Central Axis Depth Dose Curves Research Articles

Dose Profiles Distribution of 6 MV Linear Accelerator Using Monte Carlo Code System for Optimum Treatment planning in Radiotherapy Department

Monte Carlo (MC) radiotherapy treatment planning has become practical in medical physics, particularly with the rapid development of computer technology. Requires detailed knowledge of linear medical accelerators (Linacs) beam radiation. In the present work the basic dosimetric properties of the 6-MV photon beam and the Varian Clinic 600CLinac were examined. The dosimetric features of interest are, the absorbed dose from the central axis, beam profiles, photon blockers, electron and positron, mean energy, spectral distribution, and isodose curves. Varian Clinic 600C Linac is simulated using the OMEGA BEAMnrc MC code system. Detailed spectra of phase space profiles of a photon beam with a field of view of 10 cm × 10 cm were modeled, simulated and finally analyzed using BEAMdp. The central axis depth dose curves, dose profiles and isodose curves for photon beams in the aqueous phantom were also recorded and analyzed using the DOSXYZnrc, STATDOSE and DOSXYZ_SHOW code. This study demonstrates that MC can generate phase space data files that can be used to generate accurate MC dose distributions for photon beams produced using a high-powered clinical linear accelerator in a water phantom or in patients for radiotherapy.

Electron contamination for 6 MV photon beams from an Elekta linac: Monte Carlo simulation

In external beam radiotherapy, the photons from a linear accelerator (linac) machine undergo multiple interactions, not only in the patient but also in the linac head and the air column between the linac head and the patient. Electrons are released from these interactions and contaminate the beams. The current study evaluates electron contamination for 6 MV photon beams from an Elekta linac using Monte Carlo simulation. The linac head was simulated by the BEAMnrc code and the absorbed dose in a phantom was calculated using the DOSXYZnrc code. The parameters of the initial electron beams on the target, such as mean energy and radial intensity distribution, were determined by matching the calculated dose distributions with the measured dose (at 10 x 10 cm2 field size and 90 cm source-skin distance). The central axis depth-dose curves of electron contamination were calculated for various field sizes from 5 x 5 cm2 to 40 x 40 cm2. We investigated the components that generated the electron contamination for a field size of 10 x 10 cm2. The optimal initial electron beam energy was 6.3 MeV with a full-width half maximum (FWHM) of the radial intensity distribution of 1.0 mm. These parameters were found to be in good agreement with the measured data. Electron contamination increased as the field size increased. At a depth of 1.0 mm and field sizes of 5 x 5, 10 x 10, 20 x 20, 30 x 30, and 40 x 40 cm2, the doses from electron contamination were 3.71, 5.19, 14.39, 18.97 and 20.89 %, respectively. Electron contamination decreased with increased depth. At a depth of 15 mm, the electron contamination was about 1 %. It was mainly generated in the air column between the linac head and the phantom (3.65 %), the mirror (0.99 %), and the flattening filter (0.59 %) (for the depth of 1.0 mm and the field size of 10 x 10 cm2).

SU‐E‐T‐257: A Monte Carlo Simulation Study of Photon Beam with Energies 6X, 10X, 6FFF, 10FFF From a TrueBeam Linear Accelerator

