID: 15 Experimental verification of 4D Monte Carlo calculations of dose delivered to a deforming anatomy

The aim of this work is to validate 4D Monte Carlo (MC) simulation method [1] for reconstructing dose delivered to a breathing patient. Static and VMAT plans were delivered to a deformable lung phantom by an Elekta Agility linear accelerator and measured doses were compared with simulations. Measurements were performed in a deformable lung phantom containing a 2.6 cm diameter tumour with the phantom in stationary and moving (sinusoidal) states. Dose within the tumor was measured using EBT3 film and a RADPOS detector connected to the RADPOS 4D dosimetry system [2] . Dose inside the lung was measured by another RADPOS detector mounted outside the tumor at 1.5 cm from the tumor center. A single 6 MV 3 × 3 cm 2 square field and a VMAT plan, both covering the tumour, were created on the end-of-inhale CT scans using Monaco V.5.10.02. A validated BEAMnrc model of our Elekta linac was used for all MC simulations. For 4D simulations, deformation vectors were generated by deformable registration of end-of-exhale to end-of-inhale 4DCT images using Velocity AI 3.2.0 as input to the 4DdefDOSXYZnrc code along with the phantom motion trace recorded with RADPOS. Dose values from MC simulations and measurements were found to be within 3.5% of each other. The passing rate for a gamma comparison of 3%/2 mm between Monte Carlo simulations and film measurements were found to be better than 98%. In conclusion, our 4D Monte Carlo simulations using the defDOSXYZnrc code accurately calculates dose delivered to a deforming anatomy. Future work will focus on irregular respiratory motion patterns.

Validation of BEAMnrc Monte Carlo model for a 12 MV photon beam

Accurate dose calculation in the treatment planning system (TPS) process is the most important step to succeed the radiation therapy. For this purpose, Monte Carlo method is a powerful tool for dose calculation. This study aims to validate Monte Carlo BEAMnrc model of Saturne43 Linac head to simulate 12MV photon beam. To validate MC model, the dose distributions were calculated by BEAMnrc simulation and then the results obtained were compared against measurements. This requires to adjust the parameters of the initial electron beam incident on the target, such as mean energy, beam radius and mean angular spread. Our approach has been suggested to determine the initial electron beam parameters. The dose distribution (percent depth dose and lateral profile) has been calculated for 10×10cm2 field size in a homogeneous water phantom of 40×40×40cm3. The results obtained are compared with measured data using gamma index criteria which were fixed within 1.5%-1mm accuracy. Using phase space technique as a sub-source allows us to reduce the simulation time by a factor of 6. Good agreement between calculated and measured dose has been achieved when the mean energy, beam width and mean angular spread were 11.8MeV, 1.5mm and 0.5°, respectively. So, Monte Carlo based- BEAMnrc code is suitable to be used in the process of treatment planning system for calculating dose distribution.

MO-FG-BRA-01: 4D Monte Carlo Simulations for Verification of Dose Delivered to a Moving Anatomy

Purpose: To validate 4D Monte Carlo (MC) simulations of dose delivery by an Elekta Agility linear accelerator to a moving phantom. Methods: Monte Carlo simulations were performed using the 4DdefDOSXYZnrc/EGSnrc user code which samples a new geometry for each incident particle and calculates the dose in a continuously moving anatomy. A Quasar respiratory motion phantom with a lung insert containing a 3 cm diameter tumor was used for dose measurements on an Elekta Agility linac with the phantom in stationary and moving states. Dose to the center of tumor was measured using calibrated EBT3 film and the RADPOS 4D dosimetry system. A VMAT plan covering the tumor was created on the static CT scan of the phantom using Monaco V.5.10.02. A validated BEAMnrc model of our Elekta Agility linac was used for Monte Carlo simulations on stationary and moving anatomies. To compare the planned and delivered doses, linac log files recorded during measurements were used for the simulations. For 4D simulations, deformation vectors that modeled the rigid translation of the lung insert were generated as input to the 4DdefDOSXYZnrc code as well as the phantom motion trace recorded with RADPOS during the measurements. Results: Monte Carlo simulations and film measurements were found to agree within 2mm/2% for 97.7% of points in the film in the static phantom and 95.5% in the moving phantom. Dose values based on film and RADPOS measurements are within 2% of each other and within 2σ of experimental uncertainties with respect to simulations. Conclusion: Our 4D Monte Carlo simulation using the defDOSXYZnrc code accurately calculates dose delivered to a moving anatomy. Future work will focus on more investigation of VMAT delivery on a moving phantom to improve the agreement between simulation and measurements, as well as establishing the accuracy of our method in a deforming anatomy. This work was supported by the Ontario Consortium of Adaptive Interventions in Radiation Oncology (OCAIRO), funded by the Ontario Research Fund Research Excellence program.

