Water Tank Measurements Research Articles

Monte Carlo source verification of Elekta Synergy for pMLC collimated electron beams

IntroductionIn previous works, an electron beam source was developed that models its energy spectrum with a Lévy distribution. This source is used in BEAMnrc Monte Carlo simulation. It was benchmarked against water tank measurements for electrons delivered through an applicator collimation system, and it was found to agree with 2%/2 mm. This study uses this electron source further by investigating its ability to simulate electrons collimated by the photon multi-leaf collimators for MERT applications. Simulated and Gafchromic EBT3 depth dose and profile data were compared. MethodMeasured data were obtained from film exposures in a mini phantom with water-equivalent slabs at SSDs of 60 and 70 cm, respectively. Output factors and R90 values were calculated from film strip data taken in the depth direction. Corresponding dose data were simulated in BEAMnrc to obtain phase-space files for each beam and used in DOSXYZnrc to obtain dose data in a 3D water tank with 2 mm side length subvoxels. ResultsSimulation data corresponded with film data within 3 %/2 mm in most cases. Uncertainties in film measurements prevent a more accurate agreement. The output factors showed good agreement and could be fitted satisfactorily with an analytic function. A clinical example of a scalp treatment showed good agreement between simulated and film data. ConclusionIt is concluded that the electron source with the Lévy-based energy distribution can model electron dose for photon multi-leaf collimated electron beams within 3 %/2 mm.

Comparative assessment and QA measurement array validation of Monte Carlo and Collapsed Cone dose algorithms for small fields and clinical treatment plans.

Many studies have demonstrated superior performance of Monte Carlo (MC) over type B algorithms in heterogeneous structures. However, even in homogeneous media, MC dose simulations should outperform type B algorithms in situations of electronic disequilibrium, such as small and highly modulated fields. Our study compares MC and Collapsed Cone (CC) dose algorithms in RayStation 12A. Under consideration are 6 MV and 6 MV flattening filter-free (FFF) photon beams, relevant for VMAT plans such as head-and-neck and stereotactic lung treatments with heterogeneities, as well as plans for multiple brain metastases in one isocenter, involving highly modulated small fields. We aim to investigate collimator angle dependence of small fields and performance differences between different combinations of ArcCHECK configuration and dose algorithm. Several verification tests were performed, ranging from simple rectangular fields to highly modulated clinical plans. To evaluate and compare the performance of the models, the agreements between calculation and measurement are compared between MC and CC. Measurements include water tank measurements for test fields, ArcCHECK measurements for test fields and VMAT plans, and film dosimetry for small fields. In very small or narrow fields, our measurements reveal a strong dependency of dose output to collimator angle for VersaHD with Agility MLC, reproduced by both dose algorithms. ArcCHECK results highlight a suboptimal agreement between measurements and MC calculations for simple rectangular fields when using inhomogeneous ArcCHECK images. Therefore, we advocate for the use of homogeneous phantom images, particularly for static fields, in ArcCHECK verification with MC. MC might offer performance benefits for more modulated treatment fields. In ArcCHECK results for clinical plans, MC performed comparable to CC for 6 MV. For 6 MV FFF and the preferred homogeneous phantom image, MC resulted in consistently better results (13%-64% lower mean gamma index) compared to CC.

Proton Beam Range and Charge Verification Using Multilayer Faraday Collector.

A daily quality assurance (QA) check in proton therapy is ensuring that the range of each proton beam energy in water is accurate to 1 mm. This is important for ensuring that the tumor is adequately irradiated while minimizing damage to surrounding healthy tissue. It is also important to verify the total charge collected against the beam model. This work proposes a time-efficient method for verifying the range and total charge of proton beams at different energies using a multilayer Faraday collector (MLFC). We used an MLFC-128-250 MeV comprising 128 layers of thin copper foils separated by thin insulating KaptonTM layers. Protons passing through the collector induce a charge on the metallic foils, which is integrated and measured by a multichannel electrometer. The charge deposition on the foils provides information about the beam range. Our results show that the proton beam range obtained using MLFC correlates closely with the range obtained from commissioning water tank measurements for all proton energies. Upon applying a range calibration factor, the maximum deviation is 0.4 g/cm2. The MLFC range showed no dependence on the number of monitor units and the source-to-surface distance. Range measurements collected over multiple weeks exhibited stability. The total charge collected agrees closely with the theoretical charge from the treatment planning system beam model for low- and mid-range energies. We have calibrated and commissioned the use of the MLFC to easily verify range and total charge of proton beams. This tool will improve the workflow efficiency of the proton QA.