Purpose: To compare the quality of Monte Carlo simulation results of photon beams with multiple energies with the corresponding measured beam commissioning data from a Varian TrueBeam accelerator. Methods: IAEA phase space files for photon beam energies 6X, 10X, 6FFF, 10FFF from TrueBeam linear accelerator provided by Varian were implemented as source files in the GEANT4 Monte Carlo code. The simulation geometry consisted of upper and lower tungsten alloy jaws and a cubic water phantom (50 × 50 × 50 cm3). Central axis depth dose curves (PDD) for multiple field sizes (1×1, 2×2, 4×4, 6×6, 10×10, 20×20, 30×30 cm2) and radial and transverse dose profiles for identical field sizes at 5 different depths (Dmax, 5, 10, 20, 30 cm) for all four energies were recorded in the phantom. Each simulation was performed with 2.0E9 incident particles. The results of PDD and profiles from simulations were then compared with the corresponding beam commissioning data collected with a Wellhoffer Blue Phantom using a 0.13cc ion‐chamber and a 0.8 × 0.8 mm2 diode. Results: The GEANT4 simulated PDD curve for photon beam compared favorably well, within ∼2%, against the corresponding measured ion‐chamber PDD for all energies and field sizes. The simulated in‐plane and cross‐plane profiles also compared well, within 2 mm, at the 50% level against the measured profiles for all energies, field sizes and depths. Conclusion: For the commissioning of a linear accelerator, the feasibility of utilizing Monte‐Carlo simulated beam data has been demonstrated in this study. The increase of computer speed and capabilities may Result in the adoption of Monte‐Carlo techniques for future machine commissioning and dose calculations.

Precision Fitting to Depth-Dose Curves of Electron Beams in the Water Phantom

A function to express depth—dose curves of electron beams in the water phantom is given. This function is free from the deficiencies of the function recently proposed by Meigooni and Das. The function has been tested mainly by fitting it to the Monte-Carlo data reported by Berger and Seltzer. An energy-dependent expression obtained for the broad-beam depth—dose curve reproduces the data in the energy region from 1 to 60 MeV within 2N. The MonteCarlo data for central-axis depth—dose curves of narrow beams could also be reproduced well. An example of fit to the central-axis depth—dose curve specific to machine conditions is also given. The results of the test indicate that the function would be useful for electron-beam treatment planning.

MO‐G‐BRC‐02: Patient Specific Out‐Of‐Field Dose Calculation Tool for 6MV and 18MV: Development and Validation

Purpose: A factor that greatly limits the use of Monte Carlo methods for patient specific dose calculations in research is the substantial amount of effort required to define the Monte Carlo geometry of the actual treatment and patient setup. The purpose of this study was to develop and validate computational infrastructure to automatically convert radiation field parameters and computed tomography (CT) data from Digital Imaging and Communications (DICOM)format to Monte Carlo input format, and automate a Monte Carlo based dose calculation system for external beam photon radiation therapy. Methods: Computational infrastructure was developed using DCMTK, the DICOM tool kit. The dose calculation system (ADCS) was automated using a shell script. For validation of the ADCS, infield doses calculated by the ADCS were compared with those calculated by the treatment planning software using an eight‐field, 6 MV, step‐and‐shoot intensity modulated radiation therapy plan. Doses were calculated in a water phantom, and an anthropomorphic phantom based on CT data. Results: For each field, more than 95% of the dose voxels passed 3% and 3‐mm criteria of gamma‐index analysis of 3‐D dose distributions in the water phantom. The central axis depth dose curve agreed with in 15% (1stardard deviation) to the TPS calculations in the anthropomorphic phantom for a single representative field of the RT plan. Conclusions: The ADCS can accurately extract patient‐specific radiation treatment parameters and automatically incorporate them into the Monte Carlo format and create final dose distributions compatible with commercial treatment planning systems. This automated calculation infrastructure reduces the time required to define the Monte Carlo geometry and potential for human error. This system is capable of automating and calculating doses for conventional radiation therapy and 18MV. It is also capable of calculating out‐of‐field doses (region of interest in patient anatomy can be selected prior to running ADCS).

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.

Electron dosimetry in the presence of small cavities

Tissue inhomogeneities such as bones or air cavities give rise to significant perturbations of dose during electron radiotherapy. Whilst these can be calculated using a variety of computational methods, accurate experimental verification has hitherto been difficult. In this study, we used 3-D optical computed tomography (CT) dosimetry of PRESAGETM samples to obtain central-axis depth dose curves and to study the dose distribution around a simple air cavity. Some concerning anomalous results were obtained for the build-up region of the depth-dose curve, which are currently under investigation. However, despite this, the ability to measure rapidly varying dose distributions show great promise.