Evaluation of variance reduction techniques in BEAMnrc Monte Carlo simulation to improve the computing efficiency

The application of Variance Reduction Techniques (VRT) in Monte Carlo (MC) codes brings significant improvement in efficiency and a significant profit in the time of simulation. Monte Carlo radiation transport codes have provided by these techniques to solve the time problematic. BEAMnrc Monte Carlo code has several techniques, that have enabled to be the best and fastest code, Including rang rejection, photon forcing, splitting of photon or electron, (Direction, Selective and Uniform) Bremsstrahlung Photon Splitting and Russian roulette (RR). This paper aims to analysis the variance reduction techniques available in BEAMnrc code, validation BEAMnrc model for simulation a Saturne43 accelerator of 12 MV photon beam, and comparing between MCNPX and BEAMnrc.The results obtained show that employing of Direction Bremsstrahlung Photon Splitting (DBS) technique alone or combined with others techniques lead to enhance the efficiency in BEAMnrc simulation. It improves by 16 times compared with Selective Bremsstrahlung Photon Splitting (SBS), 3.7 times (without RR) and it increases about 29 times (with RR) compared to Uniform Bremsstrahlung Photon Splitting (UBS), and also the efficiency increases by factor of 1130 compared to analogue simulation.A good agreement between measurement and calculation of PDD curves for 10×10 cm2 of 12 MV photon beam with local differences less than 2% has been obtained.BEAMnrc has an efficiency greater than MCNPX code in case of analogue simulation (without VRTs).

TU-F-CAMPUS-T-04: Variations in Nominally Identical Small Fields From Photon Jaw Reproducibility and Associated Effects On Small Field Dosimetric Parameters

Purpose: To investigate uncertainties in small field output factors and detector specific correction factors from variations in field size for nominally identical fields using measurements and Monte Carlo simulations. Methods: Repeated measurements of small field output factors are made with the Exradin W1 (plastic scintillation detector) and the PTW microDiamond (synthetic diamond detector) in beams from the Elekta Precise linear accelerator. We investigate corrections for a 0.6x0.6 cm2 nominal field size shaped with secondary photon jaws at 100 cm source to surface distance (SSD). Measurements of small field profiles are made in a water phantom at 10 cm depth using both detectors and are subsequently used for accurate detector positioning. Supplementary Monte Carlo simulations with EGSnrc are used to calculate the absorbed dose to the detector and absorbed dose to water under the same conditions when varying field size. The jaws in the BEAMnrc model of the accelerator are varied by a reasonable amount to investigate the same situation without the influence of measurements uncertainties (such as detector positioning or variation in beam output). Results: For both detectors, small field output factor measurements differ by up to 11 % when repeated measurements are made in nominally identical 0.6x0.6 cm2 fields. Variations in the FWHM of measured profiles are consistent with field size variations reported by the accelerator. Monte Carlo simulations of the dose to detector vary by up to 16 % under worst case variations in field size. These variations are also present in calculations of absorbed dose to water. However, calculated detector specific correction factors are within 1 % when varying field size because of cancellation of effects. Conclusion: Clinical physicists should be aware of potentially significant uncertainties in measured output factors required for dosimetry of small fields due to field size variations for nominally identical fields.

SU‐E‐T‐195; An Algorithm for the Reconstruction of An Entrance Beam Fluence Using a Simulated Exit Phantom Fluence

Purpose: To design and test an algorithm for reconstructing entrance fluence from an exit fluence through a cylindrical phantom using fluence kernels generated in the BEAMnrc monte carlo code. This work may be extended to replace the simulated fluence with direct measurements on an EPID where dosimetry data from treatment can be acquired and compared with direct measures of fluence from devices used commonly for IMRT verification. Method and Materials: A parameter space array of monte carlo fluence kernels is calculated for each voxel in a 256x256 pixel grid measuring across 16.384cm2 at 60cm SPD, which corresponds to a 40.96cm2 imager at 150cm SPD. An array of coefficients of parameter space elements is then used to calculate a fluence at 150cm SPD, and an iterative minimization solver is configured to minimize the sum of the square of the differences between the calculated fluence and the measured fluence. Results: Comparison between two IMRT beam sequences simulated using the same BEAMnrc model used to generate the parameter space array were performed. The iterative solver was allowed to run for 15 minutes on a 128 core cluster on each phase space generated. A comparison of directly simulated entrance fluence and entrance fluence calculated by the proposed algorithm was performed, with a comparison criterion of the percentage of points in the fluence array with a threshold of 10% of maximum intensity, within 1% of the simulated fluence intensity; passing rates of 94.5% and 97.9% were achieved. Conclusion: These results demonstrate that the algorithm is capable of reconstructing entrance fluence from simulated exit fluence within a reasonable tolerance for verification of delivery. Additionally, patient and beam specific parameter space arrays may be generated, allowing live measurements during treatment to be used for the reconstruction of entrance fluence with a high degree of accuracy.