Investigations of the acoustic-field autoproducts in three dimensions

Acoustic remote sensing tasks, such as remote unknown source localization, are commonly completed via signal processing schemes that are implemented in the bandwidth of the acoustic recordings. Interestingly, many such schemes can also be implemented at frequencies outside of the bandwidth of the recorded field—despite their absence from the recordings—by using the frequency-difference and frequency-sum autoproducts [Worthmann and Dowling, J. Acoust. Soc. Am., 141 (2017) 4579–4590]. Recent acoustic-field autoproduct studies have involved beamforming and source localization in shallow- and deep-ocean environments, and have addressed the impacts of refraction, shadow zones, rough surface scattering, and noise. These previous efforts primarily explored the properties of the autoproducts when the sound field primarily varies in two spatial dimensions. This presentation provides the results of a study of the autoproducts formed from the simple but fully three-dimensional acoustic fields arising from an isolated source placed in a uniform-sound-speed environment bounded by reflecting surfaces that are not parallel. This study indicates the extent to which the autoproduct properties associated with two dimensional acoustic-field variations extend to three dimensional acoustic-field variations. The results from theory and simulations are presented, and from laboratory water-tank measurements if time allows.

Optimal values of the Electron Monte Carlo dose engine parameters.

The aim of this study was to indicate the most favorable - in terms of to the time of calculation and the uncertainty of determining the dose distribution - values of the parameters for the Electron Monte Carlo (eMC) algorithm in the Eclipse treatment planning system. Using the eMC algorithm and the variability of the values of its individual parameters, calculations of the electron dose distribution in the full-scattering virtual water phantom were performed, obtaining percentage depth doses, beam profiles, absolute dose values in points and calculation times. The reference data included water tank measurements such as relative dose distributions and absolute point doses. For 63 sets of calculation data created from selected values of the parameters for the eMC algorithm, calculation times were analyzed and the absolute calculated and measured doses were compared. Performing a statistical analysis made it possible to determine whether the differences in the values of deviations between the actual dose and the calculated dose in individual regions of the percentage depth dose curve and the beam profile are statistically significant between the analyzed sets of parameters. Taking into account obtained results from the analysis of the discrepancy between the distribution of the calculated and measured dose, the correspondence of the absolute value of the calculated and measured dose and the duration of the calculation of the dose distribution, the optimal set of parameters was indicated for the eMC algorithm which allows obtaining the dose distribution and the number of monitor units in an acceptable time.

Validation of RayStation Monte Carlo dose calculation algorithm for multiple LINACs

PurposeA photon Monte Carlo (MC) model was commissioned for flattened (FF) and flattening filter free (FFF) 6 MV beam energy. The accuracy of this model, as a single model to be used for three beam matched LINACs, was evaluated. MethodsMultiple models were created in RayStation v.10A for three linacs equipped with Elekta “Agility” collimator. A clinically commissioned collapsed cone (CC) algorithm (GoldCC), a MC model automatically created from the CC algorithm without further optimization (CCtoMC) and an optimized MC model (GoldMC) were compared with measurements. The validation of the model was performed by following the recommendations of IAEA TRS 430 and comprised of basic validation in a water tank, validation in a heterogeneous phantom and validation of complex IMRT/VMAT paradigms using gamma analysis of calculated and measured dose maps in a 2D-Array. ResultsDose calculation with the GoldMC model resulted in a confidence level of 3% for point measurements in water tank and heterogeneous phantom for measurements performed in all three linacs. The same confidence level resulted for GoldCC model. Dose maps presented an agreement for all models on par to each other with γ criteria 2%/2mm. ConclusionsThe GoldMC model showed a good agreement with measured data and is determined to be accurate for clinical use for all three linacs in this study.

