Automatic determination of primary electron beam parameters in Monte Carlo simulation

Monte Carlo Model Validation of 6MV Beam of OMID, the First Iranian Linear Accelerator

Monte Carlo (MC) techniques are regarded as an accurate method to simulate the dose calculation in radiotherapy for many years. The present paper aims to validate the simulated model of the 6-MV beam of OMID linear accelerator (BEHYAAR Company) by EGSnrc codes system and also investigate the effects of initial electron beam parameters (energy, radial full width at half maximum, and mean angular spread) on dose distributions. For this purpose, the comparison between the calculated and measured percentage depth dose (PDD) and lateral dose profiles was done by gamma index (GI) with 1%-1 mm acceptance criteria. MC model validating was done for 3 cm × 3 cm, 5 cm × 5 cm, 8 cm × 8 cm, 10 cm × 10 cm, and 20 cm × 20 cm field sizes. To study the sensitivity of model to beam parameters, the field size was selected as 10 cm × 10 cm and 30 cm × 30 cm. All lateral dose profiles were obtained at 10 cm. Excellent agreement was achieved with a 99.2% GI passing percentage for PDD curves and at least 93.8% GI for lateral dose profiles for investigated field sizes. Our investigation confirmed that the lateral dose profile severely depends on the considered source parameters in this study. PDD only considerably depends on the initial electron beam energy. Therefore, source parameters should not be specified independently. These results indicate that the current model of OMID 6-MV Linac is well established, and the accuracy of the simulation is high enough to be used in various applications.

DeepBeam: a machine learning framework for tuning the primary electron beam of the PRIMO Monte Carlo software

Background

Any Monte Carlo simulation of dose delivery using medical accelerator-generated megavolt photon beams begins by simulating electrons of the primary electron beam interacting with a target. Because the electron beam characteristics of any single accelerator are unique and generally unknown, an appropriate model of an electron beam must be assumed before MC simulations can be run. The purpose of the present study is to develop a flexible framework with suitable regression models for estimating parameters of the model of primary electron beam in simulators of medical linear accelerators using real reference dose profiles measured in a water phantom.

Methods

All simulations were run using PRIMO MC simulator. Two regression models for estimating the parameters of the simulated primary electron beam, both based on machine learning, were developed. The first model applies Principal Component Analysis to measured dose profiles in order to extract principal features of the shapes of the these profiles. The PCA-obtained features are then used by Support Vector Regressors to estimate the parameters of the model of the electron beam. The second model, based on deep learning, consists of a set of encoders processing measured dose profiles, followed by a sequence of fully connected layers acting together, which solve the regression problem of estimating values of the electron beam parameters directly from the measured dose profiles. Results of the regression are then used to reconstruct the dose profiles based on the PCA model. Agreement between the measured and reconstructed profiles can be further improved by an optimization procedure resulting in the final estimates of the parameters of the model of the primary electron beam. These final estimates are then used to determine dose profiles in MC simulations.

Results

Analysed were a set of actually measured (real) dose profiles of 6 MV beams from a real Varian 2300 C/D accelerator, a set of simulated training profiles, and a separate set of simulated testing profiles, both generated for a range of parameters of the primary electron beam of the Varian 2300 C/D PRIMO simulator. Application of the two-stage procedure based on regression followed by reconstruction-based minimization of the difference between measured (real) and reconstructed profiles resulted in achieving consistent estimates of electron beam parameters and in a very good agreement between the measured and simulated photon beam profiles.

Conclusions

The proposed framework is a readily applicable and customizable tool which may be applied in tuning virtual primary electron beams of Monte Carlo simulators of linear accelerators. The codes, training and test data, together with readout procedures, are freely available at the site: https://github.com/taborzbislaw/DeepBeam.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13014-021-01847-w.

