Validation of an electron Monte Carlo dose calculation algorithm in the presence of heterogeneities using EGSnrc and radiochromic film measurements

Using PRIMO to determine the initial beam parameters of Elekta Synergy linac for electron beam energies of 6, 9, 12, and 15 MeV

Background

The purpose of this research was to establish the primary electron beam characteristics for an Elekta Synergy linear accelerator. In this task, we take advantage of the PRIMO Monte Carlo software, where the model developed contains the majority of the component materials of the Linac.

Materials and methods

For all energies, the Elekta Linac electron mode and 14 × 14 cm2 applicator were chosen. To obtain percentage depth dose (PDD) curves, a homogeneous water phantom was voxelized in a 1 × 1 × 0.1 cm3 grid along the central axis. At the reference depth, the dose profile was recorded in 0.1 × 1 × 1 cm3 voxels. Iterative changes were made to the initial beams mean energy and full width at half maximum (FWHM) of energy in order to keep the conformity of the simulated and measured dose curves within. To confirm simulation results, the Gamma analysis was performed with acceptance criteria of 2 mm - 2%. From the validated calculation, the parameters of the PDD and profile curve (R100, R50, Rp, and field size) were collected.

Results

Initial mean energies of 7.3, 9.85, 12.9, and 15.7 MeV were obtained for nominal energies of 6, 9, 12, and 15, respectively. The PRIMO Monte Carlo model for Elekta Synergy was precisely validated.

Conclusions

PRIMO is an easy-to-use software program that can calculate dose distribution in water phantoms.

CT-less electron radiotherapy simulation and planning with a consumer 3D camera

Purpose

Electron radiation therapy dose distributions are affected by irregular body surface contours. This study investigates the feasibility of three‐dimensional (3D) cameras to substitute for the treatment planning computerized tomography (CT) scan by capturing the body surfaces to be treated for accurate electron beam dosimetry.

Methods

Dosimetry was compared for six electron beam treatments to the nose, toe, eye, and scalp using full CT scan, CT scan with Hounsfield Unit (HU) overridden to water (mimic 3D camera cases), and flat‐phantom techniques. Radiation dose was prescribed to a depth on the central axis per physician’s order, and the monitor units (MUs) were calculated. The 3D camera spatial accuracy was evaluated by comparing the 3D surface of a head phantom captured by a 3D camera and that generated with the CT scan in the treatment planning system. A clinical case is presented, and MUs were calculated using the 3D camera body contour with HU overridden to water.

Results

Across six cases the average change in MUs between the full CT and the 3Dwater (CT scan with HU overridden to water) calculations was 1.3% with a standard deviation of 1.0%. The corresponding hotspots had a mean difference of 0.4% and a standard deviation of 1.9%. The 3D camera captured surface of a head phantom was found to have a 0.59 mm standard deviation from the surface derived from the CT scan. In‐vivo dose measurements (213 ± 8 cGy) agreed with the 3D‐camera planned dose of 209 ± 6 cGy, compared to 192 ± 6 cGy for the flat‐phantom calculation (same MUs).

Conclusions

Electron beam dosimetry is affected by irregular body surfaces. 3D cameras can capture irregular body contours which allow accurate dosimetry of electron beam treatment as an alternative to costly CT scans with no extra exposure to radiation. Tools and workflow for clinical implementation are provided.

Validation of a Monte Carlo model for multi leaf collimator based electron delivery

PURPOSE

To develop and validate a Monte Carlo model of the Varian TrueBeam to study electron collimation using the existing photon multi-leaf collimators (pMLC), instead of conventional electron applicators and apertures.

