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Purpose A good Monte Carlo model with an accurate head shielding model is important in estimating the long-term risks of unwanted radiation exposure during radiation therapy. The aim of this paper was to validate the Monte Carlo simulation of a TrueBeam linear accelerator (linac) head shielding model. We approach this by evaluating the accuracy of out-of-field dose predictions at extended distances which are comprised of scatter from within the patient and treatment head leakage and thus reflect the accuracy of the head shielding model. We quantify the out-of-field dose of a TrueBeam linac for low-energy photons, 6X and 6X-FFF beams, and compare measurements to Monte Carlo simulations using Varian VirtuaLinac that include a realistic head shielding model, for a variety of jaw sizes and angles up to a distance of 100 cm from the isocenter, in both positive and negative directions. Given the high value and utility of the VirtuaLinac model, it is critical that this model is validated thoroughly and the results be available to the medical physics community. Materials and method Simulations were done using VirtuaLinac, the GEANT4-based Monte Carlo model of the TrueBeam treatment head from Varian Medical Systems, and an in-house GEANT4-based code. VirtuaLinac included a detailed model of the treatment head shielding and was run on the Amazon Web Services cloud to generate spherical phase space files surrounding the treatment head. These phase space files were imported into the in-house code, which modeled the measurement setup with a solid water buildup, the carbon fiber couch, and the gantry stand. For each jaw size (2 × 2 cm2 , 4 × 4 cm2 , 10 × 10 cm2 , and 20 × 20 cm2 ) and angular setting (0°, 90°, 45°, 135°), the dose was calculated at intervals of 5 cm along each measurement direction. Results For the 10 × 10 cm2 jaw size, both 6X and 6X-FFF showed very good agreement between simulation and measurement in both in-plane directions, with no apparent systematic bias. The percentage deviations for these settings were as follows: (mean, STDEV, maximum) (8.34, 6.44, 24.84) for 6X and (13.21, 8.93, 35.56) for 6X-FFF. For all jaw sizes, simulation agreed well in the in-plane direction going away from the gantry, but, some deviations were observed moving toward the gantry at larger distances. At larger distances, for the jaw sizes smaller than 10 × 10 cm2 , the simulation underestimates the dose compared with measurement, while for jaw sizes larger than 10 × 10 cm2 , it overestimates dose. For all comparisons between ±50 cm from isocenter, average absolute agreement between simulation and measurement was better than 28%. Conclusion We have validated the Varian VirtuaLinac's head shielding model via out-of-field doses and quantified the differences between TrueBeam head shielding model created out-of-field doses and measurements for an extended distance of 100 cm.

International audience; The Geant4 electromagnetic (EM) physics sub-packages are a component of LHC experiment simulations. During long shutdown 2 for LHC, these packages are under intensive development and we report progress of EM physics in Geant4 versions 10.5 and 10.6, which includes faster computation, more accurate EM models, and extensions to the validation suite. New approaches are developed to simulate radiation damage for silicon vertex detectors and for configuration of multiple scattering per detector region. Improvements in user interfaces developed for low-energy and the Geant4-DNA project are used also for LHC simulation optimisation

PURPOSE Dynamic electron arc radiotherapy (DEAR) is a novel dynamic technique that achieves highly conformal dose through simultaneous couch and gantry motion during delivery. The purpose of this study is to develop a framework integrating a Monte Carlo dose engine (VirtuaLinac) to a treatment planning system (TPS, Eclipse) for DEAR. A quality assurance (QA) procedure is also developed. METHODS AND MATERIALS The interfaces include the following: computed tomography image export and conversion for VirtuaLinac; VirtuaLinac computation tasks management through application programming interface (API); and dose matrix processing and evaluation. The framework was validated with both static beam and DEAR plan with a 3 × 3 cm2 cutout for both 6 and 9 MeV electrons. Verification plans for DEAR were created on flat phantom and a hybrid dose calculation technique was developed which convolves precalculated small field kernel with the beam trajectory, and the resulting dose was compared with the full VirtuaLinac calculation and film measurement. RESULTS Excellent agreement between VirtuaLinac and eMC was observed with three-dimensional γ pass rate of 98% at 1%/1 mm criteria for both 6 and 9 MeV electrons. Film measurement shows two-dimensional (2D) γ passing rate of 99.8 % (6 MeV) and 97.1% (9 MeV) at 2%/2 mm criteria. For DEAR plans the comparison of VirtuaLinac and measurement shows the 2D γ passing rates of 94% at 2%/2 mm for 6 MeV. The dose distributions from hybrid method in phantom are identical to the full VirtuaLinac simulations, but can be done instantly. CONCLUSIONS A framework has been developed for DEAR dose calculation using VirtuaLinac Monte Carlo dose engine. The VirtuaLinac calculated dose was validated against measurement. A feasible and practical DEAR QA method has been developed for dose measurement in phantom. The hybrid dose calculation technique is efficient and suitable for DEAR QA purpose.

International audience; We report on developments of the Geant4 electromagnetic physics sub-libraries of Geant4 release 10.4 and beyond. Modifications are introduced to the models of photoelectric effect, bremsstrahlung, gamma conversion, single and multiple scattering. The theory-based Goudsmit-Saunderson model of electron/positron multiple scattering has been recently reviewed and a new improved version, providing the most accurate results for scattering of electrons and positrons, was made available. The updated interfaces, models and configurations have already been integrated into LHC applications and may be useful for any type of simulations

Purpose: Electronic portal imagers (EPIDs) with high detective quantum efficiencies (DQEs) are sought to facilitate the use of the megavoltage (MV) radiotherapy treatment beam for image guidance. Potential advantages include high quality (treatment) beam’s eye view imaging, and improved cone-beam computed tomography (CBCT) generating images with more accurate electron density maps with immunity to metal artifacts. One approach to increasing detector sensitivity is to couple a thick pixelated scintillator array to an active matrix flat panel imager (AMFPI) incorporating amorphous silicon thin film electronics. Cadmium tungstate (CWO) has many desirable scintillation properties including good light output, a high index of refraction, high optical transparency, and reasonable cost. However, due to the cleave plane inherent in its crystalline structure, the difficulty of cutting and polishing CWO has, in part, limited its study relative to other scintillators such as cesium iodide and bismuth germanate (BGO). The goal of this work was to build and test a focused large-area pixelated “strip” CWO detector. 0 1 0 Methods: A 361 × 52 mm scintillator assembly that contained a total of 28 072 pixels was constructed. The assembly comprised seven subarrays, each 15 mm thick. Six of the subarrays were fabricated from CWO with a pixel pitch of 0.784 mm, while one array was constructed from BGO for comparison. Focusing was achieved by coupling the arrays to the Varian AS1000 AMFPI through a piecewise linear arc-shaped fiber optic plate. Simulation and experimental studies of modulation transfer function (MTF) and DQE were undertaken using a 6 MV beam, and comparisons were made between the performance of the pixelated strip assembly and the most common EPID configuration comprising a 1 mm-thick copper build-up plate attached to a 133 mg/cm2 gadolinium oxysulfide scintillator screen (Cu-GOS). Projection radiographs and CBCT images of phantoms were acquired. The work also introduces the use of a lightweight edge phantom to generate MTF measurements at MV energies and shows its functional equivalence to the more cumbersome slit-based method. Results: Measured and simulated DQE(0)’s of the pixelated CWO detector were 22% and 26%, respectively. The average measured and simulated ratios of CWO DQE(f) to Cu-GOS DQE(f) across the frequency range of 0.0–0.62 mm−1 were 23 and 29, respectively. 2D and 3D imaging studies confirmed the large dose efficiency improvement and that focus was maintained across the field of view. In the CWO CBCT images, the measured spatial resolution was 7 lp/cm. The contrast-to-noise ratio was dramatically improved reflecting a 22 × sensitivity increase relative to Cu-GOS. The CWO scintillator material showed significantly higher stability and light yield than the BGO material. Conclusions: An efficient piecewise-focused pixelated strip scintillator for MV imaging is described that offers more than a 20-fold dose efficiency improvement over Cu-GOS.

