Towards MR-guided electron therapy: Measurement and simulation of clinical electron beams in magnetic fields

PurposeIn the current era of MRI-linac radiotherapy, dose optimization with arbitrary dose distributions is a reality. For the first time, we present new and targeted experiments and modeling to aid in evaluating the potential dose improvements offered with an electron beam mode during MRI-linac radiotherapy.MethodsSmall collimated (1 cm diameter and 1.5 × 1.5 cm² square) electron beams (6, 12 and 20 MeV) from a clinical linear accelerator (Varian Clinac 2100C) are incident perpendicular and parallel to the strong and localized magnetic fields (0–0.7 T) generated by a permanent magnet device. Gafchromic EBT3 film is placed inside a slab phantom to measure two-dimensional dose distributions. A benchmarked and comprehensive Monte Carlo model (Geant4) is established to directly compare with experiments.ResultsWith perpendicular fields a 5% narrowing of the beam FWHM and a 10 mm reduction in the 15% isodose penetration is seen for the 20 MeV beam. In the inline setup the penumbral width is reduced by up to 20%, and a local central dose enhancement of 100% is observed. Monte Carlo simulations are in agreement with the measured dose distributions (2% or 2 mm).ConclusionA new range of experiments have been performed to offer insight into how an electron beam mode could offer additional choices in MRI-linac radiotherapy. The work extends on historic studies to bring a successful unified experimental and Monte Carlo modeling approach for studying small field electron beam dosimetry inside magnetic fields. The results suggest further work, particularly on the inline magnetic field scenario.     

Comparison of the RayStation photon Monte Carlo dose calculation algorithm against measured data under homogeneous and heterogeneous irradiation geometries

PurposeThis work compares Monte Carlo dose calculations performed using the RayStation treatment planning system against data measured on a Varian Truebeam linear accelerator with 6 MV and 10 MV FFF photon beams.MethodsThe dosimetric performance of the RayStation Monte Carlo calculations was evaluated in a variety of irradiation geometries employing homogeneous and heterogeneous phantoms. Profile and depth dose comparisons against measurement were carried out in relative mode using the gamma index as a quantitative measure of similarity within the central high dose regions.ResultsThe results demonstrate that the treatment planning system dose calculation engine agrees with measurement to within 2%/1 mm for more than 95% of the data points in the high dose regions for all test cases. A systematic underestimation was observed at the tail of the profile penumbra and out of field, with mean differences generally <0.5 mm or 1% of curve dose maximum respectively. Out of field agreement varied between evaluated beam models.ConclusionsThe RayStation implementation of photon Monte Carlo dose calculations show good agreement with measured data for the range of scenarios considered in this work and is deemed sufficiently accurate for introduction into clinical use.     

Model of total skin electron treatment using the 'six-dual-field' technique

During implementation of the total skin electron treatment, using six-dual-field technique, at radiotherapy department a large number of measurements are needed. To assess depth dose curve required by clinicians and dose uniformity over a whole treatment plane, combinations of different irradiation parameters are used (electron energy, beam angle, scatterers). Measurements for each combination must be performed. One possible way to reduce number of measurements is to model the treatment using the Monte Carlo simulation of electron transport. We made a simplified multiple-source Monte Carlo model of electron beam and tested it by comparing calculations and experimental results. Calculated data differs less than 5 percent from measurements in the treatment plane. During the treatment patient can be approximated using cylinders with different diameters and orientations. We tried to model the depth dose variations in the total skin electron treatment not just around the body cross-section (simplified to cylinders of different diameters), but also along the body to account for the variations in body curvature longitudinally. This effect comes down to the problem of modeling distribution in different cylinders, but varying the longitudinal orientation of those cylinders. We compared Monte Carlo calculations and film measurements of depth dose curves for two orientations of the cylindrical phantom, which were the simplest for experimental arrangement. Comparison of the results proved accuracy of the model and we used it to calculate depth dose curves for a number of other cylinder orientations.     

