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

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

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

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

Dosimetric and Monte Carlo verification of jaws-only IMRT plans calculated by the Collapsed Cone Convolution algorithm for head and neck cancers

AimThe aim of this study is to verify the Prowess Panther jaws-only intensity modulated radiation therapy (JO-IMRT) treatment planning (TP) by comparing the TP dose distributions for head-and-neck (H&N) cancer with the ones simulated by Monte Carlo (MC).BackgroundTo date, dose distributions planned using JO-IMRT for H&N patients were found superior to the corresponding three-dimensional conformal radiotherapy (3D-CRT) plans. Dosimetry of the JO-IMRT plans were also experimentally verified using an ionization chamber, MapCHECK 2, and Octavius 4D and good agreements were shown.Materials and methodsDose distributions of 15 JO-IMRT plans of nasopharyngeal patients were recalculated using the EGSnrc Monte Carlo code. The clinical photon beams were simulated using the BEAMnrc. The absorbed dose to patients treated by fixed-field IMRT was computed using the DOSXYZnrc. The simulated dose distributions were then compared with the ones calculated by the Collapsed Cone Convolution (CCC) algorithm on the TPS, using the relative dose error comparison and the gamma index using global methods implemented in PTW-VeriSoft with 3%/3 mm, 2%/2 mm, 1%/1 mm criteria.ResultsThere is a good agreement between the MC and TPS dose. The average gamma passing rates were 93.3 ± 3.1%, 92.8 ± 3.2%, 92.4 ± 3.4% based on the 3%/3 mm, 2%/2 mm, 1%/1 mm criteria, respectively.ConclusionsAccording to the results, it is concluded that the CCC algorithm was adequate for most of the IMRT H&N cases where the target was not immediately adjacent to the critical structures.     

Dosimetric comparison between two MLC systems commonly used for stereotactic radiosurgery and radiotherapy: A Monte Carlo and experimental study

In this work dosimetric parameters of two multi-leaf collimator (MLC) systems, namely the beam modulator (BM), which is the MLC commercial name for Elekta “Synergy S” linear accelerator and Radionics micro-MLC (MMLC), are compared using measurements and Monte Carlo simulations. Dosimetric parameters, such as percentage depth doses (PDDs), in-plane and cross-plane dose profiles, and penumbras for different depths and field sizes of the 6 MV photon beams were measured using ionization chamber and a water tank. The collimator leakages were measured using radiographic films. MMLC and BM were modeled using the EGSnrc-based BEAMnrc Monte Carlo code and above dosimetric parameters were calculated. The energy fluence spectra for the two MLCs were also determined using the BEAMnrc and BEAMDP. Dosimetric parameters of the two MLCs were similar, except for penumbras. Leaf-side and leaf-end 80–20% dose penumbras at 10 cm depth for a 10 × 10 cm² field size were 4.8 and 5.1 mm for MMLC and 5.3 mm and 6.3 mm for BM, respectively. Both Radionics MMLC and Elekta BM can be used effectively based on their dosimetric characteristics for stereotactic radiosurgery and radiotherapy, although the former showed slightly sharper dose penumbra especially in the leaf-end direction.     

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

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

EGSnrc application for IMRT planning

The aim of this study was to describe a detailed instruction of intensity modulated radiotherapy (IMRT) planning simulation using BEAMnrc-DOSXYZnrc code system (EGSnrc package) and present a new graphical user interface based on MATLAB code (The MathWorks) to combine more than one. 3ddose file which were obtained from the IMRT plan.This study was performed in four phases: the commissioning of Varian Clinac iX6 MV, the simulation of IMRT planning in EGSnrc, the creation of in-house VDOSE GUI, and the analysis of the isodose contour and dose volume histogram (DVH) curve from several beam angles. The plan paramaters in sequence and control point files were extracted from the planning data in Tan Tock Seng Hospital Singapore (multileaf collimator (MLC) leaf positions – bank A and bank B, gantry angles, coordinate of isocenters, and MU indexes).VDOSE GUI which was created in this study can display the distribution dose curve in each slice and beam angle. Dose distributions from various MLC settings and beam angles yield different dose distributions even though they used the same number of simulated particles. This was due to the differences in the MLC leaf openings in every field. The value of the relative dose error between the two dose ditributions for “body” was 51.23 %. The Monte Carlo (MC) data was normalized with the maximum dose but the analytical anisotropic algorithm (AAA) data was normalized by the dose in the isocenter.In this study, we have presented a Monte Carlo simulation framework for IMRT dose calculation using DOSXYZnrc source 21. Further studies are needed in conducting IMRT simulations using EGSnrc to minimize the different dose error and dose volume histogram deviation.     