Purpose: To develop a framework for accurate electron Monte Carlo dose calculation. In this study, comprehensive validations of vendor provided electron beam phase space files for Varian TrueBeam Linacs against measurement data are presented. Methods: In this framework, the Monte Carlo generated phase space files were provided by the vendor and used as input to the downstream plan-specific simulations including jaws, electron applicators, and water phantom computed in the EGSnrc environment. The phase space files were generated based on open field commissioning data. A subset of electron energies of 6, 9, 12, 16, and 20 MeV and open and collimated field sizes 3 × 3, 4 × 4, 5 × 5, 6 × 6, 10 × 10, 15 × 15, 20 × 20, and 25 × 25 cm2 were evaluated. Measurements acquired with a CC13 cylindrical ionization chamber and electron diode detector and simulations from this framework were compared for a water phantom geometry. The evaluation metrics include percent depth dose, orthogonal and diagonal profiles at depths R 100, R 50, Rp , and R p+ for standard and extended source-to-surface distances (SSD), as well as cone and cut-out output factors. Results: Agreement for the percent depth dose and orthogonal profiles between measurement and Monte Carlo was generally within 2% or 1 mm. The largest discrepancies were observed within depths of 5 mm from phantom surface. Differences in field size, penumbra, and flatness for the orthogonal profiles at depths R 100, R 50, and Rp were within 1 mm, 1 mm, and 2%, respectively. Orthogonal profiles at SSDs of 100 and 120 cm showed the same level of agreement. Cone and cut-out output factors agreed well with maximum differences within 2.5% for 6 MeV and 1% for all other energies. Cone output factors at extended SSDs of 105, 110, 115, and 120 cm exhibited similar levels of agreement. Conclusions: We have presented a Monte Carlo simulation framework for electron beam dose calculations for Varian TrueBeam Linacs. Electron beam energies of 6 to 20 MeV for open and collimated field sizes from 3 × 3 to 25 × 25 cm2 were studied and results were compared to the measurement data with excellent agreement. Application of this framework can thus be used as the platform for treatment planning of dynamic electron arc radiotherapy and other advanced dynamic techniques with electron beams.

International audience; We report on the recent progress within the Geant4 electromagnetic physics subpackages. Several new interfaces and models recently introduced are already used in LHC applications and may be useful for any type of simulation. Significant developments were carried out to improve the user interface, develop models of single and multiple scattering, and validate high energy models. Part of these developments are included in the Geant4 10.2 release and the full set are available in the new version 10.3 of December, 2016.

Strategies for delivering radiation to a moving lesion each require a margin to compensate for uncertainties in treatment. These motion margins have been determined here by separating the total uncertainty into components. Probability density functions for the individual sources of uncertainty were calculated for ten motion traces obtained from the literature. Motion margins required to compensate for the center of mass motion of the clinical treatment volume were found by convolving the individual sources of uncertainty. For measurements of position at a frequency of 33 Hz, system latency was the dominant source of positional uncertainty. Averaged over the ten motion traces, the motion margin for tracking with a latency of 200 ms was 4.6 mm. Gating with a duty cycle of 33% required a mean motion margin of 3.2-3.4 mm, and tracking with a latency of 100 ms required a motion margin of 3.1 mm. Feasible reductions in the effects of the sources of uncertainty, for example by using a simple prediction algorithm to anticipate the lesion position at the end of the latency period, resulted in a mean motion margin of 1.7 mm for tracking with a latency of 100 ms, 2.4 mm for tracking with a latency of 200 ms, and 2.1-2.2 mm for the gating strategies with duty cycles of 33%. A crossover tracking latency of 150 ms was found, below which tracking strategies could take advantage of narrower motion margins than gating strategies. The methods described here provide a means to guide selection of a motion management strategy for a given patient.

72 1024x768 Normal 0 false false false The assumption of cylindrical symmetry in radiotherapy accelerator models can pose a challenge for precise Monte Carlo modeling. This assumption makes it difficult to account for measured asymmetries in clinical dose distributions. We have performed a sensitivity study examining the effect of varying symmetric and asymmetric beam and geometric parameters of a Monte Carlo model for a Siemens PRIMUS accelerator. The accelerator and dose output were simulated using modified versions of BEAMnrc and DOSXYZnrc that allow lateral offsets of accelerator components and lateral and angular offsets for the incident electron beam. Dose distributions were studied for 40 × 40 cm2 fields. The resulting dose distributions were analyzed for changes in flatness, symmetry, and off-axis ratio (OAR). The electron beam parameters having the greatest effect on the resulting dose distributions were found to be electron energy and angle of incidence, as high as 5% for a 0.25° deflection. Electron spot size and lateral offset of the electron beam were found to have a smaller impact. Variations in photon target thickness were found to have a small effect. Small lateral offsets of the flattening filter caused significant variation to the OAR. In general, the greatest sensitivity to accelerator parameters could be observed for higher energies and off-axis ratios closer to the central axis. Lateral and angular offsets of beam and accelerator components have strong effects on dose distributions, and should be included in any high-accuracy beam model.

Purpose: To use an imaging beam line (IBL) to obtain the first megavoltage cone-beam computed tomography (MV CBCT)images of patients with a low atomic number ( Z ) target, and to compare these images to those taken of the same patients with the 6 MV flattened beam from the treatment beam line (TBL). Methods: The IBL, which produces a 4.2 MV unflattened beam from a carbon target, was installed on a linear accelerator in use for radiotherapy. Provision was made for switching between the IBL and TBL for imaging the same patient with beams from the low- Z and high- Z targets. Dose was quoted as monitor units times the dose per monitor unit for the standard calibration geometry. Images were acquired with institutional approval and patient consent with both the IBL and TBL on a series of 23 patients undergoing radiotherapy. Patients were imaged daily to weekly and aligned to the planning CT using the images.Doses were reduced over the course of treatment to determine the minimum doses required for alignment. Images were assessed offline. Results: IBL MV CBCTimages of prostate, head and neck, lung, and abdomen showed improvement in soft tissue contrast for the same dose as the TBL images. Bony anatomy, air cavities, and fiducial markers were sharper. CBCT with a dose of 1 cGy was sufficient for alignment of prostate and head and neck patients based on bony anatomy or implanted gold seeds, 2–4 cGy for lung, abdomen, and pelvis. Photon scatter in the patient had minimal effect on image quality. The metallic hip prosthesis in one patient showed reduced artifacts compared to diagnostic CT. Conclusions: The IBL has the advantage of improved image quality at the same dose, or reduced dose for the same image quality, over the TBL.

Purpose: To determine the properties of a megavoltage cone-beam CT system using the unflattened beam from a sintered diamond target at 4 and 6 MV. Methods: A sintered diamond target was used in place of a graphite target as part of an imaging beam line (an unflattened beam from a graphite target) installed on a linear accelerator. The diamond target, with a greater density than the graphite target, permitted imaging at the lower beam energy (4 MV) required with the graphite target and the higher beam energy (6 MV) conventionally used with the tungsten/stainless steel target and stainless steel flattening filter. Images of phantoms and patients were acquired using the different beam lines and compared. The beam spectra and dose distributions were determined using Monte Carlo simulation. Results: The diamond target allowed use of the same beam energy as for treatment, simplifying commissioning and quality assurance. Images acquired with the diamond target at 4 MV were similar to those obtained with the graphite target at 4 MV. The slight reduction in low energy photons due to the higher-Z sintering material in the diamond target had minimal effect on image quality. Images acquired at 6 MV with the diamond target showedmore » a small decrease in contrast-to-noise ratio, resulting from a decrease in the fraction of photons in the beam in the energy range to which the detector is most sensitive. Conclusions: The diamond target provides images of a similar quality to the graphite target. Diamond allows use of the higher beam energy conventionally used for treatment, provides a higher dose rate for the same beam current, and potentially simplifies installation and maintenance of the beam line.« less

Purpose: Obtain an accurate simulation of the dose from the 6 and 18 MV x-ray beams from a Siemens Oncor linear accelerator by comparing simulation to measurement. Constrain the simulation by independently determining parameters of the treatment head and incident beam, in particular, the energy and spot size. Methods: Measurements were done with the treatment head in three different configurations: (1) The clinical configuration, (2) the flattening filter removed, and (3) the target and flattening filter removed. Parameters of the incident beam and treatment head were measured directly. Incident beam energy and spectral width were determined from the percent-depth ionization of the raw beam (as described previously), spot size was determined using a spot camera, and the densities of the flattening filters were determined by weighing them. Simulations were done with EGSnrc/BEAMnrc code. An asymmetric simulation was used, including offsets of the spot, primary collimator, and flattening filter from the collimator rotation axis. Results: Agreement between measurement and simulation was obtained to the least restrictive of 1% or 1 mm at 6 MV, both with and without the flattening filter in place, except for the buildup region. At 18 MV, the agreement was 1.5%/1.5 mm with the flattening filter inmore » place and 1%/1 mm with it removed, except for in the buildup region. In the buildup region, the discrepancy was 2%/2 mm at 18 MV and 1.5%/1.5 mm at 6 MV with the flattening filter either removed or in place. The methodology for measuring the source and geometry parameters for the treatment head simulation is described. Except to determine the density of the flattening filter, no physical modification of the treatment head is necessary to obtain those parameters. In particular, the flattening filter does not need to be removed as was done in this work. Conclusions: Good agreement between measured and simulated dose distributions was obtained, even in the buildup region. The simulation was tightly constrained by independent measurements of parameters of the incident beam and treatment head. The method of obtaining the input parameters is described, and can be carried out on a clinical linear accelerator.« less