On pathlength and energy straggling of megavoltage electrons slowing down

Purposeto elucidate the effects of multiple scattering and energy-loss straggling on electron beams slowing down in materials.MethodsEGSnrc Monte Carlo simulations are done using a purpose-written user-code.ResultsPlots are presented of the primary electron’s energy as a function of pathlength for 20 MeV electrons incident on water and tantalum as are plots of the overall distribution of pathlengths as the 20 MeV electrons slow down under various Monte Carlo scenarios in water and tantalum. The distributions range from 1 % to 135 % of the CSDA range in water and from 1 % to 186 % in tantalum. The effects of energy-loss straggling on energy spectra at depth and electron fluence at depth are also presented.ConclusionsThe role of energy-loss straggling and multiple scattering are shown to play a significant role in the range straggling which determines the dose fall-off region in electron beam dose vs depth curves and a significant role in the energy distributions as a function of depth.     

Integration of the M6 Cyberknife in the Moderato Monte Carlo platform and prediction of beam parameters using machine learning

PurposeThis work describes the integration of the M6 Cyberknife in the Moderato Monte Carlo platform, and introduces a machine learning method to accelerate the modelling of a linac.MethodsThe MLC-equipped M6 Cyberknife was modelled and integrated in Moderato, our in-house platform offering independent verification of radiotherapy dose distributions. The model was validated by comparing TPS dose distributions with Moderato and by film measurements. Using this model, a machine learning algorithm was trained to find electron beam parameters for other M6 devices, by simulating dose curves with varying spot size and energy. The algorithm was optimized using cross-validation and tested with measurements from other institutions equipped with a M6 Cyberknife.ResultsOptimal agreement in the Monte Carlo model was reached for a monoenergetic electron beam of 6.75 MeV with Gaussian spatial distribution of 2.4 mm FWHM. Clinical plan dose distributions from Moderato agreed within 2% with the TPS, and film measurements confirmed the accuracy of the model. Cross-validation of the prediction algorithm produced mean absolute errors of 0.1 MeV and 0.3 mm for beam energy and spot size respectively. Prediction-based simulated dose curves for other centres agreed within 3% with measurements, except for one device where differences up to 6% were detected.ConclusionsThe M6 Cyberknife was integrated in Moderato and validated through dose re-calculations and film measurements. The prediction algorithm was successfully applied to obtain electron beam parameters for other M6 devices. This method would prove useful to speed up modelling of new machines in Monte Carlo systems.     

Influence of optional measurement parameters in the Eclipse treatment planning system on the quality of the dosimetric model of the biomedical accelerator using the Acuros XB algorithm

BackgroundTo properly configure a treatment planning system, a measurement data set is needed, which consists of the values required for its configuration. The aim is to obtain a dosimetric model of the beam that is as compatible as possible with the measured values. The set of required data can be supplemented with optional values. The aim of the study was to assess the influence of optional measurement data on the compliance of the calculations with the measurements.Materials and methodsDosimetric measurements, model configuration and dose distribution calculations were performed for the photon radiation beams generated by the VMS TrueBeam^® linear accelerator. Beams were configured on an Eclipse™ v. 15.6 system using the Acuros v. 15.6 algorithm. The measured and calculated data were entered into the Alfard™ software for comparison with the calculated dose distributions. In the last stage, the absolute dose values at the designated points were also compared. The obtained data were statistically analysed with Statistica™ v. 13.3.ResultsThe work showed that the differences in the shape of the beam profile, depth dose and the dose value in points were not related to the use of optional data. Differences in dose distributions are within the tolerance. It cannot be determined under which conditions the use of optional data has a more favourable effect on the reflection of the actual dose values.ConclusionsThe use of optional data in modelling photon radiation beams does not significantly improve the compliance of the calculated and measured dose values.     

