Simulated Dose Profiles Research Articles

Dosimetric characteristics of LinaTech DMLC H multi leaf collimator: Monte Carlo simulation and experimental study.

This study evaluated the basic dosimetric characteristics of a Dynamic Multi Leaf Collimator (DMLC) using a diode detector and film measurements for Intensity Modulated Radiation Therapy Quality Assurance (IMRT QA). The EGSnrc Monte Carlo (MC) simulation system was used for the determination of MLC characteristics. Radiation transmission and abutting leaf leakage relevant to the LinaTech DMLC H were measured using an EDGE detector and EBT3 film. In this study, the BEAMnrc simulation code was used for modeling. The head of Siemens PRIMUS linac (6 MV) with external DMLC H was entered into a BEAMnrc Monte Carlo model using practical dosimetry data. Leaf material density, as well as interleaf and abutting air gaps were determined according to the computed and measured dose profiles. The IMRT QA field was used to evaluate the dose distribution of the simulated DMLC H. According to measurements taken with the EDGE detector and film, the total average measured leakage was 1.60 ± 0.03% and 1.57 ± 0.05%, respectively. For these measurements, abutting leaf transmission was 54.35 ± 1.85% and 53.08 ± 2.05%, respectively. To adapt the simulated leaf dose profiles with measurements, leaf material density, interleaf and abutting air gaps were adjusted to 18 g/cm3, 0.008 cm and 0.108 cm, respectively. Thus, the total average leakage was estimated to be about 1.59 ± 0.02%. The step‐and‐shoot IMRT was implemented and 94% agreement was achieved between the film and MC, using 3%‐3 mm gamma criteria. The results of this study showed that the dosimetric characteristics of DMLC H satisfied international standards.

Experimental verification of 4D Monte Carlo simulations of dose delivery to a moving anatomy.

To evaluate a novel 4D Monte Carlo simulation tool by comparing calculations to physical measurements using a respiratory motion phantom. We used a dynamic Quasar phantom in both stationary and breathing states (sinusoidal motion of amplitude of 1.8cm and period of 3.3s) for dose measurements on an Elekta Agility linear accelerator. Gafchromic EBT3 film and the RADPOS 4D dosimetry system were placed inside the lung insert of the phantom to measure dose profiles and point-dose values at the center of the spherical tumor inside the insert. Both a static 4×4cm2 field and a VMAT plan were delivered. Static and 4D Monte Carlo simulations of the treatment deliveries were performed using DOSXYZnrc and a modified version of the defDOSXYZnrc user code that allows modeling of the continuous motion of both machine and patient. DICOM treatment plan files and linac delivery log files were used to generate corresponding input files. The phantom motion recorded by RADPOS during beam delivery was incorporated into the input files for the 4DdefDOSXYZnrc simulations. For stationary phantom simulations, all point-dose values from MC simulations at the tumor center agreed within 1% with film and within 2% with RADPOS. More than 98% of the voxels from simulated dose profiles passed a 1D gamma of 2%/2-mm criteria against measured dose profiles. Similar results were observed when applying a 2D gamma analysis with a 2%/2-mm criteria to compare 2D dose distributions of Monte Carlo simulations against measurements. For simulations on the moving phantom, MC-calculated dose values at the center of the tumor were found to be within 1% of film and within 2σ of experimental uncertainties which are 2.8% of the RADPOS measurements. 1D gamma comparisons of the dose profiles were better than 91%, and 2D gamma comparisons of the 2D dose distributions were found to be better than 94%. Our 4D Monte Carlo method using defDOSXYZnrc can be used to accurately calculate the dose distribution in continuously moving anatomy for various treatment techniques. This work, if extended to deformable anatomies, can be used to reconstruct patient delivered dose for use in adaptive radiation therapy.

EGS_cbct: Simulation of a fan beam CT and RMI phantom for measured HU verification