Modeling linear accelerator (Linac) beam data by implicit neural representation learning for commissioning and quality assurance applications.

Linear accelerator (Linac) beam data commissioning and quality assurance (QA) play a vital role in accurate radiation treatment delivery and entail a large number of measurements using a variety of field sizes. How to optimize the effort in data acquisition while maintaining high quality of medical physics practice has been sought after. We propose to model Linac beam data through implicit neural representation (NeRP) learning. The potential of the beam model in predicting beam data from sparse measurements and detecting data collection errors was evaluated, with the goal of using the beam model to verify beam data collection accuracy and simplify the commissioning and QA process. NeRP models with continuous and differentiable functions parameterized by multilayer perceptrons (MLPs) were used to represent various beam data including percentage depth dose (PDD) and profiles of 6 MV beams with and without flattening filter. Prior knowledge of the beam data was embedded into the MLP network by learning the NeRP of a vendor-provided "golden" beam dataset. The prior-embedded network was then trained to fit clinical beam data collected at one field size and used to predict beam data at other field sizes. We evaluated the prediction accuracy by comparing network-predicted beam data to water tank measurements collected from 14 clinical Linacs. Beam datasets with intentionally introduced errors were used to investigate the potential use of the NeRP model for beam data verification, by evaluating the model performance when trained with erroneous beam data samples. Linac beam data predicted by the model agreed well with water tank measurements, with averaged Gamma passing rates (1%/1mm passing criteria) higher than 95% and averaged mean absolute errors less than 0.6%. Beam data samples with measurement errors were revealed by inconsistent beam predictions between networks trained with correct versus erroneous data samples, characterized by a Gamma passing rate lower than 90%. A NeRP beam data modeling technique has been established for predicting beam characteristics from sparse measurements. The model provides a valuable tool to verify beam data collection accuracy and promises to simplify commissioning/QA processes by reducing the number of measurements without compromising the quality of medical physics service.

Improving Online Adaptive Radiotherapy Quality Assurance with Streamlined Clinical Workflow through In-House Development

<h3>Purpose/Objective(s)</h3> Online adaptive radiotherapy has been clinically feasible through decades of research and development and is now available in the clinical routine with dedicated platforms using CBCT-LINAC or MR-LINAC. The quality assurance (QA) of online plans has become a bottleneck in the clinical workflow as it often requires manual data export/import through third party software, lengthy wait for the secondary dose calculation, and tedious work on report generation and checking, etc. The QA bottleneck not only impedes clinical workflow but also risks plan and treatment quality due to the time pressure with the patient waiting on the treatment table. We have developed and implemented in-house solutions to address such a bottleneck. <h3>Materials/Methods</h3> We developed dedicated in-house infrastructure to facilitate online adaptive plan QA and clinical workflow: 1) RT-PACK with SQLite database, which receives and organizes DICOM-RT data from any DICOM sender (e.g., clinical TPS); 2) a general purpose dose calculation server powered by the in-house GPU-based Monte Carlo particle transport engine with electromagnetic field modelling; 3) a report generator that automatically generates reports for secondary dose and plan quality check; 4) a web-portal that allows clinicians to review and sign the report anywhere with a network and VPN connection, even from a mobile phone. The reports and related documents will be automatically sent to the Record & Verify system after obtaining both physician and physicist signatures. An auto-commissioner was also developed to facilitate beam modeling based on water-tank measurements, which serves the purpose of fully independent verification. The dose calculation was performed on the whole patient volume with grid resolution of 2 mm x 2 mm x 2 mm. The 3D dose volume, treatment fields, point dose, Gamma Index with global 3%, 2mm criteria, three orthogonal dose profiles, dose volume histograms, and dosimetric constraints are metrics to compare the plan retrieved from TPS to the secondary dose verification and reported for clinicians' review. The whole in-house infrastructure went through extensive unit tests, dosimetric accuracy verifications, and alpha and beta tests before adopted for clinical use. <h3>Results</h3> The secondary dose and plan check infrastructure went live for CBCT-LINAC and MR-LINAC in our clinic and has since performed secondary check for every plan generated (846 plans and 622 plans, respectively, in less than 6 months.) The average processing time, from DICOM receiving to report generation, was 62 seconds for CBCT-LINAC and 130 seconds for MR-LINAC, which was 3-5 minutes shorter compared to using commercial software. More than 95% of CBCT-LINAC (MR-LINAC) plans had Gamma passing rates >99% (>95%). <h3>Conclusion</h3> The in-house developed software, through strict verification and validation, has been integrated into the clinical system. It has greatly improved online adaptive quality assurance with efficient and streamlined clinical workflow.