Monte Carlo Study of Fetal Dosimetry Parameters for 6 MV Photon Beam

Because of the adverse effects of ionizing radiation on fetuses, prior to radiotherapy of pregnant patients, fetal dose should be estimated. Fetal dose has been studied by several authors in different depths in phantoms with various abdomen thicknesses (ATs). In this study, the effect of maternal AT and depth in fetal dosimetry was investigated, using peripheral dose (PD) distribution evaluations. A BEAMnrc model of Oncor linac using out of beam components was used for dose calculations in out of field border. A 6 MV photon beam was used to irradiate a chest phantom. Measurements were done using EBT2 radiochromic film in a RW3 phantom as abdomen. The followings were measured for different ATs: Depth PD profiles at two distances from the field's edge, and in-plane PD profiles at two depths. The results of this study show that PD is depth dependent near the field's edge. The increase in AT does not change PD depth of maximum and its distribution as a function of distance from the field's edge. It is concluded that estimating the maximum fetal dose, using a flat phantom, i.e., without taking into account the AT, is possible. Furthermore, an in-plane profile measured at any depth can represent the dose variation as a function of distance. However, in order to estimate the maximum PD the depth of D max in out of field should be used for in-plane profile measurement.

Validation of a commercial TPS based on the VMC++ Monte Carlo code for electron beams: Commissioning and dosimetric comparison with EGSnrc in homogeneous and heterogeneous phantoms

The aim of the present work was the validation of the VMC++ Monte Carlo (MC) engine implemented in the Oncentra Masterplan (OMTPS) and used to calculate the dose distribution produced by the electron beams (energy 5-12 MeV) generated by the linear accelerator (linac) Primus (Siemens), shaped by a digital variable applicator (DEVA). The BEAMnrc/DOSXYZnrc (EGSnrc package) MC model of the linac head was used as a benchmark.Commissioning results for both MC codes were evaluated by means of 1D Gamma Analysis (2%, 2 mm), calculated with a home-made Matlab (The MathWorks) program, comparing the calculations with the measured profiles. The results of the commissioning of OMTPS were good [average gamma index (γ) > 97%]; some mismatches were found with large beams (size ≥ 15 cm). The optimization of the BEAMnrc model required to increase the beam exit window to match the calculated and measured profiles (final average γ > 98%).Then OMTPS dose distribution maps were compared with DOSXYZnrc with a 2D Gamma Analysis (3%, 3 mm), in 3 virtual water phantoms: (a) with an air step, (b) with an air insert, and (c) with a bone insert.The OMTPD and EGSnrc dose distributions with the air-water step phantom were in very high agreement (γ ∼ 99%), while for heterogeneous phantoms there were differences of about 9% in the air insert and of about 10–15% in the bone region. This is due to the Masterplan implementation of VMC++ which reports the dose as “dose to water”, instead of “dose to medium”.

Sci-Thur AM: Planning - 08: Validation of a commercial Monte Carlo code used for stereotactic radiosurgery and stereotactic body radiation therapy.

Our project consisted of validating the BrainLab iPlan Monte Carlo algorithm, used in conjunction with the stereotactic radiosurgery (SRS) mode of the Varian Novalis TX linear accelerator, for clinical use. Our approach was to "benchmark" the iPlan algorithm by comparing dose distributions with those obtained using a BEAMnrc model of the Novalis SRS mode. The BEAMnrc model was obtained by modifying an existing accelerator model to include the SRS flattening filter and source characteristics of the Novalis TX, and by reprogramming a component module to model the high definition 120-leaf multi-leaf collimator. The free parameters of interleaf air gap and leaf density were adjusted by matching to interleaf leakage profiles measured with EBT2 film. The BEAMnrc model was used to perform comparisons of depth dose curves and planar distributions for fields in homogeneous and heterogeneous slab phantoms between both MC codes and film. The source parameters of electron beam energy, size and angular spread were determined to be 6.6 MeV, 0.7 mm and 0.8 mm (cross and in-plane), and 1.27°, respectively. Comparisons between iPlan and EGSnrc MC codes show agreement within 2% for PDD curves, and a high pass rate (>98%) on gamma analysis (3%/3mm) for planar distributions, when the scored quantity is dose to medium. Discrepancies between both MC codes and film measurements were seen near bone inhomogeneities, where the film trend agrees somewhat with iPlan MC reporting dose-to-water. Further work is being performed to understand these differences and how film is used to measure dose near bone.