IntroductionA mathematical 3D model of an existing computed tomography (CT) scanner was created and used in the EGSnrc-based BEAMnrc and egs_cbct Monte Carlo codes. Simulated transmission dose profiles of a RMI-465 phantom were analysed to verify Hounsfield numbers against measured data obtained from the CT scanner. Methods and materialsThe modelled CT unit is based on the design of a Toshiba Aquilion 16 LB CT scanner. As a first step, BEAMnrc simulated the X-ray tube, filters, and secondary collimation to obtain phase space data of the X-ray beam. A bowtie filter was included to create a more uniform beam intensity and to remove the beam hardening effects. In a second step the Interactive Data Language (IDL) code was used to build an EGSPHANT file that contained the RMI phantom which was used in egs_cbct simulations. After simulation a series of profiles were sampled from the detector model and the Feldkamp-Davis-Kress (FDK) algorithm was used to reconstruct transversal images. The results were tested against measured data obtained from CT scans. ResultsThe egs_cbct code can be used for the simulation of a fan beam CT unit. The calculated bowtie filter ensured a uniform flux on the detectors. Good correlation between measured and simulated CT numbers was obtained. ConclusionsIn principle, Monte Carlo codes such as egs_cbct can model a fan beam CT unit. After reconstruction, the images contained Hounsfield values comparable to measured data.

Is grid therapy useful for all tumors and every grid block design?

Grid therapy is a treatment technique that has been introduced for patients with advanced bulky tumors. The purpose of this study is to investigate the effect of the radiation sensitivity of the tumors and the design of the grid blocks on the clinical response of grid therapy. The Monte Carlo simulation technique is used to determine the dose distribution through a grid block that was used for a Varian 2100C linear accelerator. From the simulated dose profiles, the therapeutic ratio (TR) and the equivalent uniform dose (EUD) for different types of tumors with respect to their radiation sensitivities were calculated. These calculations were performed using the linear quadratic (LQ) and the Hug‐Kellerer (H‐K) models. The results of these calculations have been validated by comparison with the clinical responses of 232 patients from different publications, who were treated with grid therapy. These published results for different tumor types were used to examine the correlation between tumor radiosensitivity and the clinical response of grid therapy. Moreover, the influence of grid design on their clinical responses was investigated by using Monte Carlo simulations of grid blocks with different hole diameters and different center‐to‐center spacing. The results of the theoretical models and clinical data indicated higher clinical responses for the grid therapy on the patients with more radioresistant tumors. The differences between TR values for radioresistant cells and radiosensitive cells at 20 Gy and 10 Gy doses were up to 50% and 30%, respectively. Interestingly, the differences between the TR values with LQ model and H‐K model were less than 4%. Moreover, the results from the Monte Carlo studies showed that grid blocks with a hole diameters of 1.0 cm and 1.25 cm may lead to about 19% higher TR relative to the grids with hole diameters smaller than 1.0 cm or larger than 1.25 cm (with 95% confidence interval). In summary, the results of this study indicate that grid therapy is more effective for tumors with radioresistant characteristics than radiosensitive tumors.PACS number(s): 87.55.‐x

MCNP6 model of the University of Washington clinical neutron therapy system (CNTS)

A MCNP6 dosimetry model is presented for the Clinical Neutron Therapy System (CNTS) at the University of Washington. In the CNTS, fast neutrons are generated by a 50.5 MeV proton beam incident on a 10.5 mm thick Be target. The production, scattering and absorption of neutrons, photons, and other particles are explicitly tracked throughout the key components of the CNTS, including the target, primary collimator, flattening filter, monitor unit ionization chamber, and multi-leaf collimator. Simulations of the open field tissue maximum ratio (TMR), percentage depth dose profiles, and lateral dose profiles in a 40 cm × 40 cm × 40 cm water phantom are in good agreement with ionization chamber measurements. For a nominal 10 × 10 field, the measured and calculated TMR values for depths of 1.5 cm, 5 cm, 10 cm, and 20 cm (compared to the dose at 1.7 cm) are within 0.22%, 2.23%, 4.30%, and 6.27%, respectively. For the three field sizes studied, 2.8 cm × 2.8 cm, 10.4 cm × 10.3 cm, and 28.8 cm × 28.8 cm, a gamma test comparing the measured and simulated percent depth dose curves have pass rates of 96.4%, 100.0%, and 78.6% (depth from 1.5 to 15 cm), respectively, using a 3% or 3 mm agreement criterion. At a representative depth of 10 cm, simulated lateral dose profiles have in-field (⩾10% of central axis dose) pass rates of 89.7% (2.8 cm × 2.8 cm), 89.6% (10.4 cm × 10.3 cm), and 100.0% (28.8 cm × 28.8 cm) using a 3% and 3 mm criterion. The MCNP6 model of the CNTS meets the minimum requirements for use as a quality assurance tool for treatment planning and provides useful insights and information to aid in the advancement of fast neutron therapy.