An EGS Monte Carlo model for Varian TrueBEAM treatment units: Commissioning and experimental validation of source parameters

In this work we have created and commissioned a Monte Carlo model of 6FFF Varian TrueBeam linear accelerator using BEAMnrc. For this purpose we have experimentally measured the focal spot size and shape of three Varian TrueBeam treatment units in 6FFF modality with a slit collimator and several depth dose and lateral beam profiles in a water phantom. The Monte Carlo model of a 6FFF TrueBeam machine was implemented with a primary electron source commissioned as a 2D Gaussian with Full Width Half Maximum selected by comparison of simulated and measured narrow beam profiles. The energy of the primary electron beam was optimized through a simultaneous fit to the measured beam depth dose profiles. Special attention was paid to evaluation of uncertainties of the selected Monte Carlo source parameters. These uncertainties were calculated by analysing the sensitivity of the commissioning process to changes in both primary beam size and energy. Both experimental and Monte Carlo commissioned focus size values were compared and found to be in excellent agreement. The commissioned Monte Carlo model reproduces within 1% accuracy the dose distributions of radiation field size from 3 cm × 3 cm to 15 cm × 15 cm.

Benchmarking of Monte Carlo model of Siemens Oncor® linear accelerator for 18MV photon beam: Determination of initial electron beam parameters

OBJECTIVE

This study aims to benchmark a Monte Carlo (MC) model of the 18 MV photon beam produced by the Siemens Oncor® linac using the BEAMnrc and DOSXYZnrc codes.

METHODS

By matching the percentage depth doses and beam profiles calculated by MC simulations with measurements, the initial electron beam parameters including electron energy, full width at half maximum (spatial FWHM), and mean angular spread were derived for the 10×10 cm2 and 20×20 cm2 field sizes. The MC model of the 18 MV photon beam was then validated against the measurements for different field sizes (5×5, 30×30 and 40×40 cm2) by gamma index analysis.

RESULTS

The optimum values for electron energy, spatial FWHM and mean angular spread were 14.2 MeV, 0.08 cm and 0.8 degree, respectively. The MC simulations yielded the comparable measurement results of these optimum parameters. The gamma passing rates (with acceptance criteria of 1% /1 mm) for percentage depth doses were found to be 100% for all field sizes. For cross-line profiles, the gamma passing rates were 100%, 97%, 95%, 96% and 95% for 5×5, 10×10, 20×20, 30×30 and 40×40 cm2 field sizes, respectively.

CONCLUSIONS

By validation of the MC model of Siemens Oncor® linac using various field sizes, it was found that both dose profiles of small and large field sizes were very sensitive to the changes in spatial FWHM and mean angular spread of the primary electron beam from the bending magnet. Hence, it is recommended that both small and large field sizes of the 18 MV photon beams should be considered in the Monte Carlo linac modeling.

A photon source model based on particle transport in a parameterized accelerator structure for Monte Carlo dose calculations

PURPOSE

An accurate source model of a medical linear accelerator is essential for Monte Carlo (MC) dose calculations. This study aims to propose an analytical photon source model based on particle transport in parameterized accelerator structures, focusing on a more realistic determination of linac photon spectra compared to existing approaches.

METHODS

We designed the primary and secondary photon sources based on the photons attenuated and scattered by a parameterized flattening filter. The primary photons were derived by attenuating bremsstrahlung photons based on the path length in the filter. Conversely, the secondary photons were derived from the decrement of the primary photons in the attenuation process. This design facilitates these sources to share the free parameters of the filter shape and be related to each other through the photon interaction in the filter. We introduced two other parameters of the primary photon source to describe the particle fluence in penumbral regions. All the parameters are optimized based on calculated dose curves in water using the pencil-beam-based algorithm. To verify the modeling accuracy, we compared the proposed model with the phase space data (PSD) of the Varian TrueBeam 6 and 15 MV accelerators in terms of the beam characteristics and the dose distributions. The EGS5 Monte Carlo code was used to calculate the dose distributions associated with the optimized model and reference PSD in a homogeneous water phantom and a heterogeneous lung phantom. We calculated the percentage of points passing 1D and 2D gamma analysis with 1%/1 mm criteria for the dose curves and lateral dose distributions, respectively.

RESULTS

The optimized model accurately reproduced the spectral curves of the reference PSD both on- and off-axis. The depth dose and lateral dose profiles of the optimized model also showed good agreement with those of the reference PSD. The passing rates of the 1D gamma analysis with 1%/1 mm criteria between the model and PSD were 100% for 4 × 4, 10 × 10, and 20 × 20 cm² fields at multiple depths. For the 2D dose distributions calculated in the heterogeneous lung phantom, the 2D gamma pass rate was 100% for 6 and 15 MV beams. The model optimization time was less than 4 min.

