Radiation Transport Algorithm Research Articles

A Comparative Analysis of Illustris TNG and THESAN Cosmological Simulations

Cosmological simulations play a crucial role in understanding the evolution of the universe, offering valuable insights when compared with observational data. With advancements in computational capabilities and data-gathering techniques, simulations have evolved to model larger volumes and finer details of cosmic structures. This review compares two prominent simulations, Illustris TNG and THESAN, focusing on their algorithms and factors considered. Illustris TNG primarily employs smoothed particle hydrodynamics (SPH), while THESAN utilizes moving mesh hydrodynamics (MMH), each offering unique advantages in simulating different aspects of the universe. Furthermore, THESAN incorporates additional factors such as cosmological dust and sophisticated radiation transport algorithms, enhancing its accuracy in predicting early galaxies and stars. The comparison underscores the impact of algorithmic differences and the consideration of various factors on simulation accuracy. This analysis highlights the importance of understanding these specifications in designing simulations that effectively capture the complexities of the cosmos.

DblurDoseNet: A deep residual learning network for voxel radionuclide dosimetry compensating for single-photon emission computerized tomography imaging resolution.

Current methods for patient-specific voxel-level dosimetry in radionuclide therapy suffer from a trade-off between accuracy and computational efficiency. Monte Carlo (MC) radiation transport algorithms are considered the gold standard for voxel-level dosimetry but can be computationally expensive, whereas faster dose voxel kernel (DVK) convolution can be suboptimal in the presence of tissue heterogeneities. Furthermore, the accuracies of both these methods are limited by the spatial resolution of the reconstructed emission image. To overcome these limitations, this paper considers a single deep convolutional neural network (CNN) with residual learning (named DblurDoseNet) that learns to produce dose-rate maps while compensating for the limited resolution of SPECT images. We trained our CNN using MC-generated dose-rate maps that directly corresponded to the true activity maps in virtual patient phantoms. Residual learning was applied such that our CNN learned only the difference between the true dose-rate map and DVK dose-rate map with density scaling. Our CNN consists of a 3D depth feature extractor followed by a 2D U-Net, where the input was 11 slices (3.3cm) of a given Lu-177 SPECT/CT image and density map, and the output was the dose-rate map corresponding to the center slice. The CNN was trained with nine virtual patient phantoms and tested on five different phantoms plus 42 SPECT/CT scans of patients who underwent Lu-177 DOTATATE therapy. When testing on virtual patient phantoms, the lesion/organ mean dose-rate error and the normalized root mean square error (NRMSE) relative to the ground truth of the CNN method was consistently lower than DVK and MC, when applied to SPECT images. Compared to DVK/MC, the average improvement for the CNN in mean dose-rate error was 55%/53% and 66%/56%; and in NRMSE was 18%/17% and 10%/11% for lesion and kidney regions, respectively. Line profiles and dose-volume histograms demonstrated compensation for SPECT resolution effects in the CNN-generated dose-rate maps. The ensemble noise standard deviation, determined from multiple Poisson realizations, was improved by 21%/27% compared to DVK/MC. In patients, potential improvements from CNN dose-rate maps compared to DVK/MC were illustrated qualitatively, due to the absence of ground truth. The trained residual CNN took about 30 s on a single GPU (Tesla V100) to generate a 512 512 130 dose-rate map for a patient. The proposed residual CNN, trained using phantoms generated from patient images, has potential for real-time patient-specific dosimetry in clinical treatment planning due to its demonstrated improvement in accuracy, resolution, noise, and speed over the DVK/MC approaches.

Historical Reconstruction of Astronaut Cancer Risk: Context for Recent Solar Minima

AbstractAs NASA prepares for longer duration missions beyond the geomagnetic field, the possibility of intensifying galactic cosmic ray (GCR) exposure raises possible concerns for mission planning and crew health and safety. Spaceborne satellite measurements have exhibited increasing GCR intensities and dose‐rates over the past few solar minima, consistent with related heliospheric magnetic field data. Surrogate measures of solar activity (e.g., sunspot number records and radionuclide yields from ice cores and/or tree rings) combined with model predictions have also been used to infer that the Sun may be entering into a quiet period similar to the Dalton or Gleissberg minima when GCR fluences reached historically high levels. Using the most up‐to‐date models for characterizing the ambient GCR environment, physical interactions with shielding, and cancer risk, it is found that projected risks vary by only ±10% over all historical minima since 1750. To provide additional context for this level of variation, two different nuclear interaction models are used within the radiation transport algorithms used to propagate GCR ions and secondary particles through shielding and tissue. It is shown that switching the nuclear interaction model also introduces 10% difference, on average, in projected risks over all historical minima. Furthermore, it is noted that the quantified historical variation is within combined environmental and physics uncertainties already accounted for in the probabilistic cancer risk model used by NASA for crew health and safety considerations.