Purpose: The purposes of this study are to improve the accuracy of source and geometry parameters used in the simulation of large electron fields from a clinical linear accelerator and to evaluate improvement in the accuracy of the calculated dose distributions. Methods: The monitor chamber and scattering foils of a clinical machine not in clinical service were removed for direct measurement of component geometry. Dose distributions were measured at various stages of reassembly, reducing the number of geometry variables in the simulation. The measured spot position and beam angle were found to vary with the beam energy. A magnetic field from the bending magnet was found between the exit window and the secondary collimators of sufficient strength to deflect electrons 1 cm off the beam axis at 100 cm from the exit window. The exit window was 0.05 cm thicker than manufacturer’s specification, with over half of the increased thickness due to water pressure in the channel used to cool the window. Dose distributions were calculated with Monte Carlo simulation of the treatment head and water phantom using EGSnrc, a code benchmarked at radiotherapy energies for electron scatter and bremsstrahlung production, both critical to the simulation. The secondary scattering foil and monitor chamber offset from the collimator rotation axis were allowed to vary with the beam energy in the simulation to accommodate the deflection of the beam by the magnetic field, which was not simulated. Results: The energy varied linearly with bending magnet current to within 1.4% from 6.7 to 19.6 MeV, the bending magnet beginning to saturate at the highest beam energy. The range in secondary foil offset used to account for the magnetic field was 0.09 cm crossplane and 0.15 cm inplane, the range in monitor chamber offset was 0.14 cm crossplane and 0.07 cm inplane. A 1.5%/0.09 cm match or better was obtained to measured depth dose curves. Profiles measured at the depth of maximum dose matched the simulated profiles to 2.6% or better at doses of 80% or more of the dose on the central axis. The profiles along the direction of MLC motion agreed to within 0.16 cm at the edge of the field. There remained a mismatch for the lower beam energies at the edge of the profile that ran parallel to the direction of jaw motion of up to 1.4 cm for the 6 MeV beam, attributed to the MLC support block at the periphery of the field left out of the simulation and to beam deflection by the magnetic field. The possibility of using these results to perform accurate simulation without disassembly is discussed. Phase-space files were made available for benchmarking beam models and other purposes. Conclusions: The match to measured large field dose distributions from clinical electron beams with Monte Carlo simulation was improved with more accurate source details and geometry details closer to manufacturer’s specification than previously achieved.

Monte Carlo simulations of x-ray beams typically take parameters of the electron beam in the accelerating waveguide to be free parameters. In this paper, a methodology is proposed and implemented to determine the energy, spectral width, and beam divergence of the electron source. All treatment head components were removed from the beam path, leaving only the exit window. With the x-ray target and flattener out of the beam, uncertainties in physical characteristics and relative position of the target and flattening filter, and in spot size, did not contribute to uncertainty in the energy. Beam current was lowered to reduce recombination effects. The measured dose distributions were compared with Monte Carlo simulation of the electron beam through the treatment head to extract the electron source characteristics. For the nominal 6 and 18 MV x-ray beams, the energies were 6.51±0.15 and 13.9±0.2 MeV, respectively, with the uncertainties resulting from uncertainties in the detector position in the measurement and in the stopping power in the simulations. Gaussian spectral distributions were used, with full widths at half maximum ranging from 20±4% at 6 MV to 13±4% at 18 MV required to match the fall-off portion of the percent-depth ionization curve. Profiles at the depth of maximum dose from simulations that used the manufacturer-specified exit window geometry and no beam divergence were 2–3 cm narrower than measured profiles. Two simulation configurations yielding the measured profile width were the manufacturer-specified exit window thickness with electron source divergences of 3.3° at 6 MV and 1.8° at 18 MV and an exit window 40% thicker than the manufacturer’s specification with no beam divergence. With the x-ray target in place (and no flattener), comparison of measured to simulated profiles sets upper limits on the electron source divergences of 0.2° at 6 MV and 0.1° at 18 MV. A method of determining source characteristics without mechanical modification of the treatment head, and therefore feasible in clinics, is presented. The energies and spectral widths determined using this method agree with those determined with only the exit window in the beam path.

Geant4 is a software toolkit for the simulation of the passage of particles through matter. It is used by a large number of experiments and projects in a variety of application domains, including high energy physics, astrophysics and space science, medical physics and radiation protection. Over the past several years, major changes have been made to the toolkit in order to accommodate the needs of these user communities, and to efficiently exploit the growth of computing power made available by advances in technology. The adaptation of Geant4 to multithreading, advances in physics, detector modeling and visualization, extensions to the toolkit, including biasing and reverse Monte Carlo, and tools for physics and release validation are discussed here., Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 835, ISSN:0168-9002, ISSN:1872-9576

Monte Carlo simulations can provide powerful insight into the physical phenomena and geometrical interactions of linear accelerator beams. This insight can be used to understand the phenomena that govern the beam characteristic and, for instance, to guide the development of treatment planning systems. In this study, we use the VirtuaLinac, a cloud-based application to model the treatment head of the Varian TrueBeam linear accelerator. VirtuaLinac implements the treatment head geometry into the Monte Carlo code Geant4, which is then utilized to provide the physics and numerical engine for the simulations. We consider both open fields and fields limited by multi-leaf collimators and compute the dose deposited in a water phantom. We then compare the simulation results with experimental measurements. The simulated data are also used to extract some of the characteristics of the multi-leaf collimators and to evaluate their impact on the beam properties and the dose distribution.

Journal of Physics: Conference Series In this work we report on recent improvements in the electromagnetic (EM) physics models of Geant4 and new validations of EM physics. Improvements have been made in models of the photoelectric effect, Compton scattering, gamma conversion to electron and muon pairs, fluctuations of energy loss, multiple scattering, synchrotron radiation, and high energy positron annihilation. The results of these developments are included in the new Geant4 version 10.1 and in patches to previous versions 9.6 and 10.0 that are planned to be used for production for run-2 at LHC. The Geant4 validation suite for EM physics has been extended and new validation results are shown in this work. In particular, the effect of gamma-nuclear interactions on EM shower shape at LHC energies is discussed.

URL: http://link.aps.org/abstract/PRL/v96/e215301; International audience; We present the first measurements of the thermal conductivity of spin-polarized normal liquid 3He. Using the rapid melting technique to produce nuclear polarizations up to 0.7, and a vibrating wire both as a heater and a thermometer, we show that, unlike the viscosity, the conductivity increases much less than predicted for s-wave scattering. We suggest that this might be due to a small probability for head-on collisions between quasiparticles.

The most recent upgrades of the electromagnetic (EM) physics “standard” and “low energy” sub-libraries of the general purpose Geant4 Monte Carlo simulation toolkit are described. These upgrades are relevant to different application domains including high energy physics, medical physics and space science. Validation results are presented and discussed.