Parameterization of electron beam output factor

Electron beam dose distribution is dependent on the beam energy and complicated trajectory of particles. Recent treatment planning systems using Monte Carlo calculation algorithm provide accurate dose calculation. However, double check of monitor units (MUs) based on an independent algorithm is still required. In this study, we have demonstrated single equation that reproduces the measured relative output factor (ROF) that can be used for MU calculation for electron radiotherapy. Electron beams generated by an iX (Varian Medical Systems) and a PRIMUS (Siemens) accelerator were investigated. For various energies of electron beams, the ROF at respective d_max were measured using diode detector in a water phantom at SSD of 100 cm. Curve fitting was performed with an exponential generalized equation ROF = α(β – e^−γR) including three variables (α, β, γ) as a function of field radius and electron energy. The correlation coefficients between the ROF measured and that calculated by the equation were greater than 0.998. For ROF of Varian electron beams, the average values of all fitting formulas were applied for two of the constants; α and β. The parameter γ showed good agreement with the quadratic approximation as a function of mean energy at surface (E₀). The differences between measured and calculated ROF values were within ±3% for beams with cutout radius of ≥1.5 cm for electron beams with energies from 6 MeV to 15 MeV. The proposed formula will be helpful for double-check of MUs, as it requires minimal efforts for MU calculation.     

Monte Carlo comparison of superficial dose between flattening filter free and flattened beams

This study investigates the superficial dose from FFF beams in comparison with the conventional flattened ones using a Monte Carlo (MC) method. Published phase-space files which incorporated real geometry of a TrueBeam accelerator were used for the dose calculation in phantom and clinical cases. The photon fluence on the central axis is 3 times that of a flattened beam for a 6 MV FFF beam and 5 times for a 10 MV beam. The mean energy across the field in air at the phantom surface is 0.92–0.95 MeV for the 6 MV FFF beam and 1.18–1.30 MeV for the corresponding flattened beam. At 10 MV, the values are 1.52–1.72 and 2.15–2.87 MeV for the FFF and flattened beams, respectively. The phantom dose at the depth of 1 mm in the 6 MV FFF beam is 6% ± 2.5% (of the maximum dose) higher compared to the flattened beam for a 25 × 25 cm² field and 14.6% ± 1.9% for the 2 × 2 cm² field. For the 10 MV beam, the corresponding differences are 3.4% ± 1.5% and 10.7% ± 0.6%. The skin dose difference at selected points on the patient's surface between the plans using FFF and flattened beams in the head-and-neck case was 6.5% ± 2.3% (1SD), and for the breast case it was 6.4% ± 2.3%. The Monte Carlo simulations showed that due to the lower mean energy in the FFF beam, the clinical superficial dose is higher without the flattening filter compared to the flattened beam.     

Influence of the Presence of Tissue Expanders on Energy Deposition for Post-Mastectomy Radiotherapy

An increasing number of studies have shown that post-mastectomy radiotherapy presents benefits associated with the patients survival and a significant fraction of the treated patients makes use of tissue expanders for breast reconstruction. Some models of tissue expanders have a magnetic disk on their surface that constitutes heterogeneity in the radiation field, which can affect the dose distribution during the radiotherapy treatment. In this study, the influence of a metallic heterogeneity positioned in a breast tissue expander was evaluated by means of Monte Carlo simulations using the MCNPX code and using Eclipse treatment planning system. Deposited energy values were calculated in structures which have clinical importance for the treatment. Additionally, the effect in the absorbed energy due to backscattering and attenuation of the incident beam caused by the heterogeneity, as well as due to the expansion of the prosthesis, was evaluated in target structures for a 6 MV photon beam by simulations. The dose distributions for a breast treatment were calculated using a convolution/superposition algorithm from the Eclipse treatment planning system. When compared with the smallest breast expander volume, underdosage of 7% was found for the largest volume of breast implant, in the case of frontal irradiation of the chest wall, by Monte Carlo simulations. No significant changes were found in dose distributions for the presence of the heterogeneity during the treatment planning of irradiation with an opposed pair of beams. Even considering the limitation of the treatment planning system, the results obtained with its use confirm those ones found by Monte Carlo simulations for a tangent beam irradiation. The presence of a heterogeneity didńt alters the dose distributions on treatment structures. The underdosage of 7% observed with Monte Carlo simulations were found for irradiation at 0°, not used frequently in a clinical routine.     