Relativistic Abrasion–Ablation De-excitation Fragmentation (RAADFRG) model

Astronauts are exposed to ionizing radiation that may pose significant health risks from missions to low Earth orbit (LEO) and beyond. The National Aeronautics and Space Administration (NASA) uses the deterministic radiation transport code, High charge (Z) and Energy TRaNsport (HZETRN), to estimate particle fluxes inside shielded vehicles to evaluate risk of radiation exposure to crew members. Highly efficient radiation transport algorithms and cross section models are needed to perform calculations in realistic vehicles with complex geometrical configurations. The HZETRN code uses the NUClear FRaGmentation (NUCFRG) model to evaluate fragmentation cross section products from nucleus–nucleus collisions. Although highly efficient, the NUCFRG model has some limitations that are based on its unique implementation of the abrasion–ablation formalism. NUCFRG performs well in predicting fragmentation cross sections on the average when compared to experimental data; however, even–odd nuclear structure effects observed in laboratory measurements are absent. The aim of the present work is to formulate a self-consistent theory that produces accurate nuclear fragmentation cross sections while maintaining numerical efficiency. To that end, the Relativistic Abrasion–Ablation FRaGmentation (RAADFRG) model has been developed. The theoretical framework for nuclear interaction is multiple scattering theory (MST), where relativistic kinematics may be included in the momentum–space representation of the Lippmann–Schwinger equation. The nuclear abrasion model employs the Eikonal (Eik) approximation and is used to predict prefragment cross sections. A novel approach is utilized for the excitation energy of prefragment, where in addition to differences of binding energies between two nuclei, energy is transferred to the prefragment from subsequent multiple scattering of abraded nucleons with the spectator nucleon constituents of the prefragment. Next, the excited prefragment liberates particles through the nuclear ablation process, and a nuclear coalescence model that forms aggregate particles for each prefragment channel is included in the yield. The ElectroMagnetic Dissociation FRaGmentation (EMDFRG) model is also included for peripheral interactions that stimulates particle emission via nuclear-photon field interactions. When compared to NUCFRG3, uncertainty quantification analysis shows that RAADFRG is better able to predict experimental nuclear fragmentation cross sections. RAADFRG is also shown to produce the even–odd nuclear structure effects, which is achieved by modification of isospin pairing correction in the prefragment excitation energy model.

Impact of neutrino pair-production rates in core-collapse supernovae

In this paper, we present a careful study on the impact of neutrino pair-production on core-collapse supernovae via spherically-symmetric, general-relativistic simulations of two different massive star progenitors with energy-dependent neutrino transport. We explore the impact and consequences of both the underlying microphysics and the implementation in the radiation transport algorithms on the supernova evolution, neutrino signal properties, and the explosion dynamics. We consider the two dominant neutrino pair-production processes found in supernovae, electron-positron annihilation as well as nucleon-nucleon bremsstrahlung in combination with both a simplified and a complete treatment of the processes in the radiation transport algorithms. We find that the use of the simplified prescription quantitatively impacts the neutrino signal at the 10\% level and potentially the supernova dynamics, as we show for the case of a 9.6M$_\odot$ progenitor. We also show that the choice of nucleon-nucleon bremsstrahlung interaction can also have a quantitative impact on the neutrino signal. A self-consistent treatment with state-of-the-art microphysics is suggested for precision simulations of core collapse, however the simplified treatment explored here is both computationally less demanding and results in a qualitatively similar evolution.