Designing a range modulator wheel to spread-out the Bragg peak for a passive proton therapy facility

In proton beam therapy, a Spread-Out Bragg peak (SOBP) is used to establish a uniform dose distribution in the target volume. In order to create a SOBP, several Bragg peaks of different ranges, corresponding to different entrance energies, with certain intensities (weights) should be combined each other. In a passive beam scattering system, the beam is usually extracted from a cyclotron at a constant energy throughout a treatment. Therefore, a SOBP is produced by a range modulator wheel, which is basically a rotating wheel with steps of variable thicknesses, or by using the ridge filters. In this study, we used the Geant4 toolkit to simulate a typical passive scattering beam line. In particular, the CATANA transport beam line of INFN Laboratori Nazionali del Sud (LNS) in Catania has been reproduced in this work. Some initial properties of the entrance beam have been checked by benchmarking simulations with experimental data. A class dedicated to the simulation of the wheel modulators has been implemented. It has been designed in order to be easily modified for simulating any desired modulator wheel and, hence, any suitable beam modulation. By using some auxiliary range-shifters, a set of pristine Bragg peaks was obtained from the simulations. A mathematical algorithm was developed, using the simulated pristine dose profiles as its input, to calculate the weight of each pristine peak, reproduce the SOBP, and finally generate a flat dose distribution. Therefore, once the designed modulator has been realized, it has been tested at CATANA facility, comparing the experimental data with the simulation results.

WE-AB-BRB-07: Alanine and Monte Carlo Determined Beam Quality Corrections for Nonstandard Fields of the Varian TrueBeam Accelerator

Purpose: To determine beam quality correction factors for flattened and unflattened beams of the Varian TrueBeam™ accelerator using alanine measurements and Monte Carlo calculations. Methods: Measurements were performed using L-α-alanine pellets encased in Virtual Water™ paddles. These were irradiated in liquid water using various field sizes of the 6MV and 10MV beam energies of the TrueBeam™, with both flattening-filter-free (FFF) and flattened beam modes. Measurements were also performed in a 10×10 cm2 60Co field. Pellets were read out by the National Physical Laboratory (NPL, Teddington, UK) using electron paramagnetic resonance (EPR) spectrometry, and results were corrected for energy dependence using graphite calorimetry. Beam quality correction factors (kQ, kQmsr,Q, and kQclin,Qmsr) were determined for three ionization chambers: an Exradin A12 Farmer-type chamber, an Exradin A1SL scanning chamber, and an Exradin A26 microchamber. All chambers were modeled in the egs_chamber user code of EGSnrc. Sources were created using Varian-supplied IAEA-compliant phase spaces and the BEAMnrc user code, and were validated by comparing measured and simulated dose profiles. Results: Measured and calculated kQ values agreed within uncertainties, showing the chamber models to be reliable. A comparison of measured and calculated kQmsr,Q results with TG-51-based values showed that TG-51 adequately accounts for variations in beam quality between flattened and unflattened 10×10 cm2 fields. Small field corrections, kQclin,Qmsr, were determined to be up to 5%, referenced to a 10×10 cm2 field of the same energy and mode. Conclusion: Beam quality corrections were determined for several beam energies of the TrueBeam™ accelerator using Monte Carlo calculations and were validated using measurements with alanine. Values of kQ determined using TG-51 were found to be adequate for FFF reference fields. However, small field corrections are necessary for both flattened and unflattened beams to ensure greater treatment planning accuracy.

SU‐E‐T‐47: A Monte Carlo Model of a Spot Scanning Proton Beam Based On a Synchrotron Proton Therapy Accelerator

Purpose:To build the model of a spot scanning proton beam for the dose calculation of a synchrotron proton therapy accelerator, which is capable of accelerating protons from 50 up to 221 MeV.Methods:The spot scanning beam nozzle is modeled using TOPAS code, a simulation tool based on Geant4.9.6. The model contained a beam pipe vacuum window, a beam profile monitor, a drift chamber, two plane‐parallel ionization chambers, and a spot‐position monitor consisted of a multiwire ionization chamber. A water phantom is located with its upstream surface at the isocenter plane. The initial proton beam energy and anglar deflection are modeled using a Gaussian distribution with FWHM (Full Widths at Half Maximum) deponding on its beam energy. The phase space file (PSF) on a virtual surface located at the center between the two magnets is recorded. PSF is used to analyze the pencil beam features and offset the pencil beam position. The source model parameters are verificated by fitting the simulated Result to the measurement.Results:The simulated percentage depth dose (PDD) and lateral profiles of scanning pencil beams of various incident proton energies are verificated to the measurement. Generally the distance to agreement (DTA) of Bragg peaks is less than 0.2cm. The FWHM of Gaussian anglar distribution was adjusted to fit the lateral profile difference between the simulation and the measurement to less than 2∼3cm.Conclusion:A Monte Carlo model of a spot scanning proton beam was bullt based on a synchrotron proton therapy accelerator. This scanning pencil beam model will be as a block to build the broad proton beam as a proton TPS dose verification tool.