Assessment of Radioactivity Dispersion Model for Surface Water

Uranium is an element commonly found in natural groundwater can be measured up to 100-500 µg per L without impact from mining. Determination of radionuclide concentrations by dispersion in the surface water is based on several mathematical models. In this paper, the main purpose of this research is to estimate the distribution of radionuclide contamination in the water body under laboratory conditions. The simplest approach in deriving radioactivity dispersion was evaluated by a laboratory-scale glass tank (60 x 60 x 60 cm) which was used for dispersion measurements in the laboratory. The box model transition test has taken into account several parameters; these are mixing ratio, transit time and decay to predict radionuclide concentrations in the water at the location of interest. Radioactive solution (650 Bq/liter) was released in the box and samples were taken from 10 cm intervals every 5 minutes. Results show that radioactivity dispersion can be predicted briefly by using this model for surface waters. Laboratory water tank measurements show that radioactivity dispersion of uranium-bearing solution in surface water compatible with the equation. Measured transition time in the field is the most valuable information that improves the ability to predict the transport and dispersion in surface water.

Three-dimensional simulation of building thermal plumes merging in calm conditions: Turbulence model evaluation and turbulence structure analysis

A 3D numerical modelling is conducted to investigate the thermal plumes merging and buoyancy-driven airflow in urban areas, in which poor natural ventilation causes both thermal comfort and air quality issue. The performances of five different turbulence models, i.e., three URANS (Unsteady Reynolds-averaged Navier–Stokes equations) models and two LES (Large-Eddy Simulation) models are evaluated by comparing the velocity field with previous water tank measurements. Validation results show that all four turbulence models can capture the bending of thermal plumes toward the centre, and LES models provide a better prediction on the vertical velocity profiles, while both URANS models show underestimation. The plume merging mechanism is analysed with the high accuracy LES results. Both pressure difference induced mean horizontal flow and shear instability induced turbulence are the main causes of plume merging. The coherent structure of plume merging is analysed by a quadrant analysis, which shows ejection and sweep events dominate the turbulence momentum exchange. A case study with complex urban geometry is conducted to show the impact of thermal plumes merging in the real high-density urban areas. The convergence airflow at the pedestrian level is estimated to 2 m/s, which is comparable to wind-driven ventilation and beneficial to thermal comfort and air quality.

Tungsten carbide and LMPA electron cutouts: comparison and validation using Monte Carlo modelling and measurement of dose

Electron applicator cutouts for radiation therapy electron beam shaping are typically cast from Low Melting Point Alloys (LMPA), such as cerrobend. In this work, we describe the Monte Carlo modelling of novel 3D printed cutouts based on tungsten carbide powder and the resulting dose profiles subject to Elekta Agility electron beams. Cerrobend cutouts were also modelled using the Monte Carlo code EGSnrc. Cerrobend and tungsten carbide cutouts were found to have the same dose profiles within model variance. Computed profiles and percentage depth dose (PDDs) curves of the Elekta Agility accelerator model using standard cutouts in water were found to agree with water tank measurements using gamma criteria of 2%/2mm. The Monte Carlo computed dose profiles of the tungsten carbide cutouts in a polystyrene phantom were also found to agree with liquid-filled ionization chamber array measurements using gamma criteria of 2%/2mm. We conclude that the tungsten carbide cutouts are clinically equivalent to LMPA cutouts.

