Simulated Percent Depth Dose Research Articles

Validation of the Elekta Synergy Platform Linac at 6 MV Photon Beam using PRIMO Monte Carlo Software.

The objective was to validate the initial beam parameters of the Davao Doctors Hospital's 6 MV Elekta Synergy Platform linac, which performs to the specification of the commissioning data per our records using the gamma-index analysis toolkit integrated inside PRIMO software. In PRIMO, a sequence of optimization processes is performed, in which the measured and simulated percent depth dose (PDD) and lateral beam profiles at various depths are compared, using the stringent gamma-index passing rate at 1%/1 mm criteria (GPR11). Using four fields of sizes 3 cm × 3 cm, 4 cm × 4 cm, 5 cm × 5 cm, and 10 cm × 10 cm, the dose is calculated on a water phantom measuring 16.2 cm × 16.2 cm × 30.0 cm. In addition, one field of size 20 cm × 20 cm is used on a 50.0 cm × 50.0 cm × 30.0 cm water phantom with a bin size of 0.2 cm × 0.2 cm × 0.2 cm at a source-surface distance of 100.0 cm. For PDD and beam profiles comparison at the largest field size, the 6.5 MeV initial electron beam energy, 0.25 MeV full-width-half-maximum energy, 0.20 cm focal spot size, and 3° beam divergence tuned configuration yield GPR11 values of 94.0% and 97.7% (PRIMO PDD and lateral beam profile at 200 mm scan depth, respectively) with a statistical uncertainty of 2.9%. For lower field sizes, the GPR11 values are consistent at more than 90% for the PDD, whereas GPR11 values of 80.3% and 70.6% for the lateral beam profiles (at 15 mm and 200 mm scan depths) at 10 cm × 10 cm and 5 cm × 5 cm, respectively. The percentage difference between the measured and simulated PDD 20,10 ratios of not more than 2.45% is observed in all field settings. These tuned beam parameters are remarkably in agreement with the suggested beam parameters listed on the PRIMO website for the 6 MV Elekta linac which was optimized with a different set of measurements.

Using EBT3 Films and Monte Carlo Simulations to Determine the Percent Depth Dose of a New Y-90 Disc Source for Episcleral Brachytherapy

<h3>Purpose/Objective(s)</h3> A novel Y-90-disc beta source was recently developed for episcleral eye plaque brachytherapy. The purpose of this study was to determine the percent depth dose (PDD) of this new source using the EBT3 film measurements and Monte Carlo simulations. <h3>Materials/Methods</h3> Radiographic films of the same batch were calibrated using the 6MV beam of a linear accelerator, and used to measure the dose distribution of this Y-90-disc source in water. Monte Carlo simulations were performed to calculate the in-water dose distribution using the "GATE," a software based on a Monte Carlo simulation toolkit developed by the international OpenGATE collaboration. The simulation codes were first validated with the TG43, ICRU56, and ICRU72 results, and then used to simulate the radiation transport in the Y-90-disc source, its capsule and the surrounding water medium. The standard Y-90 beta spectrum from ICRU 72 was used, and the physics processes for beta and photon interactions with matters were all included. Three-dimensional dose distribution was simulated, from which the PDD curve along the central axis perpendicular to the disk surface was extracted and compared with that obtained from the film measurements. <h3>Results</h3> The measured PDD curve, normalized to the point at 1 mm above the source capsule, agreed well with the simulated PDD curve with an average difference of -0.27±1.5%. Since the eye plaque brachytherapy might be prescribed to a point with a low PDD (e.g., 5%-10%), we further analyzed the agreement in this area. The difference between the measured and simulated PDDs gradually increases for depths larger than 3.5 mm (measured 16.7% vs. simulated 16.6 %). At 5 millimeters (5.9% vs. 4.3%), the simulated PDD is 26.2% lower than the measured, which we believe is still a clinically acceptable agreement. For larger depths, the difference increased quickly to a level that we could not confidently conclude that the measured PDD is accurate. <h3>Conclusion</h3> PDD of this new Y-90 source was successfully measured with the EBT3 films and confirmed by Monte Carlo simulations for depth up to 5 mm from the surface of the plaque. Large discrepancies were observed for depths beyond 5 mm. More studies are needed to confirm the PDD if the treatment needs to be prescribed to a depth larger than 5 mm.