On the parametrization of lateral dose profiles in proton radiation therapy

PurposeThe accurate evaluation of the lateral dose profile is an important issue in the field of proton radiation therapy. The beam spread, due to Multiple Coulomb Scattering (MCS), is described by the Molière's theory. To take into account also the contribution of nuclear interactions, modern Treatment Planning Systems (TPSs) generally approximate the dose profiles by a sum of Gaussian functions. In this paper we have compared different parametrizations for the lateral dose profile of protons in water for therapeutical energies: the goal is to improve the performances of the actual treatment planning. MethodsWe have simulated typical dose profiles at the CNAO (Centro Nazionale di Adroterapia Oncologica) beamline with the FLUKA code and validated them with data taken at CNAO considering different energies and depths. We then performed best fits of the lateral dose profiles for different functions using ROOT and MINUIT. ResultsThe accuracy of the best fits was analyzed by evaluating the reduced χ2, the number of free parameters of the functions and the calculation time. The best results were obtained with the triple Gaussian and double Gaussian Lorentz–Cauchy functions which have 6 parameters, but good results were also obtained with the so called Gauss–Rutherford function which has only 4 parameters. ConclusionsThe comparison of the studied functions with accurate and validated Monte Carlo calculations and with experimental data from CNAO lead us to propose an original parametrization, the Gauss–Rutherford function, to describe the lateral dose profiles of proton beams.

SU‐F‐18C‐09: Assessment of OSL Dosimeter Technology in the Validation of a Monte Carlo Radiation Transport Code for CT Dosimetry

Purpose:To assess the utility of optically stimulated luminescent (OSL) dosimeter technology in calibrating and validating a Monte Carlo radiation transport code for computed tomography (CT).Methods:Exposure data were taken using both a standard CT 100‐mm pencil ionization chamber and a series of 150‐mm OSL CT dosimeters. Measurements were made at system isocenter in air as well as in standard 16‐cm (head) and 32‐cm (body) CTDI phantoms at isocenter and at the 12 o' clock positions. Scans were performed on a Philips Brilliance 64 CT scanner for 100 and 120 kVp at 300 mAs with a nominal beam width of 40 mm. A radiation transport code to simulate the CT scanner conditions was developed using the GEANT4 physics toolkit. The imaging geometry and associated parameters were simulated for each ionization chamber and phantom combination. Simulated absorbed doses were compared to both CTDI100 values determined from the ion chamber and to CTDI100 values reported from the OSLs. The dose profiles from each simulation were also compared to the physical OSL dose profiles.Results:CTDI100 values reported by the ion chamber and OSLs are generally in good agreement (average percent difference of 9%), and provide a suitable way to calibrate doses obtained from simulation to real absorbed doses. Simulated and real CTDI100 values agree to within 10% or less, and the simulated dose profiles also predict the physical profiles reported by the OSLs.Conclusion:Ionization chambers are generally considered the standard for absolute dose measurements. However, OSL dosimeters may also serve as a useful tool with the significant benefit of also assessing the radiation dose profile. This may offer an advantage to those developing simulations for assessing radiation dosimetry such as verification of spatial dose distribution and beam width.

SU-E-T-384: Experimental Verification of a Monte Carlo Linear Accelerator Model Using a Radiochromic Film Stack Dosimeter