CONCLUSION

The proposed source model optimization process accurately produces photon fluence spectra from a linac using valid physical properties, without detailed knowledge of the geometry of the linac head, and with minimal optimization time.

Monte Carlo Investigation of Photon Beam Characteristics and its Variation with Incident Electron Beam Parameters for Indigenous Medical Linear Accelerator

Purpose

A Monte Carlo model of a 6 MV medical linear accelerator (linac) unit built indigenously was developed using the BEAMnrc user code of the EGSnrc code system. The model was benchmarked against the measurements. Monte Carlo simulations were carried out for different incident electron beam parameters in the study.

Materials and Methods

Simulation of indigenously developed linac unit has been carried out using the Monte Carlo based BEAMnrc user-code of the EGSnrc code system. Using the model, percentage depth dose (PDD), and lateral dose profiles were studied using the DOSXYZnrc user code. To identify appropriate electron parameters, three different distributions of electron beam intensity were investigated. For each case, the kinetic energy of the incident electron was varied from 6 to 6.5 MeV (0.1 MeV increment). The calculated dose data were compared against the measurements using the PTW, Germany make RFA dosimetric system (water tank MP3-M and 0.125 cm³ ion chamber).

Results

The best fit of incident electron beam parameter was found for the combination of beam energy of 6.2 MeV and circular Gaussian distributed source in X and Y with FWHM of 1.0 mm. PDD and beam profiles (along both X and Y directions) were calculated for the field sizes from 5 cm × 5 cm to 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 2%.

Conclusions

A Monte Carlo model of indigenous linac (6 MV) has been developed and benchmarked against the measured data.

CPU time optimization and precise adjustment of the Geant4 physics parameters for a VARIAN 2100 C/D gamma radiotherapy linear accelerator simulation using GAMOS

We have verified the GAMOS/Geant4 simulation model of a 6 MV VARIAN Clinac 2100 C/D linear accelerator by the procedure of adjusting the initial beam parameters to fit the percentage depth dose and cross-profile dose experimental data at different depths in a water phantom. Thanks to the use of a wide range of field sizes, from 2  ×  2 cm² to 40  ×  40 cm², a small phantom voxel size and high statistics, fine precision in the determination of the beam parameters has been achieved. This precision has allowed us to make a thorough study of the different physics models and parameters that Geant4 offers. The three Geant4 electromagnetic physics sets of models, i.e. Standard, Livermore and Penelope, have been compared to the experiment, testing the four different models of angular bremsstrahlung distributions as well as the three available multiple-scattering models, and optimizing the most relevant Geant4 electromagnetic physics parameters. Before the fitting, a comprehensive CPU time optimization has been done, using several of the Geant4 efficiency improvement techniques plus a few more developed in GAMOS.

A Monte Carlo study of the effect of an ultrasound transducer on surface dose during intrafraction motion imaging for external beam radiation therapy

PURPOSE

The aim of this study was to estimate changes in surface dose due to the presence of the Clarity Autoscan™ ultrasound (US) probe during prostate radiotherapy using Monte Carlo (MC) methods.

METHODS

MC models of the Autoscan US probe were developed using the BEAMnrc/DOSXYZnrc code based on kV and MV CT images. CT datasets were converted to voxelized mass density phantoms using a CT number-to-mass density calibration. The dosimetric effect of the probe, in the contact region (an 8 mm × 12 mm single layer of voxels), was investigated using a phantom set-up mimicking two scenarios (a) a transperineal imaging configuration (radiation beam perpendicular to the central US axial direction), and (b) a transabdominal imaging configuration (radiation beam parallel to the central US axial direction). For scenario (a), the dosimetric effect was evaluated as a function of the probe to inferior radiation field edge distance. Clinically applicable distances from 5 mm separation to 2 mm overlap were determined from the radiotherapy plans of 27 patients receiving Clarity imaging. Overlaps of 3 to 14 (1 to 3 SD) mm were also considered to include the effect of interfraction motion correction. The influence of voxel size on surface dose estimation was investigated. Approved clinical plans from two prostate patients were used to simulate worst-case dosimetric impact of the probe when large couch translations were applied to correct for interfraction prostate motion.