Monte Carlo determination of a nanoDot OSLD response using quality index for diagnostic kilovoltage X-ray beams

PurposeThis study aims to investigate the energy response of an optically stimulated luminescent dosimeter known as nanoDot for diagnostic kilovoltage X-ray beams via Monte Carlo calculations.MethodsThe nanoDot response is calculated as a function of X-ray beam quality in free air and on a water phantom surface using Monte Carlo simulations. The X-ray fluence spectra are classified using the quality index (QI), which is defined as the ratio of the effective energy to the maximum energy of the photons. The response is calculated for X-ray fluence spectra with QIs of 0.4, 0.5, and 0.6 with tube voltages of 50–137.6 kVp and monoenergetic photon beams. The surface dose estimated using the calculated response is verified by comparing it with that measured using an ionization chamber.ResultsThe nanoDot response in free air for monoenergetic photon beams (QI = 1.0) varies significantly at photon energies below 100 keV and reaches a factor of 3.6 at 25–30 keV. The response differs by up to approximately 6% between QIs of 0.4 and 0.6 for the same half-value layer (HVL). The response at the phantom surface decreases slightly owing to the backscatter effect, and it is almost independent of the field size. The agreement between the surface dose estimated using the nanoDot and that measured using the ionization chamber for assessing X-ray beam qualities is less than 2%.ConclusionsThe nanoDot response is indicated as a function of HVL for the specified QIs, and it enables the direct surface dose measurement.     

Monte Carlo simulation using PRIMO code as a tool for checking the credibility of commissioning and quality assurance of 6 MV TrueBeam STx varian LINAC

AimTo validate and implement Monte Carlo simulation using PRIMO code as a tool for checking the credibility of measurements in LINAC initial commissioning and routine Quality Assurance (QA). Relative and absolute doses of 6 MV photon beam from TrueBeam STx Varian Linear Accelerator (LINAC) were simulated and validated with experimental measurement, Analytical Anisotropic Algorithm (AAA) calculation, and golden beam.Methods and MaterialsVarian phase-space files were imported to the PRIMO code and four blocks of jaws were simulated to determine the field size of the photon beam. Water phantom was modeled in the PRIMO code with water equivalent density. Golden beam data, experimental measurement, and AAA calculation results were imported to PRIMO code for gamma comparison.ResultsPRIMO simulations of Percentage Depth Dose (PDD) and in-plane beam profiles had good agreement with experimental measurements, AAA calculations and golden beam. However, PRIMO simulations of cross-plane beam profiles have a better agreement with AAA calculation and golden beam than the experimental measurement. Furthermore, PRIMO simulations of absolute dose agreed well with experimental results with ±0.8% uncertainty.ConclusionThe PRIMO code has good accuracy and is appropriate for use as a tool to check the credibility of beam scanning and output measurement in initial commissioning and routine QA.     

Determination of initial electron parameters by means of Monte Carlo simulations for the Siemens Artiste Linac 6 MV photon beam