MATERIALS AND METHODS

A complete Monte Carlo model of the Varian TrueBeam was developed using Tool for particle simulation (TOPAS) (version 3.1.p3). Vendor-supplied information was used to model the treatment head components and the source parameters. A phase space plane was setup above the collimating jaws and captured particles were reused until a statistical uncertainty of 1% was achieved in the central axis. Electron energies 6, 9, 12, 16, and 20 MeV with a jaw-defined field of 20 × 20 cm² at iso-center, pMLC-defined fields of 6.8 × 6.8 cm² and 11.4 × 11.4 cm² at 80 cm source-to-surface distance (SSD) and an applicator-defined field of 10 × 10 cm² at iso-center were evaluated. All the measurements except the applicator-defined fields were measured using an ionization chamber in a water tank using 80 cm SSD. The dose difference, distance-to-agreement and gamma index were used to evaluate the agreement between the Monte Carlo calculations and measurements. Contributions of electron scattering off pMLC leaves and inter-leaf leakage on dose profiles were evaluated and compared with Monte Carlo calculations. Electron transport through a heterogeneous phantom was simulated and the resulting dose distributions were compared with film measurements. The validated Monte Carlo model was used to simulate several clinically motivated cases to demonstrate the benefit of pMLC-based electron delivery compared to applicator-based electron delivery.

RESULTS

Calculated and measured percentage depth-dose (PDD) curves agree within 2% after normalization. The agreement between normalized percentage depth dose curves were evaluated using one-dimensional gamma analysis with a local tolerance of 2%/1 mm and the %points passing gamma criteria was 100% for all energies. For jaw-defined fields, calculated profiles agree with measurements with pass rates of >97% for 2%/2 mm gamma criteria. Calculated FWHM and penumbra width agree with measurements within 0.4 cm. For fields with tertiary collimation using an pMLC or applicator, the average gamma pass rate of compared profiles was 98% with 2%/2 mm gamma criteria. The profiles measured to evaluate the pMLC leaf scattering agreed with Monte Carlo calculations with an average gamma pass rate of 96.5% with 3%/2 mm gamma criteria. Measured dose profiles below the heterogenous phantom agreed well with calculated profiles and matched within 2.5% for most points. The calculated clinically applicable cases using TOPAS MC and Eclipse TPS for single enface electron beam, electron-photon mixed beam and a matched electron-electron beam exhibited a reasonable agreement in PDDs, profiles and dose volume histograms.

CONCLUSION

We present a validation of a Monte Carlo model of Varian TrueBeam for pMLC-based electron delivery. Monte Carlo calculations agreed with measurements satisfying gamma criterion of 1%/1 mm for depth dose curves and 2%/1 mm for dose profiles. The simulation of clinically applicable cases demonstrated the clinical utility of pMLC-based electrons and the use of MC simulations for development of advanced radiation therapy techniques.

Secondary monitor unit calculations for VMAT using parallelized Monte Carlo simulations

We have developed a fast and accurate in‐house Monte Carlo (MC) secondary monitor unit (MU) check method, based on the EGSnrc system, for independent verification of volumetric modulated arc therapy (VMAT) treatment planning system dose calculations, in accordance with TG‐114 recommendations. For a VMAT treatment plan created for a Varian Trilogy linac, DICOM information was exported from Eclipse. An open‐source platform was used to generate input files for dose calculations using the EGSnrc framework. The full VMAT plan simulation employed 10⁷ histories, and was parallelized to run on a computer cluster. The resulting 3ddose matrices were converted to the DICOM format using CERR and imported into Eclipse. The method was evaluated using 35 clinical VMAT plans of various treatment sites. For each plan, the doses calculated with the MC approach at four three‐dimensional reference points were compared to the corresponding Eclipse calculations, as well as calculations performed using the clinical software package, MUCheck. Each MC arc simulation of 10⁷ particles required 13–25 min of total time, including processing and calculation. The average discrepancies in calculated dose values between the MC method and Eclipse were 2.03% (compared to 3.43% for MUCheck) for prostate cases, 2.45% (3.22% for MUCheck) for head and neck cases, 1.7% (5.51% for MUCheck) for brain cases, and 2.84% (5.64% for MUCheck) for miscellaneous cases. Of 276 comparisons, 201 showed greater agreement between the treatment planning system and MC vs MUCheck. The largest discrepancies between MC and MUCheck were found in regions of high dose gradients and heterogeneous densities. By parallelizing the calculations, point‐dose accuracies of 2‐7%, sufficient for clinical secondary checks, can be achieved in a reasonable amount of time. As computer clusters and/or cloud computing become more widespread, this method will be useful in most clinical setups.

Comprehensive evaluation of electron radiation dose using beryllium oxide dosimeters at breast radiotherapy

Introduction:

In this study, the differences between calculated and measured dose values were then analysed to assess the performance, in terms of accuracy, of the tested treatment planning system (TPS) algorithms applied to calculate electron beam dose targeted and non-targeted the breast region.