Purpose: Patient dose far from the treatment field is comprised of scatter from within the patient, and treatment head leakage. We quantify the treatment head leakage for TrueBeam linear accelerator for 6X and 6X-FFF beams by comparing measurements to Monte Carlo simulations for a variety of jaw sizes and collimator rotations. This work is conceptually similar to that of Kry et al. (Medical Physics 2006; 33: 4405), who considered a Clinac linear accelerator. Methods: Measurements were made using an EXRADIN A101 ion chamber positioned in the patient plane, at distances up to 100 cm from isocenter. Simulations were done using VirtuaLinac, the GEANT4-based Monte Carlo model of the TrueBeam treatment head, and an in-house (U. Virginia) GEANT4-based code. In-house code modelled an ion chamber with build-up, based on a CT scan of the chamber. VirtuaLinac included a detailed model of the treatment head shielding, and was run on the Amazon Web Services cloud to generate spherical phase space files surrounding the treatment head. These phase space files were imported into the in-house code. Results: Initial comparisons between measurements and simulation showed an excess of dose in the in-plane direction, away from the gantry, in the simulations. This was traced to an incomplete model of the shielding—specifically, the component holding the primary collimator was smaller in the model than in the TrueBeam. Modifications were made to VirtuaLinac to more closely match the engineering drawings. In the in-plane direction, the lowest out of field dose was away from gantry (negative abscissa values) at around 60 cm from isocenter, for fields smaller than 10×10 cm2. Out of field dose decreased with decreasing jaw size. Flattening-filter free beam produced out-of-field doses as low as 65% of those with flattened beam. Conclusion: Doses determined from simulation and measurement were in close agreement. Funding support from the Jefferson Trust Foundation

Purpose: DEAR is a radiation therapy technique utilizing synchronized motion of gantry and couch during delivery to optimize dose distribution homogeneity and penumbra for treatment of superficial disease. Dose calculation for DEAR is not yet supported by commercial TPSs. The purpose of this study is to demonstrate the feasibility of using a web-based Monte Carlo (MC) simulation tool (VirtuaLinac) to calculate dose distributions for a DEAR delivery. Methods: MC simulations were run through VirtuaLinac, which is based on the GEANT4 platform. VirtuaLinac utilizes detailed linac head geometry and material models, validated phase space files, and a voxelized phantom. The input was expanded to include an XML file for simulation of varying mechanical axes as a function of MU. A DEAR XML plan was generated and used in the MC simulation and delivered on a TrueBeam in Developer Mode. Radiographic film wrapped on a cylindrical phantom (12.5 cm radius) measured dose at a depth of 1.5 cm and compared to the simulation results. Results: A DEAR plan was simulated using an energy of 6 MeV and a 3×10 cm2 cut-out in a 15×15 cm2 applicator for a delivery of a 90° arc. The resulting data were found to provide qualitative and quantitative evidence that the simulation platform could be used as the basis for DEAR dose calculations. The resulting unwrapped 2D dose distributions agreed well in the cross-plane direction along the arc, with field sizes of 18.4 and 18.2 cm and penumbrae of 1.9 and 2.0 cm for measurements and simulations, respectively. Conclusion: Preliminary feasibility of a DEAR delivery using a web-based MC simulation platform has been demonstrated. This tool will benefit treatment planning for DEAR as a benchmark for developing other model based algorithms, allowing efficient optimization of trajectories, and quality assurance of plans without the need for extensive measurements.

Electron scattering algorithms in Geant4 versions 9.4 and 9.5 were benchmarked by comparing scattered distributions against previously measured values at 13 and 20 MeV, for low, intermediate, and high atomic number materials. Several scattering models were used: Versions 93 and 95 of the Urban model, with different step size limits near boundaries; Goudsmit-Saunderson multiple scattering; and single scattering. The Urban93 and Urban95 models with a large step size limit (as in the Option 0 physics list) were found to give results most closely matching the experimental results. Scattered distributions using the Urban models were all narrower than measured by up to 6%, consistent with previous published simulations using EGSnrc. This is suggestive of a systematic difference between simulations and measurement. The magnitudes of the differences were similar to previously published results using Geant4, although there were differences in detail. In particular, the current results were typically 2% narrower than values. Results with the more restrictive step size limit in Option 3 were even more narrow, and close to those with single scattering. The Goudsmit-Saunderson multiple scattering model produced distributions up to 15% different from measured in Geant4 version 9.5 and up to 45% different in Geant4 version 9.4.

Journal article Monte Carlo simulation can accurately calculate electron fluence at the patient surface and the resultant dose deposition if the initial source electron beam and linear accelerator treatment head geometry parameters are well characterized. A recent approach used large electron fields to extract these simulation parameters. This method took advantage of the absence of lower energy, widely scattered electrons from the applicator resulting in more accurate data. It is important to validate these simulation parameters for clinically relevant fields. In the current study, these simulation parameters are applied to fields collimated by applicators and inserts to perform a comprehensive validation. Measurements were performed on a Siemens Oncor linear accelerator for 6 MeV, 9 MeV, 12 MeV, 15 MeV, 18 MeV and 21 MeV electron beams and collimators ranging from an open 25 x 25 cm(2) applicator to a 10 x 10 cm(2) applicator with a 1 cm diameter cerrobend insert. Data were collected for inserts placed in four square applicators. Monte Carlo simulations were performed using EGSnrc/BEAMnrc. Source and geometry parameters were obtained from previous measurements and simulations with the maximum field size (40 x 40 cm(2)). The applicators were modelled using manufacturer specifications, confirmed by direct measurements. Cerrobend inserts were modelled based on calliper measurements. Monte Carlo-calculated percentage depth dose and off-axis profiles agreed with measurements to within the least restrictive of 2%/1 mm in most cases. For the largest applicator (25 x 25 cm(2)), and 18 MeV and 21 MeV beams, differences in dose profiles of 3% were observed. Calculated relative output factors were within 2% of those measured with an electron diode for fields 1.5 cm in diameter or larger. The disagreement for 1 cm diameter fields was up to 5%. For open applicators, simulations agreed with parallel plate chamber-measured relative output factors to 1%. This work has validated a recent methodology used to extract data on the electron source and treatment head from large electron fields, resulting in a reduction in the number of unknown parameters in treatment head simulation. Applicator and insert collimated electron fields were accurately simulated without adjusting these parameters. Results demonstrate that commissioning of electron beams based on large electron field measurements is a viable option. peer-reviewed

To determine the properties of a megavoltage cone-beam CT system using the unflattened beam from a sintered diamond target at 4 and 6 MV.A sintered diamond target was used in place of a graphite target as part of an imaging beam line (an unflattened beam from a graphite target) installed on a linear accelerator. The diamond target, with a greater density than the graphite target, permitted imaging at the lower beam energy (4 MV) required with the graphite target and the higher beam energy (6 MV) conventionally used with the tungsten/stainless steel target and stainless steel flattening filter. Images of phantoms and patients were acquired using the different beam lines and compared. The beam spectra and dose distributions were determined using Monte Carlo simulation.The diamond target allowed use of the same beam energy as for treatment, simplifying commissioning and quality assurance. Images acquired with the diamond target at 4 MV were similar to those obtained with the graphite target at 4 MV. The slight reduction in low energy photons due to the higher-Z sintering material in the diamond target had minimal effect on image quality. Images acquired at 6 MV with the diamond target showed a small decrease in contrast-to-noise ratio, resulting from a decrease in the fraction of photons in the beam in the energy range to which the detector is most sensitive.The diamond target provides images of a similar quality to the graphite target. Diamond allows use of the higher beam energy conventionally used for treatment, provides a higher dose rate for the same beam current, and potentially simplifies installation and maintenance of the beam line.

This paper reports on improved simulation of small electron fields from a Siemens Primus linear accelerator. Accelerator simulation was performed using the Monte Carlo user code BEAMnrc. Source and geometry parameters used were obtained from previous measurements and simulation of the linear accelerator with maximum electron field size (40 × 40 cm2). Careful measurements of electron dose distributions were performed for fields ranging from an open 25 × 25 cm2 electron applicator to a 10 × 10 cm2 applicator containing a 1 cm diameter cerrobend insert. Monte Carlo calculated and measured data generally matched to within 2 % or 1 mm. This study has used improved source and treatment head simulation parameters and exacting measurements to closely match measured and simulated dose distributions for small electron fields.

Methodology is presented with the goal to determine fluence to 1-2% for electron and x-ray beams used in external beam radiotherapy. Fluence was determined by combining accurate and detailed treatment head simulation of a modern clinical accelerator with accurate measurements done over a wide range of operating parameters. The treatment head was disassembled for confirmation of the information provided by the manufacturer and for direct measurement of treatment head geometry. Dose measurements were made on the same accelerator with methodology to improve the accuracy of the measurements. Detailed simulation of the treatment head for the different modalities and beam energies were done with two different, accurate Monte Carlo systems. Source and geometry details in the simulation were adjusted within tight constraints set by the measurements. Results give confidence the 1-2% accuracy goal is achievable for both electron and x-ray beams.