Validation of a Lévy electron energy straggling model for an Elekta Synergy® linear accelerator

AimIn this study, an EGSnrc based Monte Carlo electron model was validated for an Elekta Synergy® 160-leaf Agility™ linear accelerator. A previously reported electron energy straggling model based on a Lévy distribution was tested against water tank measurements and a specially designed heterogeneous multi-layered phantom. This included PDD, beam profile, and relative output factor (ROF) comparison. All data passed a 2%/2 mm gamma criterion with the exception of some ROF data, which showed discrepancies of up to 2.7%. MethodsBEAMnrc was used to accurately model the linac that included the improved exit electron energy spectrum based on a Lévy distribution. The resulting BEAMnrc phase space files were used as sources in DOSXYZnrc for water tank dose distribution simulations consisting of 6 electron beam energies, 11 field sizes, and source-to-surface distances (SSDs) of 95 and 100 cm. Evaluation parameters included PDD, dose profiles, and relative output factors, as well as phantom PDD and dose profile measurements with EBT3 gafchromic film. ResultsThe improved exit electron beam energy spectrum caused simulated data to comply with measured data (PPD's and dose profiles) with a 100% pass rate using a 2%/2 mm criterion except for some relative output factors that deviated by 2.7% from measured ones in water. This was observed for both 95 and 100 cm SSD data. Good agreement was obtained between film and simulation data within 2% in more than 90% of PDD and profile measurements. ConclusionsThe Lévy based energy straggling model for electron beams allowed for accurate electron beam characterization in water tank and phantom measurements.

PLA as a suitable 3D printing thermoplastic for use in external beam radiotherapy.

The purpose of this paper is to investigate the tissue equivalence and radiological properties of Polylactic Acid (PLA) to determine if this material is suitable for use as a 3D printing thermoplastic for radiotherapy applications. Profiles and percentage depth-dose measurements (PDDs) were analysed for photon and electron modalities with PLA samples (~ 1.25g/cm3) to determine material dosimeteric characteristics. Beam profiles and PDDs from treatment planning system (TPS) simulations, water tank measurements and radiochromic film measurements were compared. Tissue equivalence was determined through CT scanning several PLA samples and measuring the Hounsfield units (HUs) to determine relative electron density (RED), mass density and mass attenuation, these results were compared to several commercial tissue phantoms with varying properties. Geometric accuracy was tested by comparing digitally planned dimensions to physical and CT image measurements. Finally, resistance to radiation damage was tested by exposing PLA samples to several thousand monitor units (MUs) over several weeks and inspecting for damage. It was determined that PLA is a safe and effective thermoplastic for use as a patient specific bolus for both electron and photon treatment modalities. The material properties have been characterised and can be accurately modelled in the MONACO TPS.

Technical Note: Use of commercial multilayer Faraday cup for offline daily beam range verification at the McLaren Proton Therapy Center.

Daily verification of the proton beam range in proton radiation therapy is a vital part of the quality assurance (QA) program. The objective of this work is to study the use of a multilayer Faraday cup (MLFC) to perform a quick and precise daily range verification of proton beams produced by a synchrotron. Proton beam depth dose measurements were performed at room iso-center in water using PTW water tank and Bragg Peak ion chamber. The IBA Giraffe, calibrated against the water tank data, was used to measure the water equivalent thickness (WET) of the sample copper plates. The WET measurements provided the range calibration factors for the MLFC. To establish a baseline for in room measurements, range measurements for energies from 70 to 250MeV in steps of 10MeV were performed using the Pyramid MLFC at room iso-center. For the daily range verification measurements, the MLFC is permanently placed at the end of the beam line, inside the accelerator vault.The daily range constancy is performed for five representative beam energies; namely 70, 100, 150, 200, and 250MeV. Data collected over a period of more than 100days are analyzed and presented. The measured WET values of the copper plates increased with increasing energy. The centroid channel number in the MLFC where the protons stop, was converted to depth in water and compared to the depth of the distal 80% (d80) obtained from the water tank measurements. The depths agreed to within 2mm, with the maximum deviation of 1.97mm observed for 250MeV beam. The daily variation in the ranges measured by the MLFC was within ±0.5mm. The total time to verify five proton beam ranges varies between 4 and 5min. Based on the result of our measurements, the MLFC can be used for a daily range constancy check with submillimeter accuracy. It is a quick and simple method to perform range constancy verification on a daily basis.