Purpose: To experimentally verify a Monte Carlo (MC) linear accelerator model for the simulation of intensity-modulated radiation therapy (IMRT) treatments of moving targets. Methods: A Varian Clinac™ 21EX linear accelerator was modeled using the EGSnrc user code BEAMnrc. The mean energy, radial-intensity distribution, and divergence of the electron beam incident on the bremsstrahlung target were adjusted to achieve agreement between simulated and measured percentage-depth-dose and transverse field profiles for a 6 MV beam. A seven-field step-and-shoot IMRT lung procedure was prepared using Varian Eclipse™ treatment planning software. The plan was delivered using a Clinac™ 21EX linear accelerator and measured with a Gafchromic™ EBT2 film stack dosimeter (FSD) in two separate static geometries: within a cylindrical water-equivalent-plastic phantom and within an anthropomorphic chest phantom. Two measurements were completed in each setup. The dose distribution for each geometry was simulated using the EGSnrc user code DOSXYZnrc. MC geometries of the treatment couch, cylindrical phantom, and chest phantom were developed by thresholding CT data sets using MATLAB™. The FSD was modeled as water. The measured and simulated dose distributions were normalized to the median dose within the FSD. Results: Using an electron beam with a mean energy of 6.05 MeV, a Gaussian radial-intensity distribution with a full width at half maximum of 1.5 mm, and a divergence of 0°, the measured and simulated dose profiles agree within 1.75% and 1 mm. Measured and simulated dose distributions within both the cylindrical and chest phantoms agree within 3% over 94% of the FSD volume. The overall uncertainty in the FSD measurements is 3.1% (k=1). Conclusion: MC simulations agree with FSD measurements within measurement uncertainty, thereby verifying the accuracy of the linear accelerator model for the simulation of IMRT treatments of static geometries. The experimental verification will be extended to treatments of targets undergoing three-dimensional motion.

Intensity-modulated arc therapy using the gate Monte Carlo simulation platform in a grid environment

Introduction The GATE Monte-Carlo (MC) simulation platform ( www.opengatecollaboration.org ) based on the GEANT4 toolkit has come into widespread use for simulating Positron Emission Tomography, Single Photon Emission Computed Tomography, Computing Tomography (CT) imaging devices and radiation therapy. In this work, we explore the simulation of an intensity-modulated arc therapy (IMAT) treatment using GATE 6.2. To reach this objective, we first performed a complete modeling of the Varian's Novalis Tx linear accelerator (15MV photon mode) using the multileaf collimator (MLC). Then, the simulation of the geometry was validated in water before experiencing a head-and-neck cancer treatment. Material and methods The first stage is to model the Novalis Tx head with the GATE MC platform using manufacturer's data. We simulated this linac in 15 MV mode and compared depth doses, in-plane and cross-plane profiles with reference measurements (performed with ionization chambers (Semiflex 31013, PTW and LA48 Linear Array, PTW) and PTW60012 Diode E) in water for several field sizes ranging from 1 × 1 to 10 × 10 cm 2 . Target and flattening filter geometries and initial electron beamare adjusted in order to fit simulation calculations with dosemeasurements. In a second stage, the MLC geometry implementation enabled the simulation of clinical IMAT treatment plans compared with the iPlan treatment planning software (BrainLab) calculations. In order to reduce the computing time of GATE calculations, the simulations are executed on a distributed infrastructure using the GateLab ( http://vip.creatis.insa-lyon.fr/ ) platform. Results Weshowa good agreement between measurements and simulated depth dose profiles in liquid water. GATE and iPlan normalized treatment plans were compared using agammaindex of ±2%/mm.The ongoing work concerning treatment plan comparisons demonstrates limits of the iPlan Pencil Beam algorithm close to heterogeneities. Using the distributed infrastructure, space phase file was produced in 14 h for 500 million primary photons generated reaching a gain of a factor 50 comparing to calculations performed on a single CPU. Conclusions This study demonstrates that GATE offers efficient tools to make possible IMAT applications. In a subsequent work a microdosimetry study should be performed for a complete validation.

FLUKA Monte Carlo simulation for the Leksell Gamma Knife Perfexion radiosurgery system: Homogeneous media

The purpose of this work is to investigate the capability of the FLUKA Monte Carlo (MC) code to simulate the Elekta Leksell Gamma Knife Perfexion (LGK-PFX) and reproduce the Treatment Planning System (TPS) Leksell GammaPlan version 8.2 (LGP) dose calculations for the case of a water equivalent phantom target. Thanks to the collaboration with Elekta Instruments AB, the collimation system geometry, the source positions and all the involved material have been simulated in detail. The relative linear dose distribution along the three coordinate axes, for each collimator size, and the Relative Output Factors (ROF) have been investigated. The simulation has been validated comparing simulated linear dose profiles with measurements performed with EBT radiochromic films. The acceptance criterion between experimental data and FLUKA results is based on the gamma index (GI) method. The FLUKA MC calculation for the ROF provided the values of 0.920 for the 8 mm collimators and 0.800 for the 4 mm collimators. These values are in good agreement with the Elekta reference data of 0.924 and 0.805 respectively. The percentage difference between calculated and reference values for the ROF is under 1% and within the FLUKA uncertainty. Also the simulated relative dose profiles show a good agreement with the LGP calculation expressed by means of the gamma index method. This established accuracy proves that FLUKA is a suitable and powerful tool in order to reproduce successfully the LGP calculations for the homogeneous media.