RESULTS

The dosimetric impact of both the MV and kV probe models agreed within ±2% for both beam configurations. For scenario (a) and 1 mm voxel model, the probe gave mean dose increases of 1.2% to 4.6% (of the dose at isocenter) for 5 mm separation to 0 mm overlap in the probe-phantom contact region, respectively. This increased to 27.5% for the largest interfraction motion correction considered (14 mm overlap). For separations of ≥ 2 mm dose differences were < 2%. Simulated dose perturbations were found to be superficial; for the 14 mm overlap the dose increase reduced to < 3% at 5.0 mm within the phantom. For scenario (b), dose increases due to the probe were < 5% in all cases. The dose increase was underestimated by up to ~13% when the voxel size was increased from 1 mm to 3 mm. MC simulated dose to the PTV and OARs for the two clinical plans considered showed good agreement with commercial treatment planning system results (within 2%). Mean dose increases due to the presence of the probe, after the maximum interfraction motion correction, were ~16.3% and ~8.0%, in the contact region, for plan 1 and plan 2, respectively.

CONCLUSIONS

The presence of the probe results in superficial dose perturbations for patients with an overlap between the probe and the radiation field present in either the original treatment plan or due to translation of the radiation field to simulate correction of interfraction internal prostate motion.

Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of magnetic field correction factors for different magnetic field strengths and field orientations

The aim of this work was to determine magnetic field correction factors that are needed for dosimetry in hybrid devices for MR-guided radiotherapy for Farmer-type ionization chambers for different magnetic field strengths and field orientations. The response of six custom-built Farmer-type chambers irradiated at a 6 MV linac was measured in a water tank positioned in a magnet with magnetic field strengths between 0.0 T and 1.1 T. Chamber axis, beam and magnetic field were perpendicular to each other and both magnetic field directions were investigated. EGSnrc Monte Carlo simulations were compared to the measurements and simulations with different field orientations were performed. For all geometries, magnetic field correction factors, [Formula: see text], and perturbation factors were calculated. A maximum increase of 8.8% in chamber response was measured for the magnetic field perpendicular to chamber and beam axis. The measured chamber response could be reproduced by adjusting the dead volume layer near the chamber stem in the Monte Carlo simulations. For the magnetic field parallel to the chamber axis or parallel to the beam, the simulated response increased by 1.1% at maximum for field strengths up to 1.1 T. A complex dependence of the response was found on chamber radius, magnetic field strength and orientation of beam, chamber axis and magnetic field direction. Especially for magnetic fields perpendicular to beam and chamber axis, the exact sensitive volume has to be considered in the simulations. To minimize magnetic field correction factors and the influence of dead volumes on the response of Farmer chambers, a measurement set-up with the magnetic field parallel to the chamber axis or parallel to the beam is recommended for dosimetry.

Monte Carlo systems used for treatment planning and dose verification

General-purpose radiation transport Monte Carlo codes have been used for estimation of the absorbed dose distribution in external photon and electron beam radiotherapy patients since several decades. Results obtained with these codes are usually more accurate than those provided by treatment planning systems based on non-stochastic methods. Traditionally, absorbed dose computations based on general-purpose Monte Carlo codes have been used only for research, owing to the difficulties associated with setting up a simulation and the long computation time required. To take advantage of radiation transport Monte Carlo codes applied to routine clinical practice, researchers and private companies have developed treatment planning and dose verification systems that are partly or fully based on fast Monte Carlo algorithms. This review presents a comprehensive list of the currently existing Monte Carlo systems that can be used to calculate or verify an external photon and electron beam radiotherapy treatment plan. Particular attention is given to those systems that are distributed, either freely or commercially, and that do not require programming tasks from the end user. These systems are compared in terms of features and the simulation time required to compute a set of benchmark calculations.