Scientific Developments in Imaging and Dosimetry for Molecular Radiotherapy

Molecular radiotherapy is a rapidly developing field with new vector and isotope combinations continually added to market. As with any radiotherapy treatment, it is vital that the absorbed dose and toxicity profile are adequately characterised. Methodologies for absorbed dose calculations for radiopharmaceuticals were generally developed to characterise stochastic effects and not suited to calculations on a patient-specific basis. There has been substantial scientific and technological development within the field of molecular radiotherapy dosimetry to answer this challenge. The development of imaging systems and advanced processing techniques enable the acquisition of accurate measurements of radioactivity within the body. Activity assessment combined with dosimetric models and radiation transport algorithms make individualised absorbed dose calculations not only feasible, but commonplace in a variety of commercially available software packages. The development of dosimetric parameters beyond the absorbed dose has also allowed the possibility to characterise the effect of irradiation by including biological parameters that account for radiation absorbed dose rates, gradients and spatial and temporal energy distribution heterogeneities. Molecular radiotherapy is in an exciting time of its development and the application of dosimetry in this field can only have a positive influence on its continued progression.

Tree-based solvers for adaptive mesh refinement code flash – I: gravity and optical depths

We describe an OctTree algorithm for the MPI-parallel, adaptive mesh-refinement code {\sc FLASH}, which can be used to calculate the gas self-gravity, and also the angle-averaged local optical depth, for treating ambient diffuse radiation. The algorithm communicates to the different processors only those parts of the tree that are needed to perform the tree walk locally. The advantage of this approach is a relatively low memory requirement, important in particular for the optical depth calculation, which needs to process information from many different directions. This feature also enables a general tree-based radiation transport algorithm that will be described in a subsequent paper, and delivers excellent scaling up to at least 1500 cores. Boundary conditions for gravity can be either isolated or periodic, and they can be specified in each direction independently, using a newly developed generalisation of the Ewald method. The gravity calculation can be accelerated with the {\em adaptive block update} technique by partially re-using the solution from the previous time-step. Comparison with the {\sc Flash} internal multi-grid gravity solver shows that tree based methods provide a competitive alternative, particularly for problems with isolated or mixed boundary conditions. We evaluate several multipole acceptance criteria (MACs) and identify a relatively simple APE MAC which provides high accuracy at low computational cost. The optical depth estimates are found to agree very well with those of the {\sc RADMC-3D} radiation transport code, with the tree solver being much faster. Our algorithm is available in the standard release of the {\sc FLASH} code in version 4.0 and later.

P-multigrid expansion of hybrid multilevel solvers for discontinuous Galerkin finite element discrete ordinate (DG-FEM-SN) diffusion synthetic acceleration (DSA) of radiation transport algorithms

Effective preconditioning of neutron diffusion problems is necessary for the development of efficient DSA schemes for neutron transport problems. This paper uses P-multigrid techniques to expand two preconditioners designed to solve the MIP diffusion neutron diffusion equation with a discontinuous Galerkin (DG-FEM) framework using first-order elements. These preconditioners are based on projecting the first-order DG-FEM formulation to either a linear continuous or a constant discontinuous FEM system. The P-multigrid expansion allows the preconditioners to be applied to problems discretised with second and higher-order elements. The preconditioning algorithms are defined in the form of both a V-cycle and W-cycle and applied to solve challenging neutron diffusion problems. In addition a hybrid preconditioner using P-multigrid and AMG without a constant or continuous coarsening is used. Their performance is measured against a computationally efficient standard algebraic multigrid preconditioner. The results obtained demonstrate that all preconditioners studied in this paper provide good convergence with the continuous method generally being the most computationally efficient. In terms of memory requirements the preconditioners studied significantly outperform the AMG.

P-Multigrid Expansion of Hybrid Multilevel Solvers for Discontinuous Galerkin Finite Element Discrete Ordinate (DG-FEM-SN ) Diffusion Synthetic Acceleration (DSA) of Radiation Transport Algorithms

P-Multigrid Expansion of Hybrid Multilevel Solvers for Discontinuous Galerkin Finite Element Discrete Ordinate (DG-FEM-SN ) Diffusion Synthetic Acceleration (DSA) of Radiation Transport Algorithms

Hybrid Multi-level solvers for discontinuous Galerkin finite element discrete ordinate diffusion synthetic acceleration of radiation transport algorithms