Evaluation of PENFAST – A fast Monte Carlo code for dose calculations in photon and electron radiotherapy treatment planning

The aim of the present study is to demonstrate the potential of accelerated dose calculations, using the fast Monte Carlo (MC) code referred to as PENFAST, rather than the conventional MC code PENELOPE, without losing accuracy in the computed dose. For this purpose, experimental measurements of dose distributions in homogeneous and inhomogeneous phantoms were compared with simulated results using both PENELOPE and PENFAST. The simulations and experiments were performed using a Saturne 43 linac operated at 12 MV (photons), and at 18 MeV (electrons). Pre-calculated phase space files (PSFs) were used as input data to both the PENELOPE and PENFAST dose simulations. Since depth–dose and dose profile comparisons between simulations and measurements in water were found to be in good agreement (within ±1% to 1 mm), the PSF calculation is considered to have been validated. In addition, measured dose distributions were compared to simulated results in a set of clinically relevant, inhomogeneous phantoms, consisting of lung and bone heterogeneities in a water tank. In general, the PENFAST results agree to within a 1% to 1 mm difference with those produced by PENELOPE, and to within a 2% to 2 mm difference with measured values. Our study thus provides a pre-clinical validation of the PENFAST code. It also demonstrates that PENFAST provides accurate results for both photon and electron beams, equivalent to those obtained with PENELOPE. CPU time comparisons between both MC codes show that PENFAST is generally about 9–21 times faster than PENELOPE.     

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

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

Validation of XiO Electron Monte Carlo-based calculations by measurements in a homogeneous phantom and by EGSnrc calculations in a heterogeneous phantom

The purpose of the present study is to perform a clinical validation of a new commercial Monte Carlo (MC) based treatment planning system (TPS) for electron beams, i.e. the XiO 4.60 electron MC (XiO eMC). Firstly, MC models for electron beams (4, 8, 12 and 18 MeV) have been simulated using BEAMnrc user code and validated by measurements in a homogeneous water phantom. Secondly, these BEAMnrc models have been set as the reference tool to evaluate the ability of XiO eMC to reproduce dose perturbations in the heterogeneous phantom. In the homogeneous phantom calculations, differences between MC computations (BEAMnrc, XiO eMC) and measurements are less than 2% in the homogeneous dose regions and less than 1 mm shifting in the high dose gradient regions. As for the heterogeneous phantom, the accuracy of XiO eMC has been benchmarked with predicted BEAMnrc models. In the lung tissue, the overall agreement between the two schemes lies under 2.5% for the most tested dose distributions at 8, 12 and 18 MeV and is better than the 4 MeV one. In the non-lung tissue, a good agreement has been found between BEAMnrc simulation and XiO eMC computation for 8, 12 and 18 MeV. Results are worse in the case of 4 MeV calculations (discrepancies ≈ 4%). XiO eMC can predict dose perturbation induced by high-density heterogeneities for 8, 12 and 18 MeV. However, significant deviations found in the case of 4 MeV demonstrate that caution is necessary in using XiO eMC at lower electron energies.     

A review on photoneutrons characteristics in radiation therapy with high-energy photon beams

In radiation therapy with high-energy photon beams (E > 10 MeV) neutrons are generated mainly in linacs head thorough (γ,n) interactions of photons with nuclei of high atomic number materials that constitute the linac head and the beam collimation system. These neutrons affect the shielding requirements in radiation therapy rooms and also increase the out-of-field radiation dose of patients undergoing radiation therapy with high-energy photon beams. In the current review, the authors describe the factors influencing the neutron production for different medical linacs based on the performed measurements and Monte Carlo studies in the literature.     