Small field electron beam dosimetry using MOSFET detector

The dosimetry of very small electron fields can be challenging due to relative shifts in percent depth‐dose curves, including the location of dmax, and lack of lateral electronic equilibrium in an ion chamber when placed in the beam. Conventionally a small parallel plate chamber or film is utilized to perform small field electron beam dosimetry. Since modern radiotherapy departments are becoming filmless in favor of electronic imaging, an alternate and readily available clinical dosimeter needs to be explored. We have studied the performance of MOSFET as a relative dosimeter in small field electron beams. The reproducibility, linearity and sensitivity of a high‐sensitivity microMOSFET were investigated for clinical electron beams. In addition, the percent depth doses, output factors and profiles have been measured in a water tank with MOSFET and compared with those measured by an ion chamber for a range of field sizes from 1 cm diameter to 10 cm× 10 cm for 6, 12, 16 and 20 MeV beams. Similar comparative measurements were also performed with MOSFET and films in solid water phantom. The MOSFET sensitivity was found to be practically constant over the range of field sizes investigated. The dose response was found to be linear and reproducible (within ±1% for 100 cGy). An excellent agreement was observed among the central axis depth dose curves measured using MOSFET, film and ion chamber. The output factors measured with MOSFET for small fields agreed to within 3% with those measured by film dosimetry. Overall results indicate that MOSFET can be utilized to perform dosimetry for small field electron beam.PACS number: 87.55.Qr

Study on energy dependence of PAGAT polymer gel dosimeter evaluated using X-Ray CT

The normoxic polymer gel dosimeter evaluated with X-Ray computed tomography has emerged as a promising tool for measuring the dose delivered during radiotherapy in three dimensions. This study presents the dependence of PAGAT normoxic polymer gel sensitivity to different photon and electron energies. PAGAT polymer gel was prepared under normal atmospheric condition and irradiated with different photon energies of 1.25 MeV from Co-60 and 6 MV and 15 MV from linear accelerator and electron energies of 6, 9, 12, 15, 18 and 21 MeV from linear accelerator. Evaluation of dosimeter was performed with an X-Ray CT scanner. Images were acquired with optimum scanning protocols to reduce the signal-to-noise ratio. The averaged image was subtracted from the unirradiated polymer gel image for background. Central axis depth dose (PDD) curves obtained for each energy and polymer gel dosimeter measurements were in good agreement with diode and film measurements. Hounsfield (HU) – dose response curve for each photon and electron energy were derived from the PDD curve obtained from the gel dosimeter measurements. From the study it was clear that the HU-dose response curve was linear in the region 1–10 Gy. The dosimeter sensitivity was defined as a slope of these linear HU-dose response curves and found that the sensitivity of polymer gel decreases with increase in both photon and electron energies. This trend in dependence of PAGAT gel dosimeter sensitivity to different photon and electron energies was not dosimetrically significant. However, to evaluate the test phantom exposed with one energy using the calibration curve derived at another energy can produce clinically significant error.

SU‐FF‐T‐339: Small Field Electron Beam Dosimetry Using MOSFET Detector

Purpose: The dosimetry of very small electron fields can be challenging due to relative shifts in percent depth‐dose curves, including the location of dmax, and lack of lateral electronic equilibrium in an ion chamber when placed in the beam. Conventionally, a small parallel plate chamber or film is utilized to perform small field electron beam dosimetry. With the advent of electronic imaging, modern radiotherapy departments are moving to a filmless environment and an alternate clinical dosimeter is required for routine clinical dosimetry. In this work, we have studied the performance of MOSFET as a relative dosimeter for small field electron beams. Methods and materials: The reproducibility, linearity and sensitivity of a high sensitivity micro MOSFET were investigated for clinical electron beams. In addition, the percent depth doses, output factors and profiles have been measured in a water tank with MOSFET and compared with those measured by an ion chamber and film for a range of field sizes from 1 cm diameter to 10×10 cm2 for 6, 12, 16 and 20 MeV beams. Similar comparative measurements were also performed with MOSFET as well as with films in solid water phantom. Results: The MOSFET sensitivity was found to be stable over the range of field sizes investigated. The dose response was found to be linear and reproducible (within ±1% for 100 cGy). For 1, 1.5, 2, 2.5, 3, 4, 5, 6 cm circular and 10 cm square field, excellent agreement (within ±2% and ±2mm) was observed among the central axis depth dose curves measured using MOSFET, film and ion chamber. The output factors measured with MOSFET for small fields agreed to within 2% with those measured by film dosimetry. Conclusion: Overall results indicate that MOSFET can be utilized to perform dosimetry of small electron fields for routine clinical use.