Adapting a generic BEAMnrc model of the BrainLAB m3 micro-multileaf collimator to simulate a local collimation device

This work is focussed on developing a commissioning procedure so that a Monte Carlo model, which uses BEAMnrc's standard VARMLC component module, can be adapted to match a specific BrainLAB m3 micro-multileaf collimator (µMLC). A set of measurements are recommended, for use as a reference against which the model can be tested and optimized. These include radiochromic film measurements of dose from small and offset fields, as well as measurements of µMLC transmission and interleaf leakage. Simulations and measurements to obtain µMLC scatter factors are shown to be insensitive to relevant model parameters and are therefore not recommended, unless the output of the linear accelerator model is in doubt. Ultimately, this note provides detailed instructions for those intending to optimize a VARMLC model to match the dose delivered by their local BrainLAB m3 µMLC device.

Poster — Thur Eve — 20: Fine Tuning the Source Parameters of a Varian 6 MV BEAMnrc Model through the Simulation of Small Jaw Collimated Photon Fields

Purpose: The goal of this work was to finalize the electron source parameters of a Varian 6 MV BEAMnrc model through the simulation of jaw collimated small fields. Methods and Materials: For nominal field sizes of 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 cm profile data was acquired in water using a stereotactic field diode (SFD) and EBT2 film. To achieve a positional error below ± 0.01 cm a stepper motor driven linear actuator device was incorporated into the Wellhofer water tank. At electron energies of 6.0, 6.1 and 6.2 MeV the spatial distribution of the electrons incident on the target was modelled as a Gaussian. The FWHM was decreased in steps of 0.010 cm from 0.150 to 0.100 cm. DOSXYZnrc simulations were run and profile data extracted for comparison. Results: The SFD and EBT2 profile data were found to be in good agreement with small differences believed to be the result of non‐uniformities inherent to the film. In all cases the measured field sizes were found to be smaller than the nominal and the simulated field sizes adjusted accordingly. At a FWHM = 0.150 cm the penumbral width was much too broad and the smallest field widths wider than experiment. As the FWHM was decreased from 0.150 to 0.110 cm the fit became progressively better, yet worsened at a FWHM = 0.100 cm for all but the smallest field size. Conclusion: Small jaw collimated fields can be used to fine tune the electron source parameters used in a BEAMnrc linac model.

SU‐FF‐T‐325: Modifying the BEAMnrc Phase‐Space to Match Monte Carlo and Measured Dose Distributions

Purpose: To present a method for improving agreement between clinically measured and Monte Carlo (MC) calculated dose distributions (profiles and PDDs) in water for the full range of field sizes. Method and Materials: Linac beam characteristics are modeled using BEAMnrc code that produces particle phase space above the secondary collimators as a new “source” in MC calculations. We assume that the photon energy spectra, represented in the phase space as achieved by the BEAMnrc model and electron beam characteristics, are reasonably accurate. The 3D dose distributions generated from MC and measured data are then used to modify the weights of each particle in the phase‐space. The weight modification occurs through a few (typically 3) iterations and new weights are proportional to the ratio of the measures to MC calculated dose at a point where primary photon crosses the dose matrix plane. One‐dimensional (assuming radial symmetry), 2D and 3D correction techniques were investigated. Denoising of the modified phase space has also been performed to reduce the “latent” dose uncertainty. Results: The method has been applied to our 6 MV and 18 MV beam models and allowed to reduce MC/measured beam profile differences from ∼3% to less than 1% for 6 MV beam, and from ∼5% to ∼1% for 18 MV in the central region of the beam. It also reduced the difference between measured and calculated dose from over 15% to less than 3% in the penumbra of the diagonal profile of the 40 × 40 cm2 field. Conclusion: The developed technique provides a relatively simple alternative to a very time‐consuming process of fine‐tuning BEAMnrc parameters. Initial BEAM model has to be “reasonably accurate” (producing results within ∼5% agreement to measured data) prior to correction. 2D correction technique showed optimal performance providing required dose agreement and performing fasted than 3D correction.