Clinical implementation of a Monte Carlo based treatment plan QA platform for validation of Cyberknife and Tomotherapy treatments

PurposeThe main focus of the current paper is the clinical implementation of a Monte Carlo based platform for treatment plan validation for Tomotherapy and Cyberknife, without adding additional tasks to the dosimetry department.MethodsThe Monte Carlo platform consists of C++ classes for the actual functionality and a web based GUI that allows accessing the system using a web browser. Calculations are based on BEAMnrc/DOSXYZnrc and/or GATE and are performed automatically after exporting the dicom data from the treatment planning system. For Cyberknife treatments of moving targets, the log files saved during the treatment (position of robot, internal fiducials and external markers) can be used in combination with the 4D planning CT to reconstruct the actually delivered dose. The Monte Carlo platform is also used for calculation on MRI images, using pseudo-CT conversion.ResultsFor Tomotherapy treatments we obtain an excellent agreement (within 2%) for almost all cases. However, we have been able to detect a problem regarding the CT Hounsfield units definition of the Toshiba Large Bore CT when using a large reconstruction diameter. For Cyberknife treatments we obtain an excellent agreement with the Monte Carlo algorithm of the treatment planning system. For some extreme cases, when treating small lung lesions in low density lung tissue, small differences are obtained due to the different cut-off energy of the secondary electrons.ConclusionsA Monte Carlo based treatment plan validation tool has successfully been implemented in clinical routine and is used to systematically validate all Cyberknife and Tomotherapy plans.     

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

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

Study of dental prostheses influence in radiation therapy

Dental prostheses made of high density material contribute to modify dose distribution in head and neck cancer treatment. Our objective is to quantify dose perturbation due to high density inhomogeneity with experimental measurements and Monte Carlo simulations.Firstly, measurements were carried in a phantom representing a human jaw with thermoluminescent detectors (GR200A) and EBT2 Gafchromic films in the vicinity of three samples: a healthy tooth, a tooth with amalgam and a Ni–Cr crown, irradiated in clinical configuration. Secondly, Monte Carlo simulations (BEAMnrc code) were assessed in an identical configuration.Experimental measurements and simulation results confirm the two well-known phenomena: firstly the passage from a low density medium to a high density medium induces backscattered electrons causing a dose increase at the interface, and secondly, the passage from a high density medium to a low density medium creates a dose decrease near the interface. So, the results show a 1.4% and 23.8% backscatter dose rise and attenuation after sample of 26.7% and 10.9% respectively for tooth with amalgam and crown compared to the healthy tooth.Although a tooth with amalgam has a density of about 12–13, the changes generated are not significant. However, the results for crown (density of 8) are very significant and the discordance observed may be due to calculation point size difference 0.8 mm and 0.25 mm respectively for TLD and Monte Carlo. The use of Monte Carlo simulations and experimental measurements provides objective evidence to evaluate treatment planning system results with metal dental prostheses.     

Monte Carlo study of the influence of energy spectra,mesh size,high Z element on dose and PVDR based on 1-D and 3-D heterogeneous mouse head phantom for Microbeam Radiation Therapy

PurposeTo evaluate the influence of energy spectra, mesh sizes, high Z element on dose and PVDR in Microbeam Radiation Therapy (MRT) based on 1-D analogy-mouse-head-model (1-D MHM) and 3-D voxel-mouse-head-phantom (3-D VMHP) by Monte Carlo simulation.MethodsA Microbeam-Array-Source-Model was implemented into EGSnrc/DOSXYZnrc. The microbeam size is assumed to be 25 μm, 50 μm or 75 μm in thickness and fixed 1 mm in height with 200 μm c-t-c. The influence of the energy spectra of ID17@ESRF and BMIT@CLS were investigated. The mesh size was optimized. PVDR in 1-D MHM and 3-D VMHP was compared with the homogeneous water phantom. The arc influence of 3-D VMHP filled with water (3-D VMHWP) was compared with the rectangle phantom.ResultsPVDR of the lower BMIT@CLS spectrum is 2.4 times that of ID17@ESRF for lower valley dose. The optimized mesh is 5 µm for 25 µm, and 10 µm for 50 µm and 75 µm microbeams with 200 µm c-t-c. A 500 μm skull layer could make PVDR difference up to 62.5% for 1-D MHM. However this influence is limited (<5%) for the farther homogeneous media (e.g. 600 µm). The peak dose uniformity of 3-D VMHP at the same depth could be up to 8% for 1.85 mm × 1 mm irradiation field, whereas that of 3-D VMHWP is <1%. The high Z element makes the dose uniformity enhance in target. The surface arc could affect the superficial PVDR (from 44% to 21% in 0.2 mm depth), whereas this influence is limited for the more depth (<1%).ConclusionAn accurate MRT dose calculation algorithm should include the influence of 3-D heterogeneous media.     