Purpose: Calculate neutron production and transport in the TrueBeam treatment head, as input for vault design and phantom dose calculations. Methods: A detailed model of the treatment head, including shielding components off the beam axis, was created from manufacturer’s engineering drawings. Simulations were done with Geant4 for the 18X, 15X, 10X and 10FFF beams, tuned to match measured dose distributions inside the treatment field. Particles were recorded on a 70 cm radius sphere surrounding the treatment head enabling input into simulations of vaults. Results: For the 18X beam, 11×10^9 neutrons/MU were observed. The energy spectrum was a broad peak with average energy 0.37 MeV. With jaws closed, 48% of the neutrons were generated in the primary collimator, 18% in the jaws, 12% in the target, and 10% in the flattening filter. With wide open jaws, few neutrons were produced in the jaws and consequently total neutron production dropped to 8.5×10^9 neutrons/MU. Angular distributions were greatest along the beam axis (12×10^9 neutrons/MU/sr, within 2 deg of the beam axis) and antiparallel to the beam axis (7×10^9 neutrons/MU/sr). Peaks were observed in the neutron energy spectrum, corresponding to elastic scattering resonances in the shielding materials. Neutron production was lower for the other beams studied: 4.1×10^9 neutrons/MU for 15X, 0.38×10^9 neutrons/MU for 10X, and 0.22×10^9 neutrons/MU for 10FFF. Despite dissimilar treatment head geometries and materials, the neutron production and energy spectrum were similar to those reported for Clinac accelerators. Conclusion: Detailed neutron production and leakage calculations for the TrueBeam treatment head were done. Unlike other studies, results are independent of the surrounding vault, enabling vault design calculations.

Purpose: To simulate and measure radiation backscattered into the monitor chamber of a TrueBeam linac; establish a rigorous framework for absolute dose calculations for TrueBeam Monte Carlo (MC) simulations through a novel approach, taking into account the backscattered radiation and the actual machine output during beam delivery; improve agreement between measured and simulated relative output factors. Methods: The ‘monitor backscatter factor’ is an essential ingredient of a well-established MC absolute dose formalism (the MC equivalent of the TG-51 protocol). This quantity was determined for the 6 MV, 6X FFF, and 10X FFF beams by two independent Methods: (1) MC simulations in the monitor chamber of the TrueBeam linac; (2) linac-generated beam record data for target current, logged for each beam delivery. Upper head MC simulations used a freelyavailable manufacturer-provided interface to a cloud-based platform, allowing use of the same head model as that used to generate the publicly-available TrueBeam phase spaces, without revealing the upper head design. The MC absolute dose formalism was expanded to allow direct use of target current data. Results: The relation between backscatter, number of electrons incident on the target for one monitor unit, and MC absolute dose was analyzed for open fields, as well as a jaw-tracking VMAT plan. The agreement between the two methods was better than 0.15%. It was demonstrated that the agreement between measured and simulated relative output factors improves across all field sizes when backscatter is taken into account. Conclusion: For the first time, simulated monitor chamber dose and measured target current for an actual TrueBeam linac were incorporated in the MC absolute dose formalism. In conjunction with the use of MC inputs generated from post-delivery trajectory-log files, the present method allows accurate MC dose calculations, without resorting to any of the simplifying assumptions previously made in the TrueBeam MC literature. This work has been partially funded by Varian Medical Systems.

Purpose: Dynamic radiotherapy involving electron beams such as Dynamic Electron Arc Radiotherapy (DEAR) requires accurate dose modelling of small field sizes, similar to the requirement of IMRT field on the small photon field. The current commercial electron Monte Carlo algorithms such as eMC v11 in Eclipse were developed for standard field sizes and do not support the planning of dynamic therapy yet. The purpose of this study is to develop a method to accurately model small field electron beam dosimetry using Monte Carlo simulations. Methods: Comparison between eMC, phantom measurements (diode), and Monte Carlo (MC) simulations (BEAMnrc/DOSYZnrc) were performed for a Varian TrueBeam linac. MC simulations utilized Varian TrueBeam phase space files which had been validated in another study. Static single small field was assessed by comparing dose distributions in water for a 16 MeV beam for circular (2 cm diameter) and rectangular (1×10 cm{sup 2}) cut-out. MC was performed with a resolution of 2.5×2.5×2 mm{sup 2} and statistical uncertainty < 4%. The dose distribution was averaged over adjacent bins to improve precision. Depth dose and orthogonal profiles were evaluated. Results: Small field PDDs differ from those with standard cones. For both circular and rectangular cutouts, the difference in rangemore » R8 0-R1 0 is less than 2 mm and in dose within 2%. For the orthogonal profiles, field size and penumbra differences were within 1 mm at depth of maximum dose. The eMC displayed a distinctive “step” in the out-field dose profile in disagreement with both measurement and MC results and needs further investigation. Conclusion: MC was able to characterize the small field dosimetry with good agreement with the measurement data, and thus offers the opportunity for treatment planning of dynamic radiotherapy. Analyses for all other electron energies and cut-out sizes are under way and results will be included in the presentation.« less

Purpose: To validate the physics parameters of a Monte Carlo model for patient plane leakage calculations on the 6MV Unique linac by comparing the simulations against IEC patient plane leakage measurements. The benchmarked model can further be used for shielding design optimization, to predict leakage in the proximity of intended treatment fields, reduce the system weight and cost, and improve components reliability. Methods: The treatment head geometry of the Unique linac was simulated in Geant4 (v9.4.p02 with “Opt3” standard electromagnetic physics list) based on CAD drawings of all collimation and shielding components projected from the target to the area within 2m from isocenter. A 4×4m2 scorer was inserted 1m from the target in the patient plane and multiple phase space files were recorded by performing a 40-node computing cluster simulation on the EC2 cloud. The photon energy fluence was calculated relative to the value at isocenter for a 10×10cm2 field using 10×10mm2 bins. Tungsten blocks were parked accordingly to represent MLC120. The secondary particle contamination to patient plane was eliminated by “killing” those particles prior to the primary collimator entrance using a “kill-plane”, which represented the upper head shielding components not being modeled. Both IEC patient-plane leakage and X/Y-jaws transmission were simulated. Results: The contribution of photons to energy fluence was 0.064% on average, in excellent agreement with the experimental data available at 0.5, 1.0, and 1.5m from isocenter, characterized by an average leakage of 0.045% and a maximum leakage of 0.085%. X- and Y-jaws transmissions of 0.43% and 0.44% were found in good agreement with measurements of 0.48% and 0.43%, respectively. Conclusion: A Geant4 model based on energy fluence calculations for the 6MV Unique linac was created and validated using IEC patient plane leakage measurements. The “kill-plane” has effectively eliminated electron contamination to patient plane in these simulations.

Purpose: The design of the linac head is different for TrueBeam than Clinac, and there are differences in measured dose distributions in water phantoms between TrueBeam and Clinac for electron beams. Therefore, MC models for Clinac may not be applied directly to the Truebeam linac. The purpose of this study is to validate a Monte Carlo (MC) dose calculation framework for electron beams on Varian TrueBeam with phase space files provided by Varian. Methods: The particle histories from the phase space file were used as input for the down-stream simulation including jaws, applicators, and water phantom. MC packages BEAMnrc/DOSYXZnrc were used. The down-stream beam components were modeled according to manufacturer specifications and the dose distributions were compared with the measured data of standard cones. The measurements were performed in a water phantom with a p-type electron field diode (diameter 0.2cm) and ion chamber (CC13). Depth dose and orthogonal profiles at depths defined by R{sub 1} {sub 0} {sub 0}, R{sub 5} {sub 0}, Rp were compared. Results: Preliminary results for a 16 MeV phase space and 10x10, 15x15, and 20x20 cm{sup 2} applicator are presented. Simulations were run for a statistical uncertainty of

Purpose: For TrueBeam Monte Carlo simulations, Varian does not distribute linac head geometry and material compositions, instead providing a phase space file (PSF) for the users. The PSF has a finite number of particle histories and can have very large file size, yet still contains inherent statistical noises. The purpose of this study is to characterize the electron beam PSF with parameters. Methods: The PSF is a snapshot of all particles' information at a given plane above jaws including type, energy, position, and directions. This study utilized a preliminary TrueBeam PSF, of which validation against measurement is presented in another study. To characterize the PSF, distributions of energy, position, and direction of all particles are analyzed as piece-wise parameterized functions of radius and polar angle. Subsequently, a pseudo PSF was generated based on this characterization. Validation was assessed by directly comparing the true and pseudo PSFs, and by using both PSFs in the down-stream MC simulations (BEAMnrc/DOSXYZnrc) and comparing dose distributions for 3 applicators at 15 MeV. Statistical uncertainty of 4% was limited by the number of histories in the original PSF. Percent depth dose (PDD) and orthogonal (PRF) profiles at various depths were evaluated. Results: Preliminary results showed thatmore » this PSF parameterization was accurate, with no visible differences between original and pseudo PSFs except at the edge (6 cm off axis), which did not impact dose distributions in phantom. PDD differences were within 1 mm for R{sub 7} {sub 0}, R{sub 5} {sub 0}, R{sub 3} {sub 0}, and R{sub 1} {sub 0}, and PRF field size and penumbras were within 2 mm. Conclusion: A PSF can be successfully characterized by distributions for energy, position, and direction as parameterized functions of radius and polar angles; this facilitates generating sufficient particles at any statistical precision. Analyses for all other electron energies are under way and results will be included in the presentation.« less