Characterization of a mini‐multileaf collimator in a proton beamline

A mini-multileaf collimator (MMLC) was mounted as a field shaping collimator in a proton beamline at the Massachusetts General Hospital. The purpose is to evaluate the device's dosimetric and mechanical properties for the use in a proton beamline. For this evaluation, the authors compared MMLC and brass aperture shaped dose distributions with regard to lateral and depth dose properties. The lateral fall off is generally broader with the MMLC, with difference varying with proton range from 0.2 to 1.2 mm. Central axis depth dose curves did not show a difference in peak-to-entrance ratio, peak width, distal fall off, or range. Two-dimensional dose distributions to investigate the conformity of MMLC shaped doses show that the physical leaf width of approximately 2.5 mm does not have a significant impact. All differences seen in dose distribution shaped by the MMLC versus brass apertures were shown to be clinically insignificant. Measured neutron doses of 0.03-0.13 mSv/Gy for a closed brass beam block (depending on range) are very low compared to the previously published data. Irradiation of the tungsten MMLC, however, produced 1.5-1.8 times more neutrons than brass apertures. Exposure of the staff resulting from activation of the device is below regulatory limits. The measurements established an equivalency between aperture and MMLC shaped dose distributions.

Segments of a commercial Ge-doped optical fiber as a thermoluminescent dosimeter in radiotherapy

Optical fibers have been proposed as dosimeters in both diagnostic and radiotherapy applications. A commercial germanium (Ge)-doped silica fiber with a 50 μm core diameter which showed good thermoluminescence (TL) properties was selected for this study. The radiation sources used were a high dose rate brachytherapy iridium-192, MV photon and MeV electron beams from a linear accelerator. The coating of the fiber was chemically removed and then annealed at 400 °C for 1 h prior to irradiation. After irradiation, the fiber was read on a Harshaw Model 3500 TLD reader. The optical fiber had one well-defined glow peak at 327 ± 2 °C at all the radiotherapy energies. The dose response was linear within the clinical relevant dose for all these energies. Reproducibility was mainly within 4–6% (one standard deviation) for high energy photons and electrons. The fiber was found to be energy independent within the MV photon energy range. At room temperature the fading up until 1 month was around 6% which was within the 6% uncertainty of the sensitivity calibration of the fiber. Re-using the fiber four times did not significantly alter the sensitivity factor. The optical fiber was found to be dose rate as well as angular independent. Central axis depth dose curves of both 10 MV photons and 12 MeV electrons using the fiber showed relatively good agreement to standard depth dose curves in water within 4%. The Ge-doped fiber is a promising TL dosimeter but improvements have to be made to reduce the reproducibility within 3% for high energy photons and electrons.