Implementation of a double Gaussian source model for the BEAMnrc Monte Carlo code and its influence on small fields dose distributions

The shape of the radiation source of a linac has a direct impact on the delivered dose distributions, especially in the case of small radiation fields. Traditionally, a single Gaussian source model is used to describe the electron beam hitting the target, although different studies have shown that the shape of the electron source can be better described by a mixed distribution consisting of two Gaussian components. Therefore, this study presents the implementation of a double Gaussian source model into the BEAMnrc Monte Carlo code. The impact of the double Gaussian source model for a 6 MV beam is assessed through the comparison of different dosimetric parameters calculated using a single Gaussian source, previously commissioned, the new double Gaussian source model and measurements, performed with a diode detector in a water phantom. It was found that the new source can be easily implemented into the BEAMnrc code and that it improves the agreement between measurements and simulations for small radiation fields. The impact of the change in source shape becomes less important as the field size increases and for increasing distance of the collimators to the source, as expected. In particular, for radiation fields delivered using stereotactic collimators located at a distance of 59 cm from the source, it was found that the effect of the double Gaussian source on the calculated dose distributions is negligible, even for radiation fields smaller than 5 mm in diameter. Accurate determination of the shape of the radiation source allows us to improve the Monte Carlo modeling of the linac, especially for treatment modalities such as IMRT, were the radiation beams used could be very narrow, becoming more sensitive to the shape of the source.

PACS number(s): 87.53.Bn, 87.55.K, 87.56.B‐, 87.56.jf

Radiation therapy dosimetry system

New therapeutic treatments generally aim to increase therapeutic efficacy while minimizing toxicity. Many aspects of radiation dosimetry have been studied and developed particularly in the field of external radiation. The success of radiotherapy relies on monitoring the dose of radiation to which the tumor and the adjacent tissues are exposed. Radiotherapy techniques have evolved through a rapid transition from conventional three-dimensional (3D) conformal radiation therapy to intensity-modulated radiation therapy (IMRT) treatments or radiosurgery and robotic radiation therapy. These advances push the frontiers in our effort to provide better patient care by improving the precision of the absorbed dose delivered. This paper presents state-of-the art radiation therapy dosimetry techniques as well as the value of integral dosimetry (INDOS), which shows promise in the fulfillment of radiation therapy dosimetry requirements.

An investigation into the use of MMCTP to tune accelerator source parameters and testing its clinical application

This paper presents an alternative method to tune Monte Carlo electron beam parameters to match measured data using a minimal set of variables in order to reduce the model setup time prior to clinical implementation of the model. Monte Carlo calculations provide the possibility of a powerful treatment planning verification technique. The nonstandardized and nonautomated process of tuning the required accelerator model is one of the reasons for delays in the clinical implementation of Monte Carlo techniques. This work aims to establish and verify an alternative tuning method that can be carried out in a minimal amount of time, allowing it to be easily implemented in a clinical setting by personnel with minimal experience with Monte Carlo methods. This tuned model can then be incorporated into the MMCTP system to allow the system to be used as a second dose calculation check for IMRT plans. The technique proposed was used to establish the primary electron beam parameters for accelerator models for the Varian Clinac 2100 6 MV photon beam using the BEAMnrc Monte Carlo system. The method is intended to provide a clear, direct, and efficient process for tuning an accelerator model using readily available clinical quality assurance data. The tuning provides a refined model, which agrees with measured dose profile curves within 1.5% outside the penumbra or 3 mm in the penumbra, for square fields with sides of 3 cm up to 30 cm. These models can then be employed as the basis for Monte Carlo recalculations of dose distributions, using the MMCTP system, for clinical treatment plans, providing an invaluable assessment tool. This was tested on six IMRT plans and compared to the measurements performed for the pretreatment QA process. These Monte Carlo values for the average dose to the chamber volume agreed with measurements to within 0.6%.

PACS number: 87.55.km

Monte Carlo modeling and simulations of the High Definition (HD120) micro MLC and validation against measurements for a 6 MV beam

PURPOSE

The most recent Varian(®) micro multileaf collimator (MLC), the High Definition (HD120) MLC, was modeled using the BEAMNRC Monte Carlo code. This model was incorporated into a Varian medical linear accelerator, for a 6 MV beam, in static and dynamic mode. The model was validated by comparing simulated profiles with measurements.