Materials and methods:

The beryllium oxide (BeO) dosimeters placed on the female RANDO phantom were irradiated 12 MeV electron energy with medical linear accelerator and repeatedly read in the Risø thermoluminescence (TL)/optically stimulated luminescence (OSL) system via OSL method at least three times.

Results:

For electron treatment, one made quantitative comparisons of the dose distributions calculated by TPSs with those from the measurements by OSL at various points in the RANDO phantom.

The mean dose measured from the dosimeters placed on the female RANDO phantom target left breast region was 160 cGy and non-target right breast region was 1·2 cGy. Analysis of Generalised Gaussian Pencil Beam (GGPB) and Electron Monte Carlo (eMC) algorithms for determined region mean point dose values, respectively, 174 and 164 cGy. Two algorithms for non-targeted region calculated same point dose values of 0·2 cGy.

Conclusions:

The results of this study showed that BeO dosimeters can be used with OSL method in radiotherapy applications and it is a very important tool for the determination of targeted/non-targeted absorbed dose.

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.

Verification measurements of an eMC algorithm using a 2D ion chamber array

The aim of this study was to assess the suitability of the Im'RT MatriXX 2D ion chamber array for performing verification measurements on the Varian Eclipse electron Monte Carlo (eMC) algorithm for a range of clinical energies (6, 12, and 20 MeV) on a Varian 2100iX linear accelerator. Firstly, the suitability of the MatriXX for measuring percentage depth doses (PDD) in water was assessed, including characterization of the inherent buildup found in the MatriXX. Secondly the suitability of the MatriXX for measuring dose distributions in homogeneous and heterogeneous phantoms was assessed using gamma analysis at 3%/3 mm. It was found that after adjusting the PDD curves for the inherent buildup, that the position of R50,D measured using the MatriXX agreed to within 1 mm to the PDDs generated using the eMC algorithm for all energies used in this study. Gamma analysis at 3%/3 mm showed very good agreement (>95%) for all cases in both homogeneous and heterogeneous phantoms. It was concluded that the Im'RT MatriXX is a suitable device for performing eMC verification and could potentially be used for routine energy checks of electron beams.

PACS number(s): 87.55.km, 87.55.Qr

Full Monte Carlo and measurement-based overall performance assessment of improved clinical implementation of eMC algorithm with emphasis on lower energy range

New version 13.6.23 of the electron Monte Carlo (eMC) algorithm in Varian Eclipse™ treatment planning system has a model for 4MeV electron beam and some general improvements for dose calculation. This study provides the first overall accuracy assessment of this algorithm against full Monte Carlo (MC) simulations for electron beams from 4MeV to 16MeV with most emphasis on the lower energy range. Beams in a homogeneous water phantom and clinical treatment plans were investigated including measurements in the water phantom. Two different material sets were used with full MC: (1) the one applied in the eMC algorithm and (2) the one included in the Eclipse™ for other algorithms. The results of clinical treatment plans were also compared to those of the older eMC version 11.0.31. In the water phantom the dose differences against the full MC were mostly less than 3% with distance-to-agreement (DTA) values within 2mm. Larger discrepancies were obtained in build-up regions, at depths near the maximum electron ranges and with small apertures. For the clinical treatment plans the overall dose differences were mostly within 3% or 2mm with the first material set. Larger differences were observed for a large 4MeV beam entering curved patient surface with extended SSD and also in regions of large dose gradients. Still the DTA values were within 3mm. The discrepancies between the eMC and the full MC were generally larger for the second material set. The version 11.0.31 performed always inferiorly, when compared to the 13.6.23.