Effects of energy spectrum on dose distribution calculations for high energy electron beams

In an early work we have demonstrated the possibility of using Monte Carlo generated pencil beams for 3D electron beam dose calculations. However, in this model the electron beam was considered as monoenergetic and the effects of the energy spectrum were taken into account by correction factors, derived from measuring central-axis depth dose curves. In the present model, the electron beam is considered as polyenergetic and the pencil beam distribution of a clinical electron beam, of a given nominal energy, is represented as a linear combination of Monte Carlo monoenergetic pencil beams. The coefficients of the linear combination describe the energy spectrum of the clinical electron beam, and are chosen to provide the best-fit between the calculated and measured central axis depth dose, in water. The energy spectrum is determined by the constrained least square method. The angular distribution of the clinical electron beam is determined by in-air penumbra measurements. The predictions of this algorithm agree very well with the measurements in the region near the surface, and the discrepancies between the measured and calculated dose distributions, behind 3D heterogeneities, are reduced to less than 10%. We have demonstrated a new algorithm for 3D electron beam dose calculations, which takes into account the energy spectra. Results indicate that the use of this algorithm leads to a better modeling of dose distributions downstream, from complex heterogeneities.

Evaluation of optically stimulated luminescence (OSL) dosimeters for passive dosimetry of high‐energy photon and electron beams in radiotherapy

This article investigates the performance of Al2O3: C optically stimulated luminescence dosimeters (OSLDs) for application in radiotherapy. Central-axis depth dose curves and optically stimulated luminescence (OSL) responses were obtained in a water phantom for 6 and 18 MV photons, and for 6, 9, 12, 16, and 20 MeV electron beams from a Varian 21EX linear accelerator. Single OSL measurements could be repeated with a precision of 0.7% (one standard deviation) and the differences between absorbed doses measured with OSLDs and an ionization chamber were within +/- 1% for photon beams. Similar results were obtained for electron beams in the low-gradient region after correction for a 1.9% photon-to-electron bias. The distance-to-agreement values were of the order of 0.5-1.0 mm for electrons in high dose gradient regions. Additional investigations also demonstrated that the OSL response dependence on dose rate, field size, and irradiation temperature is less than 1% in the conditions of the present study. Regarding the beam energy/quality dependence, the relative response of the OSLD for 18 MV was (0.51 +/- 0.48)% of the response for the 6 MV photon beam. The OSLD response for the electron beams relative to the 6 MV photon beam. The OSLD response for the electron beams relative to the 6 MV photon beam was in average 1.9% higher, but this result requires further confirmation. The relative response did not seem to vary with electron energy at dmax within the experimental uncertainties (0.5% in average) and, therefore, a fixed correction factor of 1.9% eliminated the energy dependence in our experimental conditions.

SU‐FF‐T‐85: Assessing the Reliability of Film and Semiconductor Detector in Measurement of Central Axis Depth Dose of Electron Beams

Purpose: To investigate the possibility of using the Kodak X‐Omat V film for the central axis depth‐dose measurements of electron beams.Method and materials: The X‐Omat V films were exposed in a MED‐TEC film phantom cassette with parallel orientation to produce central axis depth dose curves. These film data were compared with the measured percentage depth dose curves by using a Scanditronix scanning water phantom with both RK ion chamber and electron diode. Measurements were made at a source to surface distance (SSD) of 100 cm for the electron energies between 6 and 20 MeV on a Varian Clinac 2100C linear accelerator for field sizes of 6 × 6 cm2 to 20 × 20 cm2. Results: The comparison of the central axis depth dose parameters, R100, R90, R50, and RP measured by the film and diode with respect to the IC's values showed that the difference did not indicate the dependence either on the energy or cone size. In the buildup region, the film presented higher percent depth dose values for most energies and cone sizes but beyond the depth of maximum dose (R100), the results were opposite. The difference of every parameter for both film and diode was within the acceptable value of ±2 mm. Conclusions: The ionization chamber and diode measurements appear to be more reliable than the film measurements, but film is particularly useful for new electron cutout data where access to accelerator beam time is limited.