Evaluation of the Varian TrueBeam™ 6 MV phase-space files for the Monte Carlo simulation in small field dosimetry

Background: The Monte Carlo (MC) simulation is an effective tool for determining the absorbed dose in small field sizes. To calculate accurate results, the MC simulation requires precise geometric and material descriptions of the linear accelerator head. Due to proprietary information issues, the description of the Varian TrueBeam™ linear accelerator (Varian Medical Systems, Palo Alto, CA) head geometry and material information are not available. Instead, the manufacturer provided a phase-space file just above the jaw for each photon energy level. Although several studies have validated the accuracy of this phase-space file, to the best of our knowledge, there are no reported data for a small field size (&lt;2x2 cm2) of 6 MV photon beams. Objectives: The purpose of this study was to evaluate the Varian TrueBeam™ phase-space file of the 6 MV photon beam provided by the manufacturer for the Monte Carlo (MC) simulation in small field dosimetry. Materials and methods: The TrueBeam™ linear accelerator was simulated using an EGSnrc MC code with a Varian phase-space file as the input. The simulation was compared with the measurement using percent depth dose (PDD) and beam profile, and the field output factor (FOF) for the 0.6x0.6, 1x1, 2x2, 3x3, 4x4, 6x6, and 10x10 cm2 field sizes. Results: The agreement between the measurements and simulated PDD data was under 2.2% beyond the buildup region. The distance to agreement (DTA) in the buildup region was within 1.0 mm. The simulation data presented identical profiles with the measurement within 1.0% of the dose difference or 1.2 mm of the DTA. The mean dose difference in the radiation field was ≤1.5% for the ≥1x1 cm2 field size. The largest deviation was observed in the 0.6x0.6 cm2 inline beam profile. The deviation of the penumbra and full width at half maximum (FWHM) between simulation and measurement was &lt;2 mm. The agreement of the simulated and measured FOF was within 1.0%, except for the 0.6x0.6 cm2 field size. Conclusion: Overall, the MC simulation demonstrates data that is consistent with the measurement for the ≥1x1 cm2 field sizes. These data assure that the 6 MV Varian phase-space file can be used as a radiation source for accurate MC dose calculation in a small field. However, a large discrepancy in beam profiles was observed at the 0.6x0.6 cm2 field size due to the different primary source sizes among TruebeamTM machines.

Characterizing a PTW microDiamond detector in kilovoltage radiation beams.

The aim of this work was to characterize the dosimetric properties of the PTW microDiamond (60019) single crystal synthetic diamond detector (DD) in kilovoltage x-ray beams. The following characteristics were addressed in this study: required preirradiation dose, dose-rate linearity, energy dependence, and percent depth dose response of the DD. UWADCL x-ray beams, characterized by NIST-traceable ionization chambers, were used in this study. Preirradiation dose required by the DD, in order to stabilize the detector's response to within 0.1%, was quantitated. Dose-rate dependence was also investigated using the UW250-M and UW50-M beams, where the dose rate was varied by changing the tube current. and calibration coefficients for all the available M series beams at UWADCL were obtained to determine the energy dependence of the DD, Diode E, Diode P, and P11 parallel-plate ionization chamber. A custom-built water tank was utilized to measure the percent depth dose (PDD) response of the DD, Diode E, Diode P, and P11 chamber in UW250-M, UW100-M, and UW50-M beams. The measured PDD response of the detectors was compared with the simulated PDD data using EGSnrc Monte Carlo code. A 1.5Gy dose-to-water or air-kerma was found to be sufficient for the given DD's response to stabilize to within 0.1% in all of the beams used in this study. The dose-rate dependence parameter, Δ, was found to be 1.00±0.02 and 1.016±0.05 for the UW250-M and UW50-M beams, respectively. Relative to the Co calibration coefficients, the DD was found to under-respond relative to calculated absorbed dose to water response and over-respond relative to the calculated air-kerma response in the M-series beams. Agreement of 1.5% was found between the measured PDD values and Monte Carlo simulated PDD values for UW250-M, UW100-M, and UW50-M beams. In order to stabilize the response, the DD needs a preirradiation dose, which is unique to every DD. A linear relationship between detector response and dose rate was found within the evaluated uncertainty. An energy dependence of the DD was studied, which is more pronounced in the low-energy beams and can be partially attributed to the metal contact material around the sensitive volume of the DD. Overall, the DD was found to be suitable for kilovoltage x-ray dosimetry.