Monte Carlo dose verification of VMAT treatment plans using Elekta Agility 160-leaf MLC

In this study, we verified volumetric modulated arc therapy (VMAT) plans in an Elekta Synergy system with an integrated Agility 160-leaf multileaf collimator (MLC) by comparing them with Monte Carlo (MC)-calculated dose distributions using the AAPM TG-119 structure sets. The head configuration of the linear accelerator with the integrated MLC was simulated with the EGSnrc/BEAMnrc code. Firstly, the dosimetric properties of the MLC were evaluated with the MC technique and film measurements. Next, VMAT plans were created with the Pinnacle³ treatment planning system (TPS) for four regions in the AAPM TG-119 structures. They were then verified by comparing them with MC-calculated dose distributions using dose volume histograms (DVHs) and three-dimensional (3D) gamma analysis. The MC simulations for the Agility MLC dosimetric properties were in acceptable agreement with measurements. TPS-VMAT plans using TG-119 structure sets agreed with MC dose distributions within 2% in the comparison of D₉₅ in planning target volumes (PTVs) evaluated from DVHs. In contrast, higher dose regions such as D₂₀, D₁₀, and D₅ in PTVs for TPS tended to be smaller than MC values. This tendency was particularly noticeable for mock head and neck with complicated structures. In 3D gamma analysis, the passing rates with 3%/3mm criteria in PTVs were ≥99%, except for mock head and neck (89.5%). All passing rates for organs at risk (OARs) were in acceptable agreement of >96%. It is useful to verify dose distributions of PTVs and OARs in TPS-VMAT plans by using MC dose calculations and 3D gamma analysis.     

Development of an x-ray-opaque-marker system for quantitative phantom positioning in patient-specific quality assurance

PurposeWe developed an x-ray-opaque-marker (XOM) system with inserted fiducial markers for patient-specific quality assurance (QA) in CyberKnife (Accuray) and a general-purpose linear accelerator (linac). The XOM system can be easily inserted or removed from the existing patient-specific QA phantom. Our study aimed to assess the utility of the XOM system by evaluating the recognition accuracy of the phantom position error and estimating the dose perturbation around a marker.MethodsThe recognition accuracy of the phantom position error was evaluated by comparing the known error values of the phantom position with the values measured by matching the images with target locating system (TLS; Accuray) and on-board imager (OBI; Varian). The dose perturbation was evaluated for 6 and 10 MV single-photon beams through experimental measurements and Monte Carlo simulations.ResultsThe root mean squares (RMSs) of the residual position errors for the recognition accuracy evaluation in translations were 0.07 mm with TLS and 0.30 mm with OBI, and those in rotations were 0.13° with TLS and 0.15° with OBI. The dose perturbation was observed within 1.5 mm for 6 MV and 2.0 mm for 10 MV from the marker.ConclusionsSufficient recognition accuracy of the phantom position error was achieved using our system. It is unnecessary to consider the dose perturbation in actual patient-specific QA. We concluded that the XOM system can be utilized to ensure quantitative and accurate phantom positioning in patient-specific QA with CyberKnife and a general-purpose linac.     

Beam quality and dose perturbation of 6 MV flattening-filter-free linac

The aim of this study is twofold: (a) determination of the spectral differences for flattening-filter-free (FFF) versus standard (STD) linac under various clinical conditions, (b) based on an extensive list of clinically important beam configurations, identification of clinical scenarios that lead to higher macroscopic dose perturbations due to the presence of high-Z material. The focus is on dose enhancement due to contrast agents including high-Z elements such as gold or gadolinium.EGSnrc was used to simulate clinical beams under various irradiation conditions: open/IMRT/spit-IMRT fields, in/out-off-field areas, different depths and field sizes. Spectra were calculated and analyzed for about 80 beams and for a total of 480 regions. Quantitative differential effects in beam quality were characterized using energy-dependent and cumulative dose perturbation metrics.Analysis of the spectral database showed that even though the general trends for both linacs (FFF/STD) were the same, there were crucial differences. In general, the relative changes between different conditions were smaller for FFF spectra. This was because of the higher component of low-energy photons of the FFF linac, which already lead to higher dose enhancement than for the STD linac (photon energies were more “uniformly” distributed for FFF spectra and henceforth their perturbation resulted in lesser relative changes). For out-of-field FFF spectra and split-IMRT fields the strongest enhancement were observed (∼25 and ∼5 respectively). Different spectral scenarios lead to different dose enhancements, however, they scale with the higher effective-Z of the materials and were directly related to the lower range of the spectra (<200 keV).     