METHODS

The Varian(®) Trilogy(®) (2300C/D) accelerator model was accurately implemented using the state-of-the-art Monte Carlo simulation program BEAMNRC and validated against off-axis and depth dose profiles measured using ionization chambers, by adjusting the energy and the full width at half maximum (FWHM) of the initial electron beam. The HD120 MLC was modeled by developing a new BEAMNRC component module (CM), designated HDMLC, adapting the available DYNVMLC CM and incorporating the specific characteristics of this new micro MLC. The leaf dimensions were provided by the manufacturer. The geometry was visualized by tracing particles through the CM and recording their position when a leaf boundary is crossed. The leaf material density and abutting air gap between leaves were adjusted in order to obtain a good agreement between the simulated leakage profiles and EBT2 film measurements performed in a solid water phantom. To validate the HDMLC implementation, additional MLC static patterns were also simulated and compared to additional measurements. Furthermore, the ability to simulate dynamic MLC fields was implemented in the HDMLC CM. The simulation results of these fields were compared with EBT2 film measurements performed in a solid water phantom.

RESULTS

Overall, the discrepancies, with and without MLC, between the opened field simulations and the measurements using ionization chambers in a water phantom, for the off-axis profiles are below 2% and in depth-dose profiles are below 2% after the maximum dose depth and below 4% in the build-up region. On the conditions of these simulations, this tungsten-based MLC has a density of 18.7 g cm(- 3) and an overall leakage of about 1.1 ± 0.03%. The discrepancies between the film measured and simulated closed and blocked fields are below 2% and 8%, respectively. Other measurements were performed for alternated leaf patterns and the agreement is satisfactory (to within 4%). The dynamic mode for this MLC was implemented and the discrepancies between film measurements and simulations are within 4%.

CONCLUSIONS

The Varian(®) Trilogy(®) (2300 C/D) linear accelerator including the HD120 MLC was successfully modeled and simulated using the Monte Carlo BEAMNRC code by developing an independent CM, the HDMLC CM, either in static and dynamic modes.

A modification of flattening filter free linac for IMRT

PURPOSE

This study investigates the benefits of a modified flattening filter free (FFF) linac over the standard (STD) linac equipped with the flattening filter. Energy and angular spread of the electron beam of the FFF linac were modified. Modification of FFF beam parameters is explored to maximize the monitor unit efficiency and to minimize the head scatter in IMRT delivery for large target volumes or targets lying away from the central axis.

METHODS

The EGSnrc code is used to model FFF and STD linacs and study basic beam properties for both linac types in various beam configurations. Increasing energy of FFF linac results in similar beam attenuation properties and maximized dose rate compared to STD linac. Matching beam attenuation properties allows a more direct exploration of beam flatness of FFF linac in regard to IMRT delivery, especially away from the central axis where the effective dose rate is considerably smaller than the one at the central axis. Flatness of open beam dose profile of FFF linac is improved by increasing the angular spread of the electron beam. The resulting dose rate within the treatment field and outside of the field (peripheral dose) are characterized and compared to the unmodified FFF and STD linacs,

RESULTS

In order to match beam penetration properties, the energy of FFF is adjusted from 6.5 to 8.0 MeV for small to medium field sizes and from 6.5 to 8.5 MeV for larger ones. Dose rate of FFF vs STD linac increased by a factor of 1.9 (6.5 MeV) and 3.4-4.1 (8.0-8.5 MeV). Adjusting the mean angular spread of the electron beam from 0 degrees to 5 degrees-10 degrees resulted in complete flattening of photon beam for field sizes between 10 x 10 cm2 and 15 x 15 cm2 and partial flattening for field sizes from 15 x 15 cm2 to 30 x 30 cm2. Values of angular spread > or =14 degrees are not recommended as they exceed the opening of the primary collimator, affecting the area at the edges of the field. FFF fields of sizes smaller than 6 x 6 cm2 are already flat and beam flattening is not necessary. Overall, the angular spread of 5 degrees-10 degrees is sufficient and can satisfactorily flatten open beam dose profiles even for larger field sizes. Increasing the electron beam angular spread amounts to a slight decrease of dose rate of FFF linac. However, for angular spread, 5 degrees-10 degrees dose rate factor of FFF vs STD is still about 1.6-2.6, depending on the field size (and the adjusted energy). Similarly, in case of peripheral dose, a moderate increase in dose can be observed for angular spread of 5 degrees-10 degrees and for field sizes 10 x 10 cm2 to 30 x 30 cm2. Lastly, beam flatness of not modified FFF linac can be conveniently described by an analytical function representing a ratio of STD vs FFF doses: 1 + b|r|(n).