Purpose: Verify that Monte Carlo simulations of clinically‐relevant x‐ray fields based on open‐field simulations agree with measured dose distributions. Incident beam energy and spot‐sized were determined independently, previously; the only simulation parameters varied here were the jaw/MLC positions. Methods & Materials:Monte Carlo simulations using EGSnrc/BEAMnrc of small x‐ray fields and fields modified with wedges were compared to measured dose distributions and output factors at 6 and 18 MV. Simulation parameters were derived from a match of open field simulation to measurement using independently measured energy of the electron beam in the waveguide and spot size, and measured jaw/MLC positions. Asymmetry, for example in the positioning of the flattening filter, was accounted for in the simulations. Output factors were measured with a CC13 ion chamber for large field sizes and an Exradin A14 for small field sizes. Transmission through the jaw was determined. Results: Simulated and measured dose distributions were in close agreement. Profiles and PDD curves agreed as well as the open field measurements, to within 1.5%/1.5 mm. Output factors were in agreement to within 2% down to a square field of size of 1.6 cm. Measured and simulated penumbra agreed, as expected because the spot size was measured. Conclusions: Simulations of clinically‐relevant x‐ray fields based on open field measurements were in close agreement with measurement. This validated both the simulations and the methods used to determine them, namely matching open field measurements with independently measured spot size and incident beam energy. Support from NIH R01 CA104777‐01A2.

Purpose: Design a retrofit to Siemens electron applicators meeting IEC leakage standards with minimal effect on treatment field. Method and Materials: Dose to air measured on a Siemens Oncor linac with electron applicators, both at the patient plane and outside the applicator enclosure, by scanning an ion chamber and with film. Material added to applicator and change in dose measured. Monte Carlo simulations of modifications done to guide measurement, using either a virtual CT of the applicator or a new applicator component module. Both these methods allow addition of extra shielding outside the existing applicator. Measurements and simulation were done at 6 energies from 6 to 21 MeV, and 4 applicator sizes, from 10 to 25 cm. Results: Leakage is predominantly scatter from the stainless/aluminum scraper bar and from the top of the Al sidewall. A 1 cm thick Al shield over the entire side of the applicator, although not practical, reduces leakage to nearly acceptable levels while only affecting the treatment field at the 1% level. Shielding around the outside of the stainless/aluminum scraper and at the top of the sidewall is sufficient for some applicators. For the 15 cm applicator, with its thinner sidewall, additional shielding is necessary. It is not necessary to shield the air gaps in the applicator. A discrepancy exists between simulation and data, with measured leakage values several percent greater than simulation. Conclusion: A retrofit design is presented such that the applicators meet IEC specifications without requiring recommissioning. Minimization of material used, for esthetic and practical reasons, is ongoing. Conflict of Interest: Support from Siemens and NIH R01 CA104777‐01A2.

Purpose: The detective quantum efficiency (DQE(f)) is a critical metric to evaluate detector performance. For diagnostic imagers operating at keV energies, there are clear standards from the International Electrotechnical Commission (IEC) using patient equivalent filters to appropriately harden the beam. Such standards do not exist for MeV imaging. Through a systematic investigation comparing simulated and measured DQE(f) we have demonstrated both the need and plausibility for such a standard at MV energies. Methods: The DQE(f) of a Varian AS1000 EPID imaged at 6 MeV by a Varian TrueBeam linac was both measured and simulated. The spectra necessary were extracted from the TrueBeam phase space files available from MyVarian.com website, then hardened through 20 cm of water. From this an equivalent thickness of aluminum was calculated, and the DQE(f) simulated and measured. The Geant4 simulation replicated the Varian AS1000 portal imager geometry, with optical blur in the Gd2O2S scintillator modeled using isotropic scattering. For the measurement of DQE, a narrow, angled slit was formed using two parallel tungsten blocks placed immediately before the imager from which the Fujita over‐sampled MTF was calculated. The NPS was generated from an ensemble of gain and offset corrected flood fields per the IEC guidelines. Results: Simulation indicates a 40% attenuation of the un‐flattened 6 MeV spectrum by 20 cm of water, the equivalent aluminum thickness of which is 85 mm. Simulation and measurement show, with 10% agreement, as much as a 70% decrease in DQE(f) for the hardened beamConclusion: We have demonstrated, through simulation and experiment, as much as a 70% reduction in DQE for a 6 MeV un‐flattened beam through 20 cm of water. We believe it plausible, therefore, to establish a measurement standard similar to that used for keV imaging to fairly evaluate imagers under realistic imaging conditions. This research was funded in part by Varian Medical Systems, Inc. and by NIH grant R01 CA138426

Purpose: Field‐independent phase‐space files are an effective means to reduce the time to simulate particle transport through medical linac treatment head components. The purpose of this work is to: (a) introduce the methodology used to newly create the full TrueBeam phase‐space library; (b) generate ∼50GB of data for each of seven photon beams; (c) present the results of the validation against recent gold beam data (GBD) measurements. Methods: An accurate representation of the TrueBeam field‐independent treatment head components was implemented in Geant4 (v.9.4.p02) and deployed on a large cloud‐based cluster with >500 computing cores. The incident electron beam was modeled by assuming Gaussian distributions for the energy spectrum, beam divergence, and spot size. Spot dimensions were fixed by measurements of the electron spots on the same three machines on which GBD was measured. Three remaining electron beam parameters (mean energy, energy spread, beam divergence) were varied one‐by‐one until the slopes of the least‐squares fits to the differences between Monte Carlo and measurement were

Purpose: Monte Carlo simulations of linear accelerator treatment heads have traditionally involved building the simulated treatment head from specifications provided by the manufacturer. Here, we make pre‐compiled applications available through a web interface, with processors running in the cloud. Methods: Simulations are run on computers on Amazon Web Services, a cloud provider. A machine image (AMI) was created from which multiple instances can be used on a pay‐as‐you‐go basis. The instances present a web interface, where input parameters (such as the target and flattening filter, code version, and parameters of the incident electron beam) are entered. The web form is converted to an input file for the simulation code by a php script. Simulations of the Varian TrueBeam accelerator using Geant4 simulation code are currently available. Phase space files can be written either above the jaws, or below the MLC, and then imported into a separate simulation. Alternatively the dose deposited in a phantom can be calculated. Modifications to the treatment head, for example custom targets, are also possible. The AMI is available to researchers through their own Amazon account. We share the AMI without additional cost; the user is responsible for the Amazon charges (typically USD 0.01/hr/core). Results: The code was verified by matching commissioning data. The web interface was used by collaborators who both matched commissioning data and investigate treatment head modifications. The treatment head was simulated, and modified in the simulation, without disclosure of proprietary information. Input of phase spaces files into EGSnrc was verified. Conclusion: Geant4‐based simulations of the Varian TrueBeam can be run in the cloud, without need for treatment head specifications or a computer cluster. The methods readily generalize to other codes or geometries. As noted in the affiliations, the authors are employees of Varian Medical Systems.