SU-E-T-474: Monte Carlo Phase Space Production to Model Magnetically Scanned Proton Beams for IMPT.

Accurate dose predictions in proton beam therapy using magnetically scanned beams are highly dependent on the accurate modeling of the lateral dose profiles. This study was performed to provide proton phase spaces for Monte Carlo simulations, used to accurately simulate doses at distances up to 12 cm from the central axis of the beam. Measured lateral dose profiles at various depths in water were compared to Monte Carlo simulations of doses for 90 discreet initial proton energies. Phase spaces were produced using a one dimensional energy distribution, and a combination of several two dimensional spatial and directional distributions. Simulations were performed iteratively using variations in the initial phase space distributions to achieve acceptable agreement between measured and simulated lateral dose profiles, i.e. differences in FWHM < 0.5 mm and dose differences less that 0.1% at distances up to 12.5 cm. 90 phase spaces of proton sources for different initial beam energies were created for use in Monte Carlo simulations of scanned proton beam therapy patient plans. At a depth of 2 cm in water, the simulated and measured FWHM of the lateral dose profiles differed in in-plane direction by an average of 0.05 mm, in cross-plane direction by 0.13 mm. All simulated profiles were within 0.1% of the measured doses at distances between 2cm and 12.5 cm from the central beam axis. A library of 90 phase space files has been created to accurately simulate magnetically scanned proton beams for IMPT, providing accurate dose distributions up to 12 cm distance from the central beam axis. This project is supported in part by P01CA021239 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.

SU-E-I-106: Description of Energy Dose Deposition Kernel for the Diagnostic Beam.

Over the recent years, the employment of kV x-rays as a diagnostic tool into clinical routine has resulted in a significant increase in the patient's exposure to ionizing radiation. The accurate determination of the absorbed dose to patients during diagnosis is therefore necessary to avoid unnecessary exposure to the patient. This study presents an analytical model of the energy deposition kernel for the monoenergetic and polyenergetic kV beams for the fast calculation of dose in radiography. The analytical model is based on the pencil beam kernels derived from Monte Carlo simulations. DOSXYZnrc code from the EGSnrc family was employed to simulate the pencil beam of 0.1 cm width for 80, 100 and 120 keV mono-energetic and polyenergetic beams. The lateral dose profiles were calculated at different depths within a homogenous water phantom of size 50×50×50 cm3 . The evaluated dose profiles showed a high amplitude primary component at the central axis and a long range low amplitude scatter component spanning a considerable distance from the central axis. The profiles were fitted analytically with a triple exponential decay function with an offset. All coefficients of the exponential function were further fitted with appropriate analytical functions to represent their behavior relative to depth and photon energy. The accuracy of the obtained kernel was checked by the convolution of a rectangular fluence profile and comparing the calculated dose distribution with the Monte Carlo simulated dose profiles for 2×2 cm2 field size. In a homogeneous phantom, the comparisons of the convolution method and Monte Carlo simulations showed sufficient agreement except for largest depths (deviation approx. 15%). Future developments will focus on an implementation of the method for dose calculation in the patient.

SU-E-T-402: Modulated Electron Radiotherapy Dose Delivery Using Photon Multileaf Collimators

Purpose: Different designs of multileaf collimators (MLC) have been investigated for dose delivery for energy and intensity modulated electron radiotherapy (MERT). This work aims at verifying MERT dose delivery accuracy on clinical accelerators, Siemens Artiste and Varian Clinac iX, equipped with conventional photon multileaf collimators (pMLC). Method and Materials:Expensive measurements and Monte Carlo (MC) simulations were performed to acquire the electron beam data for Monte Carlo-based MERT treatment planning. The calculated percentage depth dose (PDD) and dose profiles were benchmarked with measurements. Differences of electron beam characteristics for fields shaped by the Artiste and Clinac iX pMLC were examined, demonstrating their strengths and weaknesses in MERT dose delivery. Results: MC simulated PDDs and dose profiles for electron energies, 6, 9, 12, 15 (Artiste) and 16 (Clinac iX) MeV with fields shaped by either applicators or MLCs agreed well (2%/2mm) with ion chamber measurements. For pMLC-shaped fields, no significant changes in PDDs were observed as SSD varied from 61 cm to 85 cm while the flatness and penumbra of dose profiles showed obvious deterioration as SSD increased. In comparison, the Clinac iX pMLC resulted in flatter electron beams with smaller penumbra than those collimated the Artiste pMLC most likely due to the differences of the pMLC locations. Conclusions: Both Siemens Artiste and Varian Clinac iX are capable of delivering energy and intensity modulated electron dose distributions efficiently using their existing pMLC. The electron beam properties shaped by pMLCs were better suited for MERT at smaller SSDs (with smaller gaps between the pMLC and the patient surface). For the same SSD, the Clinac iX pMLC manifested superior beam flatness and penumbra compared to the Artiste pMLC.