CONCLUSIONS

A modified FFF beamline with increased energy and electron beam angular spread results in satisfactory flattened beam and high dose rate within the field. Peripheral dose remaining at similar (or smaller) level than that of STD linac for the same delivered dose within the treatment field.

On Monte Carlo modeling of megavoltage photon beams: a revisited study on the sensitivity of beam parameters

PURPOSE

To commission Monte Carlo beam models for five Varian megavoltage photon beams (4, 6, 10, 15, and 18 MV). The goal is to closely match measured dose distributions in water for a wide range of field sizes (from 2 x 2 to 35 x 35 cm2). The second objective is to reinvestigate the sensitivity of the calculated dose distributions to variations in the primary electron beam parameters.

METHODS

The GEPTS Monte Carlo code is used for photon beam simulations and dose calculations. The linear accelerator geometric models are based on (i) manufacturer specifications, (ii) corrections made by Chibani and Ma ["On the discrepancies between Monte Carlo dose calculations and measurements for the 18 MV Varian photon beam," Med. Phys. 34, 1206-1216 (2007)], and (iii) more recent drawings. Measurements were performed using pinpoint and Farmer ionization chambers, depending on the field size. Phase space calculations for small fields were performed with and without angle-based photon splitting. In addition to the three commonly used primary electron beam parameters (E(AV) is the mean energy, FWHM is the energy spectrum broadening, and R is the beam radius), the angular divergence (theta) of primary electrons is also considered.

RESULTS

The calculated and measured dose distributions agreed to within 1% local difference at any depth beyond 1 cm for different energies and for field sizes varying from 2 x 2 to 35 x 35 cm2. In the penumbra regions, the distance to agreement is better than 0.5 mm, except for 15 MV (0.4-1 mm). The measured and calculated output factors agreed to within 1.2%. The 6, 10, and 18 MV beam models use theta = 0 degrees, while the 4 and 15 MV beam models require theta = 0.5 degrees and 0.6 degrees, respectively. The parameter sensitivity study shows that varying the beam parameters around the solution can lead to 5% differences with measurements for small (e.g., 2 x 2 cm2) and large (e.g., 35 x 35 cm2) fields, while a perfect agreement is maintained for the 10 x 10 cm2 field. The influence of R on the central-axis depth dose and the strong influence of theta on the lateral dose profiles are demonstrated.

CONCLUSIONS

Dose distributions for very small and very large fields were proved to be more sensitive to variations in E(AV), R, and theta in comparison with the 10 x 10 cm2 field. Monte Carlo beam models need to be validated for a wide range of field sizes including small field sizes (e.g., 2 x 2 cm2).

Dosimetric verification of the anisotropic analytical algorithm in lung equivalent heterogeneities with and without bone equivalent heterogeneities

PURPOSE

In this study, the authors evaluated the accuracy of dose calculations performed by the convolution/superposition based anisotropic analytical algorithm (AAA) in lung equivalent heterogeneities with and without bone equivalent heterogeneities.

METHODS

Calculations of PDDs using the AAA and Monte Carlo simulations (MCNP4C) were compared to ionization chamber measurements with a heterogeneous phantom consisting of lung equivalent and bone equivalent materials. Both 6 and 10 MV photon beams of 4 x 4 and 10 x 10 cm(2) field sizes were used for the simulations. Furthermore, changes of energy spectrum with depth for the heterogeneous phantom using MCNP were calculated.