AimIn this study, we investigated initial electron parameters of Siemens Artiste Linac with 6 MV photon beam using the Monte Carlo method.BackgroundIt is essential to define all the characteristics of initial electrons hitting the target, i.e. mean energy and full width of half maximum (FWHM) of the spatial distribution intensity, which is needed to run Monte Carlo simulations. The Monte Carlo is the most accurate method for simulation of radiotherapy treatments.Materials and methodsLinac head geometry was modeled using the BEAMnrc code. The phase space files were used as input file to DOSXYZnrc simulation to determine the dose distribution in a water phantom. We obtained percent depth dose curves and the lateral dose profile. All the results were obtained at 100 cm of SSD and for a 10 × 10 cm² field.ResultsWe concluded that there existed a good conformity between Monte Carlo simulation and measurement data when we used electron mean energy of 6.3 MeV and 0.30 cm FWHM value as initial parameters. We observed that FWHM values had very little effect on PDD and we found that the electron mean energy and FWHM values affected the lateral dose profile. However, these effects are between tolerance values.ConclusionsThe initial parameters especially depend on components of a linac head. The phase space file which was obtained from Monte Carlo Simulation for a linac can be used as calculation of scattering, MLC leakage, to compare dose distribution on patients and in various studies.     

Monte Carlo simulation for Neptun 10 PC medical linear accelerator and calculations of output factor for electron beam

AimExact knowledge of dosimetric parameters is an essential pre-requisite of an effective treatment in radiotherapy. In order to fulfill this consideration, different techniques have been used, one of which is Monte Carlo simulation.Materials and methodsThis study used the MCNP-4C^b to simulate electron beams from Neptun 10 PC medical linear accelerator. Output factors for 6, 8 and 10 MeV electrons applied to eleven different conventional fields were both measured and calculated.ResultsThe measurements were carried out by a Wellhofler-Scanditronix dose scanning system. Our findings revealed that output factors acquired by MCNP-4C simulation and the corresponding values obtained by direct measurements are in a very good agreement.ConclusionIn general, very good consistency of simulated and measured results is a good proof that the goal of this work has been accomplished.     

Assessment of out-of-field doses in radiotherapy treatments of paediatric patients using Monte Carlo methods and measurements

PurposeTo assess out-of-field doses in radiotherapy treatments of paediatric patients, using Monte Carlo methods to implement a new model of the linear accelerator validated against measurements and developing a voxelized anthropomorphic paediatric phantom.MethodsCT images of a physical anthropomorphic paediatric phantom were acquired and a dosimetric planning using a TPS was obtained. The CT images were used to perform the voxelization of the physical phantom using the ImageJ software and later implemented in MCNP. In order to validate the Monte Carlo model, dose measurements of the 6 MV beam and Linac with 120 MLC were made in a clinical setting, using ionization chambers and a water phantom. Afterwards TLD measurements in the physical anthropomorphic phantom were performed in order to assess the out-of-field doses in the eyes, thyroid, c-spine, heart and lungs.ResultsThe Monte Carlo model was validated for in-field and out-of-field doses with average relative differences below 3%. The average relative differences between TLD measurements and Monte Carlo is 14,3% whilst the average relative differences between TLD and TPS is 55,8%. Moreover, organs up to 22.5 cm from PTV center show TLD and MCNP6 relative differences and TLD and TPS relative differences up to 21.2% and 92.0%, respectively.ConclusionsOur study provides a novel model that could be used in clinical research, namely in dose evaluation outside the treatment fields. This is particularly relevant, especially in pediatric patients, for studying new radiotherapy treatment techniques, since it can be used to estimate the development of secondary tumours.     

Application of Monte Carlo calculations for validation of a treatment planning system in high dose rate brachytherapy

AimThe accuracy of treatment planning systems is of vital importance in treatment outcomes in brachytherapy. In the current study the accuracy of dose calculations of a high dose rate (HDR) brachytherapy treatment planning system (TPS) was validated using the Monte Carlo method.Materials and methodsThree ⁶⁰Co sources of the GZP6 afterloading brachytherapy system were modelled using MCNP4C Monte Carlo (MC) code. The dose distribution around all the sources was calculated by MC and a dedicated treatment planning system. The results of both methods were compared.ResultsThere was good agreement (<2%) between TPS and MC calculated dose distributions except at a point near the sources (<1 cm) and beyond the tip of the sources.ConclusionsOur study confirmed the accuracy of TPS calculated dose distributions for clinical use in HDR brachytherapy.     