Evaluation of an electron Monte Carlo dose calculation algorithm for treatment planning

The purpose of this study is to evaluate the accuracy of the electron Monte Carlo (eMC) dose calculation algorithm included in a commercial treatment planning system and compare its performance against an electron pencil beam algorithm. Several tests were performed to explore the system's behavior in simple geometries and in configurations encountered in clinical practice. The first series of tests were executed in a homogeneous water phantom, where experimental measurements and eMC‐calculated dose distributions were compared for various combinations of energy and applicator. More specifically, we compared beam profiles and depth‐dose curves at different source‐to‐surface distances (SSDs) and gantry angles, by using dose difference and distance to agreement. Also, we compared output factors, we studied the effects of algorithm input parameters, which are the random number generator seed, as well as the calculation grid size, and we performed a calculation time evaluation. Three different inhomogeneous solid phantoms were built, using high‐ and low‐density materials inserts, to clinically simulate relevant heterogeneity conditions: a small air cylinder within a homogeneous phantom, a lung phantom, and a chest wall phantom. We also used an anthropomorphic phantom to perform comparison of eMC calculations to measurements. Finally, we proceeded with an evaluation of the eMC algorithm on a clinical case of nose cancer. In all mentioned cases, measurements, carried out by means of XV‐2 films, radiographic films or EBT2 Gafchromic films. were used to compare eMC calculations with dose distributions obtained from an electron pencil beam algorithm. eMC calculations in the water phantom were accurate. Discrepancies for depth‐dose curves and beam profiles were under 2.5% and 2 mm. Dose calculations with eMC for the small air cylinder and the lung phantom agreed within 2% and 4%, respectively. eMC calculations for the chest wall phantom and the anthropomorphic phantom also showed a positive agreement with the measurements. The retrospective dosimetric comparison of a clinical case, which presented scatter perturbations by air cavities, showed a difference in dose of up to 20% between pencil beam and eMC algorithms. When comparing to the pencil beam algorithm, eMC calculations are definitely more accurate at predicting large dose perturbations due to inhomogeneities.

PACS numbers: 87.55.de, 87.55.kd

Assessment of Eclipse electron Monte Carlo output prediction for various topologies

Monte Carlo simulation is deemed to be the leading algorithm for accurate dose calculation with electron beams. Patient anatomy (contours and tissue densities) as well as irradiation geometry is accounted for. The accuracy of the Monitor Unit (MU) determination is one essential aspect of a treatment planning system. Patient‐specific quality assurance of a Monte Carlo plan usually involves verification of the MUs with an independent simpler calculation approach, in which flat geometry is to be assumed. The magnitude of the discrepancies between flat and varied surfaces for a few scenarios has been investigated in this study. The ability to predict MUs for various surface topologies by the commercial electron Monte Carlo implementation from Varian Eclipse system (Eclipse eMC) has been evaluated and compared to the Generalized Gaussian Pencil Beam (GGPB) algorithm. Ten phantoms with different topologies were constructed of water‐equivalent material. Measurements with a parallel plate ionization chamber were performed using these phantoms to gauge their relative impact on outputs for 6, 9, 12, 16, and 20 MeV electron beams from a Varian TrueBeam with cone sizes ranging from 6×6 cm2 to 25×25 cm2. The corresponding Monte Carlo simulations of the measured geometries were carried out using the CT scans of these phantoms. The results indicated that the Eclipse eMC algorithm can predict these output changes within 3% for most scenarios. However, at the lowest energy, the discrepancy was the greatest, up to 6%. In comparison, the Eclipse GGPB algorithm had much worse agreement, with discrepancies up to 17% at the lowest energies.

PACS numbers: 87.55.K‐, 87.55.km

Effect of tissue composition on dose distribution in electron beam radiotherapy

OBJECTIVE

The aim of this study is to evaluate the effect of tissue composition on dose distribution in electron beam radiotherapy.

METHODS

A Siemens Primus linear accelerator and a phantom were simulated using MCNPX Monte Carlo code. In a homogeneous cylindrical phantom, six types of soft tissue and three types of tissue-equivalent materials were investigated. The tissues included muscle (skeletal), adipose tissue, blood (whole), breast tissue, soft tissue (9-components) and soft tissue (4-component). The tissue-equivalent materials were water, A-150 tissue-equivalent plastic and perspex. Electron dose relative to dose in 9-component soft tissue at various depths on the beam's central axis was determined for 8, 12, and 14 MeV electron energies.

RESULTS

The results of relative electron dose in various materials relative to dose in 9-component soft tissue were reported for 8, 12 and 14 MeV electron beams as tabulated data. While differences were observed between dose distributions in various soft tissues and tissue-equivalent materials, which vary with the composition of material, electron energy and depth in phantom, they can be ignored due to the incorporated uncertainties in Monte Carlo calculations.