Investigation of the dose distribution for a cone beam CT system dedicated to breast imaging

Cone-beam breast Computed Tomography (bCT) is an X-ray imaging technique for breast cancer diagnosis, in principle capable of delivering a much more homogeneous dose spatial pattern to the breast volume than conventional mammography, at dose levels comparable to two-view mammography. We present an investigation of the three-dimensional dose distribution for a cone-beam CT system dedicated to breast imaging. We employed Monte Carlo simulations for estimating the dose deposited within a breast phantom having a hemiellipsoidal shape placed on a cylinder of 3.5 cm thickness that simulates the chest wall. This phantom represents a pendulant breast in a bCT exam with the average diameter at chest wall, assumed to correspond to a 5-cm-thick compressed breast in mammography. The phantom is irradiated in a circular orbit with an X-ray cone beam selected from four different techniques: 50, 60, 70, and 80 kVp from a tube with tungsten anode, 1.8 mm Al inherent filtration and additional filtration of 0.2 mm Cu. Using the Monte Carlo code GEANT4 we simulated a system similar to the experimental apparatus available in our lab. Simulations were performed at a constant free-in-air air kerma at the isocenter (1 μGy); the corresponding total number of photon histories per scan was 288 million at 80 kVp. We found that the more energetic beams provide a more uniform dose distribution than at low energy: the 50 kVp beam presents a frequency distribution of absorbed dose values with a coefficient of variation almost double than that for the 80 kVp beam. This is confirmed by the analysis of the relative dose profiles along the radial (i.e. parallel to the “chest wall”) and longitudinal (i.e. from “chest wall” to “nipple”) directions. Maximum radial deviations are on the order of 25% for the 80 kVp beam, whereas for the 50 kVp beam variations around 43% were observed, with the lowest dose values being found along the central longitudinal axis of the phantom.     

The use of non-standard CT conversion ramps for Monte Carlo verification of 6 MV prostate IMRT plans

Monte Carlo (MC) dose calculation algorithms have been widely used to verify the accuracy of intensity-modulated radiotherapy (IMRT) dose distributions computed by conventional algorithms due to the ability to precisely account for the effects of tissue inhomogeneities and multileaf collimator characteristics. Both algorithms present, however, a particular difference in terms of dose calculation and report. Whereas dose from conventional methods is traditionally computed and reported as the water-equivalent dose (D_w), MC dose algorithms calculate and report dose to medium (D_m). In order to compare consistently both methods, the conversion of MC D_m into D_w is therefore necessary.This study aims to assess the effect of applying the conversion of MC-based D_m distributions to D_w for prostate IMRT plans generated for 6 MV photon beams. MC phantoms were created from the patient CT images using three different ramps to convert CT numbers into material and mass density: a conventional four material ramp (CTCREATE) and two simplified CT conversion ramps: (1) air and water with variable densities and (2) air and water with unit density. MC simulations were performed using the BEAMnrc code for the treatment head simulation and the DOSXYZnrc code for the patient dose calculation. The conversion of D_m to D_w by scaling with the stopping power ratios of water to medium was also performed in a post-MC calculation process.The comparison of MC dose distributions calculated in conventional and simplified (water with variable densities) phantoms showed that the effect of material composition on dose-volume histograms (DVH) was less than 1% for soft tissue and about 2.5% near and inside bone structures. The effect of material density on DVH was less than 1% for all tissues through the comparison of MC distributions performed in the two simplified phantoms considering water. Additionally, MC dose distributions were compared with the predictions from an Eclipse treatment planning system (TPS), which employed a pencil beam convolution (PBC) algorithm with Modified Batho Power Law heterogeneity correction. Eclipse PBC and MC calculations (conventional and simplified phantoms) agreed well (<1%) for soft tissues. For femoral heads, differences up to 3% were observed between the DVH for Eclipse PBC and MC calculated in conventional phantoms. The use of the CT conversion ramp of water with variable densities for MC simulations showed no dose discrepancies (0.5%) with the PBC algorithm. Moreover, converting D_m to D_w using mass stopping power ratios resulted in a significant shift (up to 6%) in the DVH for the femoral heads compared to the Eclipse PBC one.Our results show that, for prostate IMRT plans delivered with 6 MV photon beams, no conversion of MC dose from medium to water using stopping power ratio is needed. In contrast, MC dose calculations using water with variable density may be a simple way to solve the problem found using the dose conversion method based on the stopping power ratio.