SU‐C‐214‐03: Monte Carlo Simulation of Varian OBI Cone Beam CT (CBCT) and Dose Distribution in Head Phantom in 125kVp and 100kVp

Purpose: (1) To validate a Monte Carlo model for Varianˈs OBI CBCT in head scan mode and calculate the dose inside a Rando head phantom; (2) To compare the dose in head phantom for 125 kVp and 100 kVp scans. Methods: The BEAMnrcMP Monte Carlo package was employed and modified to simulate Varianˈs OBI CBCT system operated at 125kVp and 100kVp in full‐fan full‐bowtie mode for the same mAs and 200 deg scans only. Percentage depth dose (PDD) of a single KV beam was measured with a cc01 micro ion chamber in water tank. Dose profiles of the single KV beam at multiple depths including 0.5 cm were also measured with the same micro chamber. After the validation of the Monte‐Carlo model with the measured beam data, dose calculations were performed on a Rando head phantom. Results: The Monte Carlo simulated PDD curves agree very well with the measurements. Also, the simulated dose profiles matches well to the measured ones in both in‐plane and cross‐plane directions. Monte Carlo dose calculation made on a Rando head phantom indicated that the post right side of head get higher dose, due to the posterior KV scan from gantry 290 deg to 90 deg. The dose ratio between bone and soft tissue were ∼4.4 and ∼4.9 for 125kVp and 100kVp, respectively. The dose ratio between 125kVp and 100kVp is about 1.7, close to the theoretical value of 1.252=1.56. Conclusions: Our Monte Carlo model has been validated by measurements to simulate Varianˈs OBI CBCT system. It was shown that CBCT dose in bone is four times of that in soft tissue, such that the accumulated bone dose during a treatment course could be up to &gt;100cGy in posterior area. Investigation on patient dose deposition due to kV CBCT is in progress.

Accounting for the fringe magnetic field from the bending magnet in a Monte Carlo accelerator treatment head simulation

Monte Carlo (MC) simulation can be used for accurate electron beam treatment planning and modeling. Measurement of large electron fields, with the applicator removed and secondary collimator wide open, has been shown to provide accurate simulation parameters, including asymmetry in the measured dose, for the full range of clinical field sizes and patient positions. Recently, disassembly of the treatment head of a linear accelerator has been used to refine the simulation of the electron beam, setting tightly measured constraints on source and geometry parameters used in simulation. The simulation did not explicitly include the known deflection of the electron beam by a fringe magnetic field from the bending magnet, which extended into the treatment head. Instead, the secondary scattering foil and monitor chamber were unrealistically laterally offset to account for the beam deflection. This work is focused on accounting for this fringe magnetic field in treatment head simulation. The magnetic field below the exit window of a Siemens Oncor linear accelerator was measured with a Tesla-meter from 0 to 12 cm from the exit window and 1-3 cm off-axis. Treatment head simulation was performed with the EGSnrc/BEAMnrc code, modified to incorporate the effect of the magnetic field on charged particle transport. Simulations were used to analyze the sensitivity of dose profiles to various sources of asymmetry in the treatment head. This included the lateral spot offset and beam angle at the exit window, the fringe magnetic field and independent lateral offsets of the secondary scattering foil and electron monitor chamber. Simulation parameters were selected within the limits imposed by measurement uncertainties. Calculated dose distributions were then compared with those measured in water. The magnetic field was a maximum at the exit window, increasing from 0.006 T at 6 MeV to 0.020 T at 21 MeV and dropping to approximately 5% of the maximum at the secondary scattering foil. It was up to three times higher in the bending plane, away from the electron gun, and symmetric within measurement uncertainty in the transverse plane. Simulations showed the magnetic field resulted in an offset of the electron beam of 0.80 cm (mean) at the machine isocenter for the exit window only configuration. The fringe field resulted in a 3.5%-7.6% symmetry and 0.25-0.35 cm offset of the clinical beam R(max) profiles. With the magnetic field included in simulations, a single (realistic) position of the secondary scattering foil and monitor chamber was selected. Measured and simulated dose profiles showed agreement to an average of 2.5%/0.16 cm (maximum: 3%/0.2 cm), which is a better match than previously achieved without incorporating the magnetic field in the simulation. The undulations from the 3 stepped layers of the secondary scattering foil, evident in the measured profiles of the higher energy beams, are now aligned with those in the simulated beam. The simulated fringe magnetic field had negligible effect on the central axis depth dose curves and cross-plane dose profiles. The fringe magnetic field is a significant contributor to the electron beam in-plane asymmetry. With the magnetic field included explicitly in the simulation, realistic monitor chamber and secondary scattering foil positions have been achieved, and the calculated fluence and dose distributions are more accurate.