TU-FF-A1-02: Development of a Fluence Benchmark for Clinical Electron Beams

Purpose: A fluence benchmark for clinical electron beams with application to validation of the electron beam model of a commercial radiotherapy treatment planning system Method and Materials: A fluence benchmark for the 6–21 MeV electron beams of a clinical linear accelerator was determined for 40×40 large field. Measurements were done on a clinical accelerator that is not used to treat patients, allowing us to disassemble the accelerator and directly measure critical geometrical details and to operate the accelerator outside the normal operating range to help ascertain source and geometry details.Therefore, source and geometry in the simulation was adjusted within more restrictive limits than previous studies to match simulation results to measured central axis depth dose curves, cross-plane and in-plane profiles at the depth of maximum dose, dmax, and in the bremsstrahlung tail. Parallel to this process, a series of measurements were made on the same linac to commission a commercial planning system. The fluence calculated with the commercial beam model was validated with the fluence benchmark. Results: Agreement between measured and simulated depth-dose curves upstream of the fall-off region and depth-ionization curves in and beyond the fall-off region was excellent: within 1%/0.5 mm. The dose profiles matched within 1% in the high-dose regions of the electron field and within 2 mm at the field edge. The current benchmarks approach the goal of within 1% in fluence, given 100% on the central axis, and 2 mm in position. The electron energy distributions of the 6 electron beams calculated with the commercial beam model closely matched the benchmark. Conclusion: A high accuracy, highly detailed fluence benchmark for clinical electron beams is developed and proved to be useful for validation of the electron beam model of a commercial planning system to the accuracy required for dose calculation in radiotherapy. Support from NIH-R01-CA104777-01A2.

TU-FF-A1-01: Electron Spectra Unfolding From Open Field Depth Dose Curves

Purpose: To evaluate a new algorithm to unfold the energy spectrum of a large field electron beam from a central axis depth dose curve. Methods and Materials: As previously reported unfolding of the spectrum of electron beams is best done (1) based on large open field depth dose measurements, (2) incorporating the air space between source and phantom in the simulation of the response matrix and (3) using an inverse geometry with a pencil beam, large scoring voxels. Here an algorithm by Chvetskov based on a special form of the general Tichonov regularization function has been tested. The response matrix was created through EGSsnrc Monte Carlo Simulations of pencil beam depth doses in 0.1 MeV steps and including the air between source and phantom. The depth dose curve to be unfolded was also created through a Monte Carlo simulation which was in excellent agreement with the measurement on a clinical machine. The advantage of using a Monte Carlo based depth dose as the input is that the spectrum is precisely known. In addition it was possible to subtract the head photon component from the depth dose data and perform the algorithms on the electron only data. For later practical implementation, this approach presents a challenge, which however can be met. The Chvetskov algorithm used for the unfolding contains one smoothing parameter the impact of which was explored. For comparison, a generic set of depth dose curves was also used in the response matrix. Results and conclusions: The algorithm identifies the main energy component well. The smoothing factor impacts the waviness of the curve substantially. Optimizing this factor will be an important task. The comparison with the generic response functions shows that the use of specific response functions,including the air improves the unfolding. Support from NIH R01 CA104777‐01A2.

Monte Carlo study of a Cyberknife stereotactic radiosurgery system

This study investigated small-field dosimetry for a Cyberknife stereotactic radiosurgery system using Monte Carlo simulations. The EGSnrc/BEAMnrc Monte Carlo code was used to simulate the Cyberknife treatment head, and the DOSXYZnrc code was implemented to calculate central axis depth-dose curves, off-axis dose profiles, and relative output factors for various circular collimator sizes of 5 to 60 mm. Water-to-air stopping power ratios necessary for clinical reference dosimetry of the Cyberknife system were also evaluated by Monte Carlo simulations. Additionally, a beam quality conversion factor, kQ, for the Cyberknife system was evaluated for cylindrical ion chambers with different wall material. The accuracy of the simulated beam was validated by agreement within 2% between the Monte Carlo calculated and measured central axis depth-dose curves and off-axis dose profiles. The calculated output factors were compared with those measured by a diode detector and an ion chamber in water. The diode output factors agreed within 1% with the calculated values down to a 10 mm collimator. The output factors with the ion chamber decreased rapidly for collimators below 20 mm. These results were confirmed by the comparison to those from Monte Carlo methods with voxel sizes and materials corresponding to both detectors. It was demonstrated that the discrepancy in the 5 and 7.5 mm collimators for the diode detector is due to the water non-equivalence of the silicon material, and the dose fall-off for the ion chamber is due to its large active volume against collimators below 20 mm. The calculated stopping power ratios of the 60 mm collimator from the Cyberknife system (without a flattening filter) agreed within 0.2% with those of a 10 X 10 cm2 field from a conventional linear accelerator with a heavy flattening filter and the incident electron energy, 6 MeV. The difference in the stopping power ratios between 5 and 60 mm collimators was within 0.5% at a 10 cm depth in water. Furthermore, kQ values for the Cyberknife system were in agreement within 0.3% with those of the conventional 6 MV-linear accelerator for the cylindrical ion chambers with different wall material.