A beam model for focused proton pencil beams

IntroductionWe present a beam model for Monte Carlo simulations of the IBA pencil beam scanning dedicated nozzle installed at the Skandion Clinic. Within the nozzle, apart from entrance and exit windows and the two ion chambers, the beam traverses vacuum, allowing for a beam that is convergent downstream of the nozzle exit.Materials and methodsWe model the angular, spatial and energy distributions of the beam phase space at the nozzle exit with single Gaussians, controlled by seven energy dependent parameters. The parameters were determined from measured profiles and depth dose distributions. Verification of the beam model was done by comparing measured and GATE acquired relative dose distributions, using plan specific log files from the machine to specify beam spot positions and energy.ResultsGATE-based simulations with the acquired beam model could accurately reproduce the measured data. The gamma index analysis comparing simulated and measured dose distributions resulted in >95% global gamma index pass rates (3%/2 mm) for all depths.ConclusionThe developed beam model was found to be sufficiently accurate for use with GATE e.g. for applications in quality assurance (QA) or patient motion studies with the IBA pencil beam scanning dedicated nozzles.     

Evaluation of a commercial VMC++ Monte Carlo based treatment planning system for electron beams using EGSnrc/BEAMnrc simulations and measurements

In the present work, Monte Carlo (MC) models of electron beams (energies 4, 12 and 18 MeV) from an Elekta SL25 medical linear accelerator were simulated using EGSnrc/BEAMnrc user code. The calculated dose distributions were benchmarked by comparison with measurements made in a water phantom for a wide range of open field sizes and insert combinations, at a single source-to-surface distance (SSD) of 100 cm. These BEAMnrc models were used to evaluate the accuracy of a commercial MC dose calculation engine for electron beam treatment planning (Oncentra MasterPlan Treament Planning System (OMTPS) version 1.4, Nucletron) for two energies, 4 and 12 MeV. Output factors were furthermore measured in the water phantom and compared to BEAMnrc and OMTPS. The overall agreement between predicted and measured output factors was comparable for both BEAMnrc and OMTPS, except for a few asymmetric and/or small insert cutouts, where larger deviations between measurements and the values predicted from BEAMnrc as well as OMTPS computations were recorded. However, in the heterogeneous phantom, differences between BEAMnrc and measurements ranged from 0.5 to 2.0% between two ribs and 0.6–1.0% below the ribs, whereas the range difference between OMTPS and measurements was the same (0.5–4.0%) in both areas. With respect to output factors, the overall agreement between BEAMnrc and measurements was usually within 1.0% whereas differences up to nearly 3.0% were observed for OMTPS. This paper focuses on a comparison for clinical cases, including the effects of electron beam attenuations in a heterogeneous phantom. It, therefore, complements previously reported data (only based on measurements) in one other paper on commissioning of the VMC++ dose calculation engine.These results demonstrate that the VMC++ algorithm is more robust in predicting dose distribution than Pencil beam based algorithms for the electron beams investigated.     

Small animal irradiator dose distribution verification using radioluminescence imaging

The main objective of this work was the development of a novel 2D dosimetry approach for small animal external radiotherapy using radioluminescence imaging (RLI) with a commercial complementary metal oxide semiconductor detector. Measurements of RLI were performed on the small animal image‐guided platform SmART, RLI data were corrected for perspective distortion using Matlab. Four irradiation fields were tested and the planar 2D dose distributions and dose profiles were compared against dose calculations performed with a Monte Carlo based treatment planning system and gafchromic film. System linearity and RLI image noise against dose were also measured. The maximum difference between beam size measured with RLI and nominal beam size was less than 8% for all the tested beams. The image correction procedure was able to reduce perspective distortion. A novel RLI approach for quality assurance of a small animal irradiator was presented and tested. Results are in agreement with MC dose calculations and gafchromic film measurements.