Validation of the Swiss Monte Carlo Plan for a static and dynamic 6 MV photon beam

Monte Carlo (MC) based dose calculations can compute dose distributions with an accuracy surpassing that of conventional algorithms used in radiotherapy, especially in regions of tissue inhomogeneities and surface discontinuities. The Swiss Monte Carlo Plan (SMCP) is a GUI-based framework for photon MC treatment planning (MCTP) interfaced to the Eclipse treatment planning system (TPS). As for any dose calculation algorithm, also the MCTP needs to be commissioned and validated before using the algorithm for clinical cases. Aim of this study is the investigation of a 6 MV beam for clinical situations within the framework of the SMCP. In this respect, all parts i.e. open fields and all the clinically available beam modifiers have to be configured so that the calculated dose distributions match the corresponding measurements. Dose distributions for the 6 MV beam were simulated in a water phantom using a phase space source above the beam modifiers. The VMC++ code was used for the radiation transport through the beam modifiers (jaws, wedges, block and multileaf collimator (MLC)) as well as for the calculation of the dose distributions within the phantom. The voxel size of the dose distributions was 2 mm in all directions. The statistical uncertainty of the calculated dose distributions was below 0.4%. Simulated depth dose curves and dose profiles in terms of [Gy/MU] for static and dynamic fields were compared with the corresponding measurements using dose difference and γ analysis. For the dose difference criterion of ±1% of D max and the distance to agreement criterion of ±1 mm, the γ analysis showed an excellent agreement between measurements and simulations for all static open and MLC fields. The tuning of the density and the thickness for all hard wedges lead to an agreement with the corresponding measurements within 1% or 1 mm. Similar results have been achieved for the block. For the validation of the tuned hard wedges, a very good agreement between calculated and measured dose distributions was achieved using a 1%/1 mm criteria for the γ analysis. The calculated dose distributions of the enhanced dynamic wedges (10°, 15°, 20°, 25°, 30°, 45° and 60°) met the criteria of 1%/1 mm when compared with the measurements for all situations considered. For the IMRT fields all compared measured dose values agreed with the calculated dose values within a 2% dose difference or within 1 mm distance. The SMCP has been successfully validated for a static and dynamic 6 MV photon beam, thus resulting in accurate dose calculations suitable for applications in clinical cases.

Effects of Adaptive Section Collimation on Patient Radiation Dose in Multisection Spiral CT

To evaluate the potential effectiveness of adaptive collimation in reducing computed tomographic (CT) radiation dose owing to z-overscanning by using dose measurements and Monte Carlo (MC) dose simulations. Institutional review board approval was not necessary. Dose profiles were measured with thermoluminescent dosimeters in CT dose index phantoms and in an Alderson-Rando phantom without and with adaptive section collimation for spiral cardiac and chest CT protocols and were compared with the MC simulated dose profiles. Additional dose measurements were performed with an ionization chamber for scan ranges of 5-50 cm and pitch factors of 0.5-1.5. The measured and simulated dose profiles agreed to within 3%. By using adaptive section collimation, a substantial dose reduction of up to 10% was achieved for cardiac and chest CT when measurements were performed free in air and of 7% on average when measurements were performed in phantoms. For scan ranges smaller than 12 cm, ionization chamber measurements and simulations indicated a dose reduction of up to 38%. Adaptive section collimation allows substantial reduction of unnecessary exposure owing to z-overscanning in spiral CT. It can be combined in synergy with other means of dose reduction, such as spectral optimization and automatic exposure control.