SU‐FF‐T‐133: Commissioning Electron Beams with Monte Carlo Simulation Based On Large Field Measurements

Purpose: To commission electron beams using measurements made with the jaws wide open (with no applicator) along with state‐of‐the‐art Monte Carlo simulation. Method and Materials: Central axis depth dose curves and dose profiles of 6–21 MeV Primus electron beams were measured for the 40×40 cm field and a comprehensive set of field sizes for all square applicators. Monte Carlo treatment head and water phantom simulations were done with the EGSnrc system using the BEAM and MCRTP user codes, respectively. The measured data for the 40×40 cm field was used to estimate the source and geometry parameters used in the smaller‐field simulations. Calculated output and dose distributions were compared to measurement. Results: Dose distributions calculated with this large‐field commissioning approach generally compared to 2%/2 mm or better with diode measurement. Relative output factors compared to 2% for the largest field available for each applicator. Output with cut‐outs, relative to the field with no cutout, also agreed within 2%. Conclusion: EGSnrc is sufficiently accurate for commissioning electron beams, given a good match to a few carefully done measurements for the 40×40 cm field with no applicator. This demonstrates the use of Monte Carlo simulation to commission electron beams, with only a few measurements. Measurements needed include the 40×40 cm central axis depth dose curve and cross‐plane and in‐plane profiles at dmax, and the ROF for the largest field of each applicator for all beam energies. Output and dose distributions for small and large fields for each applicator should be measured to verify the accuracy of the simulation.

SU‐FF‐T‐293: Inter‐Linac and Intra‐Linac Variability of X‐Ray and Electron Dose Distributions

Purpose: To determine the maximum expected variation in fluence, differential in energy and position, with an accuracy of 1% for a single linac model, and eventually, combining with Monte Carlo beam models, to develop gold standard beam model benchmarks. Method and Materials: The 6 and 18 MV beams of the Siemens Primus linear accelerator (linac) were tuned through the manufacturer acceptance range in field flatness by adjusting beam energy. Dose profiles were measured at the low, high and average energy in this range for small, medium and large field sizes to quantify the intra‐linac variations caused by energy adjustment. The 6 and 18 MV x‐ray beams, and 6–21 MeV electron beams on three Primus machines, with the same treatment head, were tuned to give the same central axis depth dose curves to better than 0.5% for the largest fields available: 40×40 cm for photons, and 25×25 cm for electrons. Profiles were measured on these linacs to investigate the inter‐linac variations. Results: For the intra‐linac variations, the energy tuning has the largest effect of 7% at 10 cm on beam penetration for small fields. Scattering in the field reduces this effect for larger fields. Wedged‐field profiles are affected more than the open field, due to the dependence of transmission on energy. For inter‐linac variation, profiles measured from three different machines vary more than 2% due to beam tuning differences, lateral displacement of flatteners or scattering foils, and differences in position of components relative to the source. Conclusion: The range of inter‐linac and intra‐linac variability has been measured on a Primus linac and is clinically significant. This data will be used with Monte Carlo treatment head simulation to calculate gold standard beam model benchmarks for the development of tunable beam models of high accuracy and detail in the dose‐critical components of fluence.