This paper examines two established preconditioners which were developed to accelerate the solution of discontinuous Galerkin finite element method (DG-FEM) discretisations of the elliptic neutron diffusion equation. They are each presented here as a potential way to accelerate the solution of the Modified Interior Penalty (MIP) form of the discontinuous diffusion equation, for use as a diffusion synthetic acceleration (DSA) of DG-FEM discretisations of the neutron transport equation. The preconditioners are both two-level schemes, differing in the low-level space utilised. Once projected to the low-level space a selection of algebraic multigrid (AMG) preconditioners are utilised to obtain a further correction step, these are therefore “hybrid” preconditioners. The first preconditioning scheme utilises a continuous piece-wise linear finite element method (FEM) space, while the second uses a discontinuous piece-wise constant space. Both projections are used alongside an element-wise block Jacobi smoother in order to create a symmetric preconditioning scheme which may be used alongside a conjugate gradient algorithm. An eigenvalue analysis reveals that both should aid convergence but the piece-wise constant based method struggles with some of the smoother error modes. Both are applied to a range of problems including some which are strongly heterogeneous. In terms of conjugate gradient (CG) iterations needed to reach convergence and computational time required, both methods perform well. However, the piece-wise linear continuous scheme appears to be the more effective of the two. An analysis of computer memory usage found that the discontinuous piece-wise constant method had the lowest memory requirements.

Validation of elastic cross section models for space radiation applications

The space radiation field is composed of energetic particles that pose both acute and long-term risks for astronauts in low earth orbit and beyond. In order to estimate radiation risk to crew members, the fluence of particles and biological response to the radiation must be known at tissue sites. Given that the spectral fluence at the boundary of the shielding material is characterized, radiation transport algorithms may be used to find the fluence of particles inside the shield and body, and the radio-biological response is estimated from experiments and models. The fidelity of the radiation spectrum inside the shield and body depends on radiation transport algorithms and the accuracy of the nuclear cross sections. In a recent study, self-consistent nuclear models based on multiple scattering theory that include the option to study relativistic kinematics were developed for the prediction of nuclear cross sections for space radiation applications. The aim of the current work is to use uncertainty quantification to ascertain the validity of the models as compared to a nuclear reaction database and to identify components of the models that can be improved in future efforts.

Sensitivity Analysis of Nonequilibrium Martian Entry Flow to Chemical and Thermal Modeling

Numerical simulations were performed for a reference Martian entry test case using the nonequilibrium code URANUS. The code was further developed to allow for various chemical and thermal modelings of the nonequilibrium Martian entry flow. The flowfield around the reference vehicle was calculated for several chemical and thermal models of a carbon dioxide/nitrogen gas inflow mixture and compared to results available in the literature. A sensitivity analysis of the gas radiation and radiative heat flux were performed by employing the ESA’s plasma radiation database and the radiation transport algorithm HERTA on the computational fluid dynamics results. It was found that the radiation of the carbon dioxide molecule must be taken into consideration due to the large contribution to the overall radiation.

SU‐F‐T‐153: Experimental Validation and Calculation Benchmark for a Commercial Monte Carlo Pencil Beam Scanning Proton Therapy Treatment Planning System in Water

Purpose:Eclipse proton Monte Carlo AcurosPT 13.7 was commissioned and experimentally validated for an IBA dedicated PBS nozzle in water. Topas 1.3 was used to isolate the cause of differences in output and penumbra between simulation and experiment.Methods:The spot profiles were measured in air at five locations using Lynx. PTW‐34070 Bragg peak chamber (Freiburg, Germany) was used to collect the relative integral Bragg peak for 15 proton energies from 100 MeV to 225 MeV. The phase space parameters (σx, σθ, ρxθ) number of protons per MU, energy spread and calculated mean energy provided by AcurosPT were identically implemented into Topas. The absolute dose, profiles and field size factors measured using ionization chamber arrays were compared with both AcurosPT and Topas.Results:The beam spot size, σx, and the angular spread, σθ, in air were both energy‐dependent: in particular, the spot size in air at isocentre ranged from 2.8 to 5.3 mm, and the angular spread ranged from 2.7 mrad to 6 mrad. The number of protons per MU increased from ∼9E7 at 100 MeV to ∼1.5E8 at 225 MeV. Both AcurosPT and TOPAS agree with experiment within 2 mm penumbra difference or 3% dose difference for scenarios including central axis depth dose and profiles at two depths in multi‐spot square fields, from 40 to 200 mm, for all the investigated single‐energy and multi‐energy beams, indicating clinically acceptable source model and radiation transport algorithm in water.Conclusion:By comparing measured data and TOPAS simulation using the same source model, the AcurosPT 13.7 was validated in water within 2 mm penumbra difference or 3% dose difference. Benchmarks versus an independent Monte Carlo code are recommended to study the agreement in output, filed size factors and penumbra differences.This project is partially supported by the Varian grant under the master agreement between University of Pennsylvania and Varian