Purpose: The focus of this work was to investigate the implementation of low‐Z targets in a TrueBeam linear accelerator (linac) using Monte Carlo simulations. Methods: Simulation of a TrueBeam linac was accomplished by using Varian Virtualinac. Virtualinac is a cloud based web application utilizing GEANT4 Monte Carlo code to model TrueBeam components. Phase spaces were recorded above the jaws and used as input to BEAMnrc simulations of the lower portion of TrueBeam. Dose distributions and spectral distributions were calculated using DOSXYZnrc and BEAMdp, respectively. For validation, 6 MV flattened and FFF photon beams were generated and compared to measurement. Two low‐Z targets were investigated: a carbon target operated at 2.35 MeV and the proposed 2.50 MeV commercial imaging target. The carbon target was also simulated in a 2100EX Clinac using BEAMnrc. Contrast simulations were made by scoring the dose in the phosphor layer of an IUD20 detector after propagating through 4 and 20 cm thick water and ICRP bone phantoms. Results: Measured and modeled depth dose curves for 6 MV flattened and FFF beams agree within 2% for 99.7% of points at depths greater than 0.85 cm. The two low‐Z target photon spectra produced in TrueBeam are harder than that from a Clinac. Percent dose at depth 10 cm is greater by 8.6% and 14.2%; the fraction of photons in the diagnostic energy range is lower by 7% and 25%; and contrasts are lower by factors of 1.2 and 1.5 (thin phantom) and 1.06 and 1.4 (thick), for the TrueBeam 2.35 MV/carbon and commercial imaging beams, respectively. Conclusion: Virtualinac is effective for Monte Carlo modeling of novel target designs. A significant spectral difference is observed between the low‐Z target beam on the Clinac platform, and the proposed imaging beam line on TrueBeam, with the former providing greater diagnostic energy content. This research was supported by Varian Medical Systems and was accomplished using Varian's Virtualinac Monte Carlo application.

Purpose: Lateral dimensions of the incident electron beam (spot sizes) are essential for accurate modelling of x‐ray fields, especially for small field output factors and penumbrae. We present spot sizes measured on the Varian TrueBeam linac and propose an explanation for the observed shapes. Methods: Spot were measured on 3 TrueBeam linacs using the 4X, 6X, 6FFF, 8X, 10X, 10FFF, and 15X beams. Two methods were used: a spot camera, and an edge CT reconstruction based on images of a thin plate with edge intersecting the beam at different collimator rotations. 2‐dimensional Gaussian functions were fit to the data, with the spot orientation used as a fitting parameter. Results: Spots were consistent for the 3 linacs. Sigmas of the fitted Gaussians were between 0.47 and 1.05 mm for all the beams. The 6X, 6FFF, and 15X beams had a distinct shape: the major axes were 30–40% larger than the minor axes, and the major axes were oriented 40 degrees from the inplane direction. The remaining spots were more symmetric, with major axes at most 15% larger than minor, and a similar orientation. The spot shapes could be explained by positing a helical electron motion at the entrance to the bend magnet, and using the known (non‐achromatic) transport through the magnet. Fringe fields of the bend magnet, solenoid, or steering coils could produce the helical motion; the precise source is not known. Spots measured with the edge technique were 10% smaller in size than those measured with the camera. The 10FFF spots were 10% smaller than the 10X spots, as measured with the camera, but the 6X and 6FFF spots were similar to each other. Conclusion: Representative spots for TrueBeam are presented. Values may be used for input to modelling of fluence output, for example using Monte Carlo methods. Several of the authors are employees of Varian Medical Systems, as noted in the affiliations.

We present preliminary measurements of the thermal conductivity of spin-polarized normal liquid 3 He. Our experimental apparatus allows good thermal homogeneity of the 3 He as well as high sensitivity to the 3 He conductivity. Initial measurements show the conductivity changes by

An overview of the current status of electromagnetic physics (EM) of the Geant4 toolkit is presented. Recent improvements are focused on the performance of large scale production for LHC and on the precision of simulation results over a wide energy range. Significant efforts have been made to improve the accuracy without compromising of CPU speed for EM particle transport. New biasing options have been introduced, which are applicable to any EM process. These include algorithms to enhance and suppress processes, force interactions or splitting of secondary particles. It is shown that the performance of the EM sub-package is improved. We will report extensions of the testing suite allowing high statistics validation of EM physics. It includes validation of multiple scattering, bremsstrahlung and other models. Cross checks between standard and low-energy EM models have been performed using evaluated data libraries and reference benchmark results.

Purpose: To validate the generic phase space files for Varian TrueBeam linac head simulations. Methods: The generic phase space files include the simulation results of 6MV, 10MV, 6MV FFF, and 10MV FFF (flattening‐filter free) operating modes of TrueBeam for patient‐independent linac head components. Using the generic phase space files as the radiationsources, the BEAMnrc Monte Carlo codes are used to simulate the patient‐dependent parts of the TrueBeam linac and the resulting phase space files are generated at a plane just before entering a water phantom for 4 different field sizes (5×5, 10×10, 20×20, and 40×40 cm2). Dose distributions are calculated by DOSXYZnrc in the water phantom of size 50×50×40 cm3. The percentage‐depth‐dose (PDD) curves and lateral dose profiles at three different depths (dmax, 10cm, 20cm) are obtained. Comprehensive comparisons have been made for a total of 64 dose profiles (including PDDs) between the Monte Carlo calculations and the measured data. The gamma index analysis is performed for all the comparisons. Results: The matching of the calculated dose distributions to the measured ones is analyzed by the gamma index method with a criterion of 2% dose tolerance and 2 mm distance‐to‐agreement. Of the 64 comparisons, the minimum gamma index passing rate is at least 92%, after taking into account the statistical nature of the Monte Carlo calculated dose values. Despite the existence of latent variance of phase space files, the phantom dose calculation uncertainty can be less than 1% for field sizes as small as 5×5 cm2. The computing time saved by using phase space files could be a factor of 5–10. Conclusions: The Varian generic phase space files are accurate and efficient radiationsources for Monte Carlo calculations of radiation dose distributions for TrueBeam linac head. This work was supported in part by Varian Medical Systems and the NIH (1R01 CA104205 and 1R21 CA153587).

Purpose: Massive computing resources are needed to generate an extensive linac phase‐space (PHSP) database for different radiotherapy beams that could serve as a starting point for several user‐applications, such as patient‐dependent dose calculations or Monte‐Carlo‐based quality assurance procedures. The goal of this project is to disseminate our knowledge about performing patient‐independent Monte‐Carlo dose calculations on the Amazon Elastic Compute Cloud (EC2) and educate medical physicists on technical aspects such as simulation instance management and cluster coordination while ensuring data security for a low CPU cost. Methods: Amazon‐Web‐Services, along with Google, Microsoft, IBM, etc, offers resizable compute capacity in the cloud changing the economics of computing by eliminating the server fixed expenses. A cost‐efficient elastic cluster is available for a wide users community who can perform Monte Carlo simulations or other numerical applications using Amazon Machine Images. These virtual machines can be configured to work in a particular scientific environment, i.e. GEANT4 or PENELOPE. GEANT4 patient‐independent dose simulations were launched in parallel on EC2 using high CPU extra‐large spot instances (7 GB of memory, 8 virtual cores at 2.13 to 2.44GHz). Data storage was performed using the Simple Storage Service (S3). The S3 Organizer was used to upload the simulation or download multiple files. The 4‐layer EC2 security system, composed of the host/guest operating systems, Firewall, and signed secret access keys, ensures data privacy. Results: A local script was set‐up to launch multiple dose calculation instances on EC2 and automatically save the results on S3. Geant4‐application development, testing, and debugging were performed on a local workstation, along with data analysis and comparison with experiment. IAEA‐compliant patient‐independent PHSP files were generated and dose calculations were successfully validated against experiment. Conclusions: Cloud computing offers a low‐cost alternative for improved radiotherapy planning and optimization algorithms by enabling beam transport and Monte Carlo‐based dose calculations. Work supported by Varian Medical Systems

Purpose: To develop a patient‐independent IAEA‐compliant phase space database using accurate geometry representation of the treatment head components imported from CAD (Computer‐Aided‐Design) drawings and a physics list exploration for the specific materials of the linac treatment head. Methods: Geant4.9.4 was employed to simulate the TrueBeam 6X, 6XFFF, 10X and 10XFFF photon beams. Electron beamtransport in the linac was simulated with Parmela and used as input into dose calculations. Geometry components were tessellated solids, i.e. Generalized‐Dynamic‐Markup‐Language files, extracted from mechanical CAD drawings. A phase space scorer was implemented above the upper jaws on a cylindrical surface. The simulations were run on the Amazon Elastic Compute Cloud. Dose calculations in a water phantom (with 6×6×4 mm3 voxels) were performed using two physics lists, Standard Option‐0 and Low‐Energy Livermore/Penelope, with a 10um range‐cut. The Urban93 and Goudsmit‐Saunderson models were considered for electron multiple‐scattering, while Tsai and 2BN models for electron Bremsstrahlung angular generators. Results: The maximum dose difference between simulation and experiment was

Purpose: To account for a fringe magnetic field from the bending magnet in Monte Carlo treatment head simulation. Method and Materials: A Tesla meter was used to map the magnetic field between the exit window and secondary scattering foil of a clinical linac for 6–21 MeV electron beams. The foil and monitor chamber were removed to access this region. Measurements extended 12.0 cm along the central axis and 1.0–3.0 cm off‐axis. The Monte Carlo code EGSnrc was adapted to incorporate the magnetic field with accuracy validated in simple geometries. Results: The magnetic field was orientated crossplane, with the deflection of the electron beam inplane. The highest intensity, measured at the exit window, increased with nominal beam energy from 0.006 T for 6 MeV to 0.020 T for 21 MeV. The field falloff, approximated by a cubic function, was 5% at the secondary scattering foil. The field deflected the electron beams by 0.82–1.24 cm at isocenter. The peak position in the calculated and measured profiles, with no scattering foils in the beam, matched to 1 mm inplane and crossplane. A preliminary comparison of calculated and measured dose profiles with the foils and chamber in the beam support our contention that the simulations can accurately account for the deflection of the electron beam by the fringe magnetic field from the bending magnet. It appears the lateral variation in the magnetic field can be neglected. Conclusion: The calculated fluence is expected to have an improved uncertainty of 1%/1 mm, without the non‐physical adjustment in the monitor chamber and secondary foil positions required when the field is left out of the simulation, having a better match to the undulations in the measured profiles with the bremsstrahlung peak in the correct position.