Characterization of X-Ray Acoustic Computed Tomography for Applications in Radiotherapy Dosimetry

X-ray acoustic computed tomography (XACT) is an emerging imaging modality that detects the acoustic waves induced in an object following irradiation by X-ray photons to form an image related to the energy deposited within the object. Since XACT images can be related to deposited radiation dose, it has been proposed that XACT could be an effective dosimetry tool for radiation therapy applications. This paper aims to systematically characterize XACT to determine its capabilities as a radiation dosimetry technique. XACT images of acceptable signal to noise ratio were formed at a dose level as low as 11.6 mGy, and the spatial resolution of XACT was measured to be 6 mm at a linac pulse duration of $4 ~{\mu }\text{s}$ . XACT was experimentally shown to be energy and dose rate independent. Changes in field size of 4 mm, field location of 2 mm, and field magnitude of 3% were detectable with XACT. The characteristics of XACT images were observed to be very sensitive to the transducer used for signal detection. Based on these results, XACT has the potential to be a powerful dosimetry tool for radiotherapy dosimetry applications, specifically for relative water tank measurements.

Acoustic-based proton range verification in heterogeneous tissue: simulation studies

Acoustic-based proton range verification (protoacoustics) is a potential in vivo technique for determining the Bragg peak position. Previous measurements and simulations have been restricted to homogeneous water tanks. Here, a CT-based simulation method is proposed and applied to a liver and prostate case to model the effects of tissue heterogeneity on the protoacoustic amplitude and time-of-flight range verification accuracy. For the liver case, posterior irradiation with a single proton pencil beam was simulated for detectors placed on the skin. In the prostate case, a transrectal probe measured the protoacoustic pressure generated by irradiation with five separate anterior proton beams. After calculating the proton beam dose deposition, each CT voxel’s material properties were mapped based on Hounsfield Unit values, and thermoacoustically-generated acoustic wave propagation was simulated with the k-Wave MATLAB toolbox. By comparing the simulation results for the original liver CT to homogenized variants, the effects of heterogeneity were assessed. For the liver case, 1.4 cGy of dose at the Bragg peak generated 50 mPa of pressure (13 cm distal), a 2× lower amplitude than simulated in a homogeneous water tank. Protoacoustic triangulation of the Bragg peak based on multiple detector measurements resulted in 0.4 mm accuracy for a δ-function proton pulse irradiation of the liver. For the prostate case, higher amplitudes are simulated (92–1004 mPa) for closer detectors (<8 cm). For four of the prostate beams, the protoacoustic range triangulation was accurate to ⩽1.6 mm (δ-function proton pulse). Based on the results, application of protoacoustic range verification to heterogeneous tissue will result in decreased signal amplitudes relative to homogeneous water tank measurements, but accurate range verification is still expected to be possible.

An iterative fullwave simulation approach to multiple scattering in media with randomly distributed microbubbles

Ultrasound contrast agents (UCA), such as microbubbles, enhance the scattering properties of blood, which is otherwise hypoechoic. The multiple scattering interactions of the acoustic field with UCA are poorly understood due to the complexity of the multiple scattering theories and the nonlinear microbubble response. The majority of bubble models describe the behavior of UCA as single, isolated microbubbles suspended in infinite medium. Multiple scattering models such as the independent scattering approximation can approximate phase velocity and attenuation for low scatterer volume fractions. However, all current models and simulation approaches only describe multiple scattering and nonlinear bubble dynamics separately. Here we present an approach that combines two existing models: (1) a full-wave model that describes nonlinear propagation and scattering interactions in a heterogeneous attenuating medium and (2) a Paul–Sarkar model that describes the nonlinear interactions between an acoustic field and microbubbles. These two models were solved numerically and combined with an iterative approach. The convergence of this combined model was explored in silico for 0.5 × 106 microbubbles ml−1, 1% and 2% bubble concentration by volume. The backscattering predicted by our modeling approach was verified experimentally with water tank measurements performed with a 128-element linear array transducer. An excellent agreement in terms of the fundamental and harmonic acoustic fields is shown. Additionally, our model correctly predicts the phase velocity and attenuation measured using through transmission and predicted by the independent scattering approximation.