HIGH-TEMPERATURE PROCESSING OF SOLIDS THROUGH SOLAR NEBULAR BOW SHOCKS: 3D RADIATION HYDRODYNAMICS SIMULATIONS WITH PARTICLES

A fundamental, unsolved problem in Solar System formation is explaining the melting and crystallization of chondrules found in chondritic meteorites. Theoretical models of chondrule melting in nebular shocks has been shown to be consistent with many aspects of thermal histories inferred for chondrules from laboratory experiments; but, the mechanism driving these shocks is unknown. Planetesimals and planetary embryos on eccentric orbits can produce bow shocks as they move supersonically through the disk gas, and are one possible source of chondrule-melting shocks. We investigate chondrule formation in bow shocks around planetoids through 3D radiation hydrodynamics simulations. A new radiation transport algorithm that combines elements of flux-limited diffusion and Monte Carlo methods is used to capture the complexity of radiative transport around bow shocks. An equation of state that includes the rotational, vibrational, and dissociation modes of H$_2$ is also used. Solids are followed directly in the simulations and their thermal histories are recorded. Adiabatic expansion creates rapid cooling of the gas, and tail shocks behind the embryo can cause secondary heating events. Radiative transport is efficient, and bow shocks around planetoids can have luminosities $\sim$few$\times10^{-8}$ L$_{\odot}$. While barred and radial chondrule textures could be produced in the radiative shocks explored here, porphyritic chondrules may only be possible in the adiabatic limit. We present a series of predicted cooling curves that merit investigation in laboratory experiments to determine whether the solids produced by bow shocks are represented in the meteoritic record by chondrules or other solids.

The reliability of approximate radiation transport methods for irradiated disk studies

Context: Dynamical studies of irradiated circumstellar disks require an accurate treatment of radiation transport to, for example, properly determine cooling and fragmentation properties. The radiation transport algorithm should be as fast as the (magneto-) hydrodynamics to allow for an efficient usage of computing resources. Methods: We use a setup of a central star and a slightly flared circumstellar disk. We perform simulations for a wide range of optical depths of the disk's midplane from tau(550nm) = 0.1 up to tau(810nm) = 1 million. We check the accuracy of the gray flux-limited diffusion (FLD) approximation and a gray and frequency-dependent ray-tracing plus FLD approximation. Results: 1. For moderate optical depths, a gray approximation of the stellar irradiation yields a slightly hotter inner rim and a slightly cooler midplane of the disk at larger radii, but is otherwise in agreement with the frequency-dependent treatment. 2. The gray FLD approximation fails to compute an appropriate temperature profile in all regimes of optical depth; the maximum deviations to the comparison runs are 50 percent in the optically thin and up to 280 percent in the optically thick limit. For low optical depth, the isotropic assumption within the FLD method yields a too steep decrease of the radial temperature slope. For higher optical depths, the FLD approximation does not reproduce the shadow behind the optically thick inner rim of the circumstellar disk, yielding artificial heating at larger disk radii. 3. The frequency-dependent RT + gray FLD approximation yields remarkable accuracy for the whole range of optical depths. Conclusions: The high accuracy of the frequency-dependent hybrid radiation transport algorithm makes this method ideally suited for (magneto-) hydrodynamical studies of irradiated circumstellar disks.

Dosimetric impact of Acuros XB deterministic radiation transport algorithm for heterogeneous dose calculation in lung cancer.