Purpose: Write general‐purpose, modular code for phase space input and output in Geant4 including latch and npass. Method and Materials: Geant4.9.3 does not include the ability to read or write phase space files. These files are useful for dividing the simulation into parts, for example, into patient‐independent and patient‐specific parts. Stand‐alone classes inheriting from G4VPrimitiveScorer and G4VPrimaryGenerator were written for writing and reading phase space in IAEA format. Latch mechanisms are not stand‐alone and require implementation of (provided) G4User classes for tracking. Results: Phase space I/O is implemented in Geant4. For writing, several types of surfaces are allowed, including planes, spherical sections, cylindrical sections, surfaces enclosing a volume. Multiple surfaces of any orientation and geometry are allowed. The concept of latch in BEAMnrc (particle interacted in a region) was expanded to include: particle interacted in a region; particle passed through a region; similarly for ancestors; type of interaction in particle's history; type of ancestor particle; and user‐defined. The user defines the interactions, particles, and regions of interest via get/set functions. As in BEAMnrc, an integer npass records the number of times a particle crosses a phase space surface, which is particularly valuable in studies of backscatter. The MGH proton treatment facility was simulated with phase space files recorded at the phantom surface. Spectra using the various latch cuts were generated, demonstrating the capabilities of latching on regions, interactions, or ancestors. Conclusions: Phase space I/O capabilities have been implemented for Geant4. The classes are easy to incorporate into existing code, allowing writing on different surface geometries, and extend the Latch functionality of BEAMnrc. Support: Support from NIH grant 1R01CA140735‐01.

Purpose. Compare images and spectra using a dual energy diamond x‐ray target for megavoltage cone‐beam CT (MVCBCT) with those of carbon and tungsten targets. Materials and Methods. A Siemens Oncor linac with megavoltage cone‐beam CT was modified to use either a tungsten,carbon, or diamond target for imaging. The carbon target was used with an accelerating potential of 4.2 MV. The diamond target had a higher density than the carbon, thus reducing electron leakage and allowing the linac waveguide to operate at the same accelerating potential as the treatment beam, namely 6 MV. For this study, the diamond target was operated at both 4.2 and 6 MV. With both diamond and carbon targets, the flattening filter was out of the beam path. Images were acquired both of phantoms, to determine contrast‐to‐noise ratio (CNR), and patients. Results. CNR measured with the phantom were nearly the same with the diamond target and the carbon target when both were operated at 4.2 MV. At 6 MV, CNR with the diamond target was degraded, especially for tissue. The highest quality images of head and neck patients were with the diamond target at 4.2 MV. As compared to the carbon target, there were fewer scattering artifacts, and tissue resolution was better. At 6 MV, there were even fewer artifacts but the contrast was reduced. Conclusions. The diamond target is a viable alternative to the carbon target for the IBL. It can be used at either the same energy as the treatment beam, which simplifies commissioning and QA, or at reduced energy for enhanced contrast. Support from Siemens OCS.

Purpose: Measure and simulate dose distributions from a 6 and 18 MV x‐ray linac with open jaws, including asymmetry, with independent measurements of the source and geometry details. Materials and Methods: Energy of the incident electron beam was measured by removing the target and flattening filter and measuring the percent‐depth ionization curve of the raw beam. Spot size was measured with an alternating layer spot camera. Treatment head parameters were determined from disassembly of the treatment head, or deduced from the penumbra position. Flattening filter density was determined by weighing it. Distance between target and flattening filter determined from position of the penumbra in the diagonal profiles. Simulations were with EGSnrc. Measurements and simulations were done both with and without the flattening filters. Results: Measured and simulated dose distributions inside the treatment field agreed to 1%/1mm. The exception was the buildup region with no flattening filters, where measurements were greater than simulation by 3–5%, corresponding to a 1 mm shift. This suggests the Roos chamber over‐responded to low energy electrons. Conclusions: Close agreement betweem measurement and simulation was obtained, with simulation parameters determined independently. It was not necessary to remove the flattening filters to obtain the simulation parameters. Support from NIH R01 CA104777‐01A2 and Siemens OCS.

Purpose: To validate the use of improved source and geometry parameters, derived from Monte Carlo simulation of a Siemens Primus accelerator at maximum field size (40 × 40 cm2), for clinically relevant small electron fields. Method and Materials: Measurements were performed on a Siemens Primus linear accelerator for electron energies 6–21 MeV with open applicators and circular cerrobend inserts ranging from 1 cm to 5 cm in diameter. For the open applicators, relative output factors (ROF), depth penetration and off‐axis profiles were measured with parallel‐plate (Roos) and small volume thimble (CC13) chambers. For the cerrobend insert collimated fields a diode (EFD) was used for high spatial resolution. Off‐axis profiles were measured at 0.5 cm, dmax, in the fall‐off and in the bremsstrahlung tail of each beam. Monte Carlo simulations were performed using the EGSnrc accelerator simulation code BEAMnrc. The source and geometry parameters used were obtained from a previous study following disassembly of the treatment head, for maximum field size and no electron applicator, which resulted in an unprecedented match to measurement. Results: Good agreement was found between measured and calculated off‐axis profiles and percentage depth dose curves, to 2 % / 1 mm or better. The normalized off‐axis profiles in the bremsstrahlung tail matched to 2 – 3 %. Calculated relative output factors were within 1 % of those measured with the Roos chamber. Conclusion: This work has shown that a recent methodology used to extract accurate details on the source and geometry of the treatment head for maximum field size, is applicable for clinically relevant small electron fields. The need for careful measurements and accurate knowledge on the source and geometry of the treatment head as a precursor to accurate fluence and dose calculation is highlighted.

Purpose/Objective(s): Digital Tomosynthesis (DT) is an imaging technique for reconstructing multiple tomograms from 2D projections acquired over a limited gantry arc (20°–40°). This study evaluates the use of megavoltage cone‐beam (MVCB) DT images acquired with a high contrastimaging beam line for patient setup. The objective is to determine the optimal parameters (gantry arc, imaging dose) resulting in the least dose to normal tissue while maximizing the positioning accuracy, as compared with MVCBCT. Method and Materials: Coronal and sagittal DT tomograms of prostate (3), head and neck (2), spine (2) and lung (2) patients were reconstructed from a limited subset of projection data, and registered to the planning‐CT. The translational shifts required for patient alignment were calculated and compared with those provided by MVCBCT‐CT registration. The registration was guided by gold markers for prostate patients, bony landmark for head and neck and spine patients, bones and air cavities for lung patients. Online DT images were acquired on an anthropomorphic female thorax phantom to evaluate the applicability of DT imaging to breast patient setup. The imaging dose was measured with thermoluminescent dosimeters on the ipsilateral and contralateral breast. The measurements were compared with dose calculations. Results: DT provides positioning accuracy comparable to CBCT, with positioning difference of ⩽0.1 cm for prostate patients and ⩽0.3 cm for the other sites. Forty degrees arc DT provides the best alignment precision. Phantom studies show that DT images acquired with doses as low as 0.3 cGy are suitable for patient alignment, and the dose to the contralateral breast is negligible. Conclusion: DT is a viable alternative to CBCT for patient setup. Due to the low imaging dose and the sparing of the contralateral breast, DT has potential in breast patient setup. Conflict of Interest: Research sponsored by Siemens OCS.