RESULTS

The ionization chamber measurements and MCNP calculations in a lung equivalent phantom were in good agreement, having an average deviation of only 0.64 +/- 0.45%. For both 6 and 10 MV beams, the average deviation was less than 2% for the 4 x 4 and 10 x 10 cm(2) fields in the water-lung equivalent phantom and the 4 x 4 cm(2) field in the water-lung-bone equivalent phantom. Maximum deviations for the 10 x 10 cm(2) field in the lung equivalent phantom before and after the bone slab were 5.0% and 4.1%, respectively. The Monte Carlo simulation demonstrated an increase of the low-energy photon component in these regions, more for the 10 X 10 cm(2) field compared to the 4 x 4 cm(2) field.

CONCLUSIONS

The low-energy photon by Monte Carlo simulation component increases sharply in larger fields when there is a significant presence of bone equivalent heterogeneities. This leads to great changes in the build-up and build-down at the interfaces of different density materials. The AAA calculation modeling of the effect is not deemed to be sufficiently accurate.

eIMRT: a web platform for the verification and optimization of radiation treatment plans

The eIMRT platform is a remote distributed computing tool that provides users with Internet access to three different services: Monte Carlo optimization of treatment plans, CRT & IMRT treatment optimization, and a database of relevant radiation treatments/clinical cases. These services are accessible through a user-friendly and platform independent web page. Its flexible and scalable design focuses on providing the final users with services rather than a collection of software pieces. All input and output data (CT, contours, treatment plans and dose distributions) are handled using the DICOM format. The design, implementation, and support of the verification and optimization algorithms are hidden to the user. This allows a unified, robust handling of the software and hardware that enables these computation-intensive services. The eIMRT platform is currently hosted by the Galician Supercomputing Center (CESGA) and may be accessible upon request (there is a demo version at http://eimrt.cesga.es:8080/eIMRT2/demo; request access in http://eimrt.cesga.es/signup.html). This paper describes all aspects of the eIMRT algorithms in depth, its user interface, and its services. Due to the flexible design of the platform, it has numerous applications including the intercenter comparison of treatment planning, the quality assurance of radiation treatments, the design and implementation of new approaches to certain types of treatments, and the sharing of information on radiation treatment techniques. In addition, the web platform and software tools developed for treatment verification and optimization have a modular design that allows the user to extend them with new algorithms. This software is not a commercial product. It is the result of the collaborative effort of different public research institutions and is planned to be distributed as an open source project. In this way, it will be available to any user; new releases will be generated with the new implemented codes or upgrades.

The change of response of ionization chambers in the penumbra and transmission regions: impact for IMRT verification

Significant deviations from the expected dose have been reported in the absolute dosimetry validation of an intensity modulated radiation therapy treatment when individual segments are analyzed. However, when full treatment is considered and all segment doses are added together, these discrepancies fade out, leading to overall dose deviations below a 5% action level. This contradictory behavior may be caused by a partial compensation between detector over-responding and under-responding for measurement conditions far from radiation equilibrium. We consider three treatment verification scenarios that may lead to ionization chamber miss-responding, namely: narrow beam irradiation, field penumbra location and multi-leaf collimator transmission contribution. In this work we have analyzed the response of three different ionization chambers with different active volume under these conditions by means of Monte Carlo (MC) simulation methods. Correction factors needed to convert the detector readout into actual dose to water were calculated by inserting the specific detector geometry (carefully modeled) into the simulations. This procedure required extensive use of parallel computing resources in order to achieve the desired level of uncertainty in the final results. The analysis of the simulations shows the relative contribution of each of the three previously mentioned miss-responding scenarios. Additionally, we provide some evidence on dose deviation compensation in multi-segment radiotherapy treatment verification.

A new method for output factor determination in MLC shaped narrow beams

A new method for the measurement of output factors of narrow beams is presented in this work. By combining a new large area parallel plane ionization chamber (PTW model T34070) with a relative film dosimetry the output factors of small square fields of a 6 MV beam shaped by a MLC were measured. Several detectors (three ionization chambers, two solid state detectors and film) and Monte Carlo simulation were also employed to validate this new methodology and also to determine those detectors more suitable for narrow beam output factor determination. The proposed method for output factor measurement has shown to be in a very good agreement with diamond, diode and Monte Carlo results while it is insensitive to position displacements. Several uncertainties associated to the process of narrow beam dosimetry have also been addressed.