The Australian MRI-Linac Program: measuring profiles and PDD in a horizontal beam

The Australian MRI-Linac consists of a fixed horizontal photon beam combined with a MRI. Commissioning required PDD and profiles measured in a horizontal set-up using a combination of water tank measurements and gafchromic film. To validate the methodology, measurements were performed comparing PDD and profiles measured with the gantry angle set to 0 and 90° on a conventional linac. Results showed agreement to within 2.0% for PDD measured using both film and the water tank at gantry 90° relative to PDD acquired using gantry 0°. Profiles acquired using a water tank at both gantry 0 and 90° showed agreement in FWHM to within 1 mm. The agreement for both PDD and profiles measured at gantry 90° relative to gantry 0° curves indicates that the methodology described can be used to acquire the necessary beam data for horizontal beam lines and in particular, commissioning the Australian MRI-linac.

Implementation of the validation testing in MPPG 5.a "Commissioning and QA of treatment planning dose calculations-megavoltage photon and electron beams".

The AAPM Medical Physics Practice Guideline (MPPG) 5.a provides concise guidance on the commissioning and QA of beam modeling and dose calculation in radiotherapy treatment planning systems. This work discusses the implementation of the validation testing recommended in MPPG 5.a at two institutions. The two institutions worked collaboratively to create a common set of treatment fields and analysis tools to deliver and analyze the validation tests. This included the development of a novel, open‐source software tool to compare scanning water tank measurements to 3D DICOM‐RT Dose distributions. Dose calculation algorithms in both Pinnacle and Eclipse were tested with MPPG 5.a to validate the modeling of Varian TrueBeam linear accelerators. The validation process resulted in more than 200 water tank scans and more than 50 point measurements per institution, each of which was compared to a dose calculation from the institution's treatment planning system (TPS). Overall, the validation testing recommended in MPPG 5.a took approximately 79 person‐hours for a machine with four photon and five electron energies for a single TPS. Of the 79 person‐hours, 26 person‐hours required time on the machine, and the remainder involved preparation and analysis. The basic photon, electron, and heterogeneity correction tests were evaluated with the tolerances in MPPG 5.a, and the tolerances were met for all tests. The MPPG 5.a evaluation criteria were used to assess the small field and IMRT/VMAT validation tests. Both institutions found the use of MPPG 5.a to be a valuable resource during the commissioning process. The validation testing in MPPG 5.a showed the strengths and limitations of the TPS models. In addition, the data collected during the validation testing is useful for routine QA of the TPS, validation of software upgrades, and commissioning of new algorithms.

Monte-Carlo dose computation in radiotherapy for lung at very low density

Introduction In radiotherapy, some limits of clinical algorithms have been reported in low-density media. When patients are treated during Deep Inspiration Breath-Hold (DIBH), lung density can fall to 0.12 g cm -3 . Dose calculation with this density must be validated, in particular for Stereotactic (SBRT) treatment. Purpose We propose to evaluate the performance of Eclipse AAA algorithm (Varian) for SBRT lung treatment (including DIBH) with a Monte-Carlo model based on Geant4 (GATE). GATE, AAA, and measurements are compared for several phantoms including lung heterogeneity. Materials and methods A model of the Varian TrueBeam head was modelled using GATE. Validation was achieved by comparison with measurements in a homogeneous water tank for several static fields (3 × 3 to 20 × 20 cm 2 ) and four beam modes. Subsequently, two phantoms containing layers of water-equivalent material and cork, corresponding to lung during DIBH and free-breathing (0.12 and 0.24 g cm -3 ) were implemented. Percentage depth dose (PDD) measured by Gafchromic films were compared to AAA and GATE. Results GATE model showed a good agreement with experimental results in homogeneous and heterogeneous phantoms. In lung heterogeneity, differences were observed between AAA and measurements. These differences are higher for small fields and lower densities. As an example, for 3 × 3 field size inside a 0.12 g cm -3 heterogeneity, relative dose was 56,6%, 56,2%, and 67,5% for measurement, GATE, and AAA respectively. Conclusions In very low densities, AAA quickly showed its limits while GATE was validated. This will allow us to perform calculations for non measurable cases, for example patients treated by SBRT during DIBH.