CONCLUSION

Based on the calculations performed, differences in dose distributions in various soft tissues and tissue-equivalent materials are not significant. However, due to the difference in composition of various materials, further research in this field with lower uncertainties is recommended.

Evaluation of treatment plans using various treatment techniques for the radiotherapy of cutaneous Kaposi's sarcoma developed on the skin of feet

The purpose of this study was to investigate the plan qualities of various treatment modalities for the radiotherapy of cutaneous Kaposi's sarcoma developed on the skin of the foot. A total of six virtual targets were generated on the skin of the foot in CT images. Five types of treatment plans were generated using photon beams (PB), electron beams (EB), high‐dose‐rate (HDR) brachytherapy with a Freiburg flap applicator, intensity‐modulated radiation therapy (IMRT), and volumetric‐modulated arc therapy (VMAT) techniques. Plans for each of the six targets (single‐target plans) and also for the combined target consisting of the six single targets combined (multitarget plans) were generated. Dose‐volumetric analysis was performed for the targets and normal tissues. The averaged conformity index (CI) and homogeneity index (HI) values for each single target using PB, EB, HDR, IMRT, and VMAT techniques were 1.97, 2.39, 1.60, 4.60, and 0.80 and 1.05, 1.11, 1.52, 1.04, and 1.04, respectively. For the multitarget, the CI values were 3.99, 5.08, 1.38, 1.95, and 0.84, and the values of HI were 1.10, 1.36, 1.43, 1.06, and 1.04, respectively. The averaged mean doses to normal tissue were 2.5, 2.7, 3.6, 1.7, and 2.9 Gy for single‐target plans, and 21.3, 14.6, 14.2, 14.3, and 13.0 Gy for the multitarget plans, respectively. The VMAT demonstrated dosimetric advantages and better treatment efficiency over other techniques for the radiotherapy of multifocal skin disease of the feet.

PACS number: 87.55.dk

Comprehensive evaluation and clinical implementation of commercially available Monte Carlo dose calculation algorithm

A commercial electron Monte Carlo (eMC) dose calculation algorithm has become available in Eclipse treatment planning system. The purpose of this work was to evaluate the eMC algorithm and investigate the clinical implementation of this system. The beam modeling of the eMC algorithm was performed for beam energies of 6, 9, 12, 16, and 20 MeV for a Varian Trilogy and all available applicator sizes in the Eclipse treatment planning system. The accuracy of the eMC algorithm was evaluated in a homogeneous water phantom, solid water phantoms containing lung and bone materials, and an anthropomorphic phantom. In addition, dose calculation accuracy was compared between pencil beam (PB) and eMC algorithms in the same treatment planning system for heterogeneous phantoms. The overall agreement between eMC calculations and measurements was within 3%/2 mm, while the PB algorithm had large errors (up to 25%) in predicting dose distributions in the presence of inhomogeneities such as bone and lung. The clinical implementation of the eMC algorithm was investigated by performing treatment planning for 15 patients with lesions in the head and neck, breast, chest wall, and sternum. The dose distributions were calculated using PB and eMC algorithms with no smoothing and all three levels of 3D Gaussian smoothing for comparison. Based on a routine electron beam therapy prescription method, the number of eMC calculated monitor units (MUs) was found to increase with increased 3D Gaussian smoothing levels. 3D Gaussian smoothing greatly improved the visual usability of dose distributions and produced better target coverage. Differences of calculated MUs and dose distributions between eMC and PB algorithms could be significant when oblique beam incidence, surface irregularities, and heterogeneous tissues were present in the treatment plans. In our patient cases, monitor unit differences of up to 7% were observed between PB and eMC algorithms. Monitor unit calculations were also preformed based on point‐dose prescription. The eMC algorithm calculation was characterized by deeper penetration in the low‐density regions, such as lung and air cavities. As a result, the mean dose in the low‐density regions was underestimated using PB algorithm. The eMC computation time ranged from 5 min to 66 min on a single 2.66 GHz desktop, which is comparable with PB algorithm calculation time for the same resolution level.

PACS number: 87.55.K‐