The novel deterministic radiation transport algorithm, Acuros XB (AXB), has shown great potential for accurate heterogeneous dose calculation. However, the clinical impact between AXB and other currently used algorithms still needs to be elucidated for translation between these algorithms. The purpose of this study was to investigate the impact of AXB for heterogeneous dose calculation in lung cancer for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). The thorax phantom from the Radiological Physics Center (RPC) was used for this study. IMRT and VMAT plans were created for the phantom in the Eclipse 11.0 treatment planning system. Each plan was delivered to the phantom three times using a Varian Clinac iX linear accelerator to ensure reproducibility. Thermoluminescent dosimeters (TLDs) and Gafchromic EBT2 film were placed inside the phantom to measure delivered doses. The measurements were compared with dose calculations from AXB 11.0.21 and the anisotropic analytical algorithm (AAA) 11.0.21. Two dose reporting modes of AXB, dose-to-medium in medium (Dm,m) and dose-to-water in medium (Dw,m), were studied. Point doses, dose profiles, and gamma analysis were used to quantify the agreement between measurements and calculations from both AXB and AAA. The computation times for AAA and AXB were also evaluated. For the RPC lung phantom, AAA and AXB dose predictions were found in good agreement to TLD and film measurements for both IMRT and VMAT plans. TLD dose predictions were within 0.4%-4.4% to AXB doses (both Dm,m and Dw,m); and within 2.5%-6.4% to AAA doses, respectively. For the film comparisons, the gamma indexes (± 3%∕3 mm criteria) were 94%, 97%, and 98% for AAA, AXB_Dm,m, and AXB_Dw,m, respectively. The differences between AXB and AAA in dose-volume histogram mean doses were within 2% in the planning target volume, lung, heart, and within 5% in the spinal cord. However, differences up to 8% between AXB and AAA were found at lung∕soft tissue interface regions for individual IMRT fields. AAA was found to be 5-6 times faster than AXB for IMRT, while AXB was 4-5 times faster than AAA for VMAT plan. AXB is satisfactorily accurate for the dose calculation in lung cancer for both IMRT and VMAT plans. The differences between AXB and AAA are generally small except in heterogeneous interface regions. AXB Dw,m and Dm,m calculations are similar inside the soft tissue and lung regions. AXB can benefit lung VMAT plans by both improving accuracy and reducing computation time.

A new analytic solution of the one-speed neutron transport equation for adjacent half-spaces with isotropic scattering

This paper derives a new analytic solution of the one-speed neutron transport equation for two isotropically scattering half-spaces, where each half-space has its own isotropic internal source. The constant source problem has been studied previously by Auerbach (1961), and this analysis has been generalised to consider a source whose strength varies linearly with position. This problem arises in connection with assessing the accuracy of linear treatments of the thermal emission source in Monte Carlo thermal radiation transport algorithms. Simple expressions for the angular flux at the interface as well as its zeroth and first moments are provided as a function of the scattering ratios in the two half-spaces.

TU-C-BRA-02: Calibrating the Output of a Linear Accelerator - TG-51 Updated

Although dosimetry protocols tend to be fairly long lived it is more than ten years since the AAPM's TG‐51 protocol was published. With the developments in the technology for dose delivery and advances in the measurement and calculation of ion chambercalibration coefficients it is reasonable to question the validity of TG‐51 for the determination of reference (absolute) dose in 2011. Four areas are immediately obvious: 1. The majority of chambers available today do not have kq factors listed in TG‐51. 2. Users can obtain linac‐based absorbed dose calibration coefficients for ion chambers from calibration laboratories on a routine basis. 3. Monte‐Carlo radiation transport algorithms now allow accurate modelling of ion chamber geometries. 4. Linacs with very different beam parameters (e.g., flattening‐filter‐free) are now being introduced into clinical practice. 5. In light of these issues the AAPM set up a working group to review TG‐51 and provide clarification. The group would focus on accelerators capable of reproducing standard reference conditions (i.e., 10 cm × 10 cm field at 1 m) and the output would be an addendum that updates the TG‐51 protocol to the situation in 2011. This presentation will deal primarily with megavoltage photon beams and will discuss the content of the proposed report: i) Calculated k Q factors for new chambers developed after the publication of TG‐51 ii) A comparison of these calculations with measured k Q factors obtained at primary standards laboratories worldwide. iii) An in‐depth discussion of uncertainties with guidance on how the clinical physicist can affect the overall uncertainty in absorbed dose to water iv) Best practice recommendations to minimize errors and ensure consistent dosimetric reporting In addition there will be some discussion of the issues to be faced in the update for electron beamdosimetry. Development of electron beam standards has lagged behind that for photon beams and further experimental and theoretical work is required before an update to the electron part of TG‐ 51 can be considered. Issues include: i) status of primary standards worldwide; ii) validity of a 60Co‐based calibration (direct versus cross‐ calibration); iii) choice of chamber, cylindrical or parallel‐plate; iv) correct values of kQ factors in light of recent determinations of chamber perturbation factors. Learning objective: 1. Rationale for an update to TG‐51 2. Content of photon beam addendum 3. Timeline and research requirements for electron beam update.