Validation of the dosimetry of total skin irradiation techniques by Monte Carlo simulation.

PurposeTo validate the dose measurements for two total skin irradiation techniques with Monte Carlo simulation, providing more information on dose distributions, and guidance on further technique optimization.MethodsTwo total skin irradiation techniques (stand‐up and lay‐down) with different setup were simulated and validated. The Monte Carlo simulation was primarily performed within the EGSnrc environment. Parameters of jaws, MLCs, and a customized copper (Cu) filter were first tuned to match the profiles and output measured at source‐to‐skin distance (SSD) of 100 cm where the secondary source is defined. The secondary source was rotated to simulate gantry rotation. VirtuaLinac, a cloud‐based Monte Carlo package, was used for Linac head simulation as a secondary validation. The following quantities were compared with measurements: for each field/direction at the treatment SSDs, the percent depth dose (PDD), the profiles at the depth of maximum, and the absolute dosimetric output; the composite dose distribution on cylindrical phantoms of 20 to 40 cm diameters.ResultsCu filter broadened the FWHM of the electron beam by 44% and degraded the mean energy by 0.7 MeV. At SSD = 100 cm, MC calculated PDDs agreed with measured data within 2%/2 mm (except for the surface voxel) and lateral profiles agreed within 3%. At the treatment SSD, profiles and output factors of individual field matched within 4%; dmax and R80 of the simulated PDDs also matched with measurement within 2 mm. When all fields were combined on the cylindrical phantom, the dmax shifted toward the surface. For lay‐down technique, the maximum x‐ray contamination at the central axis was (MC: 2.2; Measurement: 2.1)% and reduced to 0.2% at 40 cm off the central axis.ConclusionsThe Monte Carlo results in general agree well with the measurement, which provides support in our commissioning procedure, as well as the full three‐dimensional dose distribution of the patient phantom.

A practical method for quantifying dose in bone and lung using TLDs when using 6 and 15 MV photon beams

This paper presents a practical method for converting dose measured with thermoluminescent dosimeters (TLD) to dose in lung and bone for 6 MV and 15 MV photon beams. Monte Carlo (MC) simulations and Burlin cavity theory calculations were performed to calculate , the dose-to-TLD to dose-to-medium conversion factor. A practical method was proposed for converting TLD-measured-dose to dose-in-medium using the TLD dose calibration in water and dose-to-medium to dose-to-water conversion factor. Theoretical calculations for were performed using photon spectrum weighted parameters and were compared with MC simulations. Verification of the proposed method was done using phantoms having either bone or lung equivalent slabs stacked in between solid water slabs. Percent depth dose (PDD) curves were measured using 0.089 cm thick LiF:Mg,Ti (TLD-100) dosemeters placed at various depths within these phantoms. They were then corrected with factors using the proposed dose conversion method, and were compared with the MC simulations. For 6 MV beam, the MC calculated factors were 0.942 and 1.002 for bone and lung, and for 15 MV it was 0.927 and 1.005 for bone and lung, respectively. The difference between the MC simulated and spectrum weighted theoretical factors were within 3% for both lung and bone. The PDD curves measured with TLD-100 chips that were corrected using the proposed method agreed well within 1.5% of the MC simulated PDD curves for both the water/lung/water and water/bone/water (WBW) phantoms. The dose-to-medium correction using MC simulated is convenient, easy, and accurate. Therefore, it can be used instead of Burlin cavity theory, especially in media with high atomic numbers such as bone for accurate dose quantification.

SU‐E‐T‐107: An Investigation Into Attenuation Effects On the Energy Spectrum for 137Cs Using Monte Carlo Techniques

Purpose:The calibration of radiation protection instrumentation including ionization chambers, scintillators, and Geiger Mueller (GM) counters used as survey meters are often done using 137Cs irradiators. During calibration, irradiators use a combination of attenuators with various thicknesses to modulate the beam to a known air‐kerma rate. The variations in energy spectra as a result of these attenuators are not accounted for and may play a role in the energy‐dependent response of survey meters. This study uses an experimentally validated irradiator geometry modeled in the MCNP5 transport code to characterize the effects of attenuation on the energy spectrum.Methods:A Hopewell Designs G‐10 137Cs irradiator with lead attenuators of thicknesses of 0.635, 1.22, 2.22, and 4.32 cm, was used in this study. The irradiator geometry was modeled in MCNP5 and validated by comparing measured and simulated percent depth dose (PDD) and cross‐field profiles. Variations in MCNP5 simulated spectra with increasing amounts of attenuation were characterized using the relative intensity of the 662 keV peak and the average energy.Results:Simulated and measured PDDs and profiles agreed within the associated uncertainty. The relative intensity of the 662 keV peak for simulated spectra normalized to the intensity of the unattenuated spectra ranged from 0.16% to 100% based on attenuation thickness. The average energy for simulated spectra for attenuators ranged from 582 keV with no attenuation to 653 keV with 5.54 cm of attenuation. Statistical uncertainty for MCNP5 simulations ranged from 0.11% to 3.69%.Conclusion:This study successfully used MCNP5 to validate a 137Cs irradiator geometry and characterize variations in energy spectra between different amounts of attenuation. Variations in the average energy of up to 12% were determined through simulations, and future work will aim to determine the effects of these differences on survey meter response and calibration.

SU-E-T-573: Monte Carlo Simulated Peripheral Doses in SRS of the Brain

Purpose: In arc based brain Stereotactic Radiation Surgery (SRS), a major concern is the accuracy of the peripheral doses to organs close to the treated lesions. This concern is more pronounced when planning software utilizes Pencil Beam algorithm. To evaluate the out-of-target radiation doses to Organs At Risk (OAR) the predicted organ doses by BrainLab Treatment Planning System (TPS) were compared with doses acquired by a Monte Carlo simulation. Methods and Materials: An arc based brain SRS treatment plan was developed in BrainLab® TPS to be treated on the Novalis® Varian linear accelerator, using 6 MV photon arcs with a 20 mm cone. The doses to different points of interest, representing major OAR were investigated. The photon spectrum in air for 20 mm cone at 100 SSD was designed and verified for the simulation with EGSnrc Monte Carlo particle transport code (DOSRZnrc user code). The resulting Percent Depth Dose (PDD) and dose profile at 75 mm depth were confirmed with respective measurements in a water phantom. Each arc will be approximated by summing simulated discrete angled beams following the arc path and accounting for the difference of their depths to the target. The doses of both calculations will be normalized to an ion chamber measurement at the isocenter. Results: The greatest difference in the PDD was less than 1%. MC simulated profile at 75 mm depth appeared to be less than 1mm wider than the measured data in the penumbra region. Conclusions: A method for simulating peripheral doses in SRS was developed. The agreement in the measured and simulated PDD and profile provide a basis for successfully utilizing this method and comparing the calculated peripheral doses by the TPS and the simulated doses by the Monte Carlo technique.

SU‐FF‐J‐157: A Monte Carlo Model for Small Animal Stereotactic Irradiation

Purpose: To develop a Monte Carlo model of the treatment beam of a small animal stereotactic irradiator. Method and Materials: A Monte Carlo source model has been created for a custom small animal stereotactic irradiation device. X‐ray generation is performed by an XRAD‐320 x‐ray tube (Precision X‐Ray Inc., North Branford, CT) operated at a potential of 250 kVp. Determination of the un‐collimated energy spectrum from the XRAD‐320 at 250 kVp was calculated using the “SpekCalc” GUI program by Poludniowski and Evans. These calculation results were used to form the photon source probability distribution in MCNPX (Los Alamos National Laboratories, Los Alamos, NM). A custom, in‐house field collimation system is used in conjunction with the x‐ray source to collimate the source into 1.0, 3.5, 5.0, 7.5, and 10.0 mm stereotactic beams. This collimation system has been incorporated into the MCNPX beam model. Percent Depth Dose (PDD) curves from the irradiator were measured using radiochromic film (Gafchromic EBT, International Specialty Products, Wayne, NJ) with all collimator apertures and two treatment SSDs of approximately 20 and 25 cm. Results: PDD curves created with film measurements and Monte Carlo simulation results were compared for agreement. Simulated PDD curves were found to agree with measured data for all collimator sizes and source‐to‐surface distances (SSDs) for 2%/2 mm normalization criteria. Conclusion: The agreement between simulated data and repeatedly measured PDD data suggests accuracy in this application of the SpekCalc GUI program. An accurate beam model of this small animal irradiator in conjunction with ongoing micro CT‐based “patient” modeling will provide a system for Monte Carlo‐based clinical and investigational research of small animal stereotactic radiotherapy.