SU‐E‐T‐700: PMMC ‐ a High‐Performance Monte Carlo Code for Proton Beam Dose Calculation

Purpose: The proton macro Monte Carlo (PMMC) method uses a fast macro Monte Carlo radiation transport algorithm for high‐energy proton transport through an absorbing medium. Methods: PMMC uses results derived from traditional Monte Carlo simulations of proton transport through macroscopic cylinders of varying length, radii and material composition. These traditional Monte Carlo simulations are done for many incident proton energies and the results are used to transport protons through an absorber geometry in a step‐by‐step manner. The absorber geometry consists of a three‐dimensional (3D) density matrix that can represent objects like a water phantom or computed tomography data. The source is specified by the user in terms of gantry angle and beam characteristics like position, energy and size. During proton transport, energy lost by the protons during each step is scored in a 3D dose grid. The creation of secondary neutrons is tracked as well to allow for estimation of the neutron dose. Results: The algorithm, which was written in C++, is currently capable of simulating 1 million protons per minute. The accuracy of the PMMC simulations was assessed by comparing the results to simulations done with the same source parameters in MCNPX. The simulations consisted of monoenergetic pencil beams and Gaussian‐shaped beams directed onto a water phantom. In general, PMMC and MCNPX are in good agreement in the high‐energy proton region before the Bragg peak, differing by less than 3%. PMMC performs worse at the depth of maximum dose, differing by as much as 10%. Conclusions: Forthcoming modifications to PMMC will address the discrepancies in the Bragg peak region and further modifications are underway to allow PMMC to be used with absorbers that are heterogeneous in density and material composition.

SU-E-T-720: Dosimetric Verification of Deterministic Acuros XB Radiation Transport Algorithm for IMRT and VMAT Plans with the RPC H&N Phantom

Purpose: Acuros XB (AXB), a novel deterministic dose calculation algorithm based on grid‐based Boltzmann transport equation solver (GBBS), was recently implemented in Eclipse treatment planning system (TPS). The purposes of this study were to verify the dosimetric performance of AXB by (1) comparing measured and calculated data and (2) comparing AXB with the existing Eclipse anisotropic analytical algorithm (AAA). Methods: Clinically equivalent intensity modulated radiation therapy(IMRT) and volumetric modulated arc therapy (VMAT) plans were created in Eclipse for the Radiological Physics Center (RPC) head and neck IMRT phantom using the RPC phantom dose prescription specifications. Each plan was calculated with two different algorithms, AXB 11.0.91 and AAA 10.0.24. Each treatment plan was delivered to the RPC phantom three times for reproducibility using a Varian Clinac21iX. Dose distributions were measured with thermoluminescent dosimeters(TLDs) and radiochromic film (GafChromic® EBT2). Gamma analysis (7%/4mmDTA) was used to quantify the agreement between AXB and AAA calculated dose distributions and those measured using films. The computation times for AAA and AXB were also evaluated. Results: In this study, good agreement was observed between measured and calculated doses both from AAA and AXB (dose‐to‐medium in medium). Comparing the calculations with TLD measurements, agreement for both AAA and AXB were < 5% (ranges from 0.2% to 4.6% for AAA, 0.1% to 3.6% for AXB) for IMRT and VMAT plans. The gamma analysis results for both AAA and AXB met the RPC 7%/4 mm criteria (over 90% passed). The AAA and AXB computation times were comparable for IMRT but AXB was ∼3 times faster than AAA for VMAT. Conclusions: The AXB was determined to be accurate using the RPC IMRT H&N phantom measurements and in good agreement with the AAA while having shorter calculation times for VMAT as compared to the AAA.