Session 3A: Radiation Dosimetry - Lombardy/Umbria
le 7 juin 2024 from 13h00 CDT to 14h30 CDT
Scientific Session 3A – Radiation Dosimetry
Friday, June 7, 2024, 13:00-14:30
Scientific Session 3A: Radiation Dosimetry– Presentation 1
Shining light on microdosimetry: A novel system for micron-scale analysis of energy deposition.
Prarthana Pasricha, Connor McNairn, Bryan Muir, Wanye Gao, Andrea Payne, Edana Cassol, Vinita Chauhan, Sanjeena Dang, Andrew Jirasek, Jeffrey L. Andrews, Sangeeta Murugkar, Rowan M. Thomson
Carleton University, National Research Council of Canada, University of British Columbia
Purpose: Develop a novel system for micron scale analysis of energy deposition in irradiated radiochromic film (RCF) using Raman Spectroscopy (RS) and Monte Carlo (MC) simulations.
Methods: Samples of RCF are irradiated to doses of 0.003 – 2 Gy with a 6 MV photon beam. Raman spectra are obtained using two independent systems, featuring varying spot sizes (1 – 30 μm) and depths of focus (6 – 34 μm) suitable for cell-scale targets. Mean intensities and standard deviations of active layer Raman peaks are determined for each irradiated sample to create a dose response curve. In parallel, MC simulations representing experimental irradiations are conducted to generate distributions of specific energy (z, energy imparted per unit mass), scored in cubic voxels with sizes matching RS focal volumes, considering varying mean specific energies (z̄, absorbed dose).
Results: The dose response curve is linear, with uncertainties of 7.7 to 12%. Significant variations are seen in intensities in Raman response, likely due to the film inhomogeneity. Specific energy distributions reveal that for lower doses, a higher fraction of voxels experiences no energy deposition, resulting in skewed energy distributions and larger relative standard deviations (σz/z̄) in specific energy across voxels. The σz/z̄ ratio is ten times greater at ~3 mGy than at 500 mGy, emphasizing relevance of microdosimetric considerations at lower doses.
Conclusion: Integrating RS with MC simulations for cell-scale energy deposition studies shows promise. Ongoing efforts focus on developing advanced machine learning techniques to correlate measurement and simulation results, with potential of direct translation to cell systems.
Scientific Session 3A:Radiation Dosimetry– Presentation 2
Development of a Model-Based Water-Equivalent EPID Dosimetry ApplicationA post-implant dosimetry simulator for permanent breast seed implant brachytherapy
Ivan Kutuzov, Dr. Ryan Rivest, Dr. Eric van Uytven, Dr. Boyd McCurdy
CancerCare Manitoba / University of Manitoba
Purpose: To extend a model-based dose calculation algorithm previously validated for a-Si EPID detectors to predict water-equivalent dose images at the EPID plane and three-dimensional dose distributions in a water phantom.
Methods: Water-equivalent dose deposition kernels were Monte Carlo simulated using the geometry of an ion chamber array (MatriXX, IBA Dosimetry) and integrated into the existing model. The new image prediction capability was validated by comparing the predicted images against in-air MatriXX measurements. Ten geometric fields and ten clinical fields were used for the extended model validation. Furthermore, the dose calculation capability was used to predict a 3D dose distribution from a 10×10 cm2 calibration field in a water tank. The percentage depth dose (PDD) and beam profiles at depth of maximum dose (14 mm) from the predicted dose distribution were compared with the measurements made in a similarly configured water tank.
Results: The comparison of the planar images demonstrated pass rates 95.9%-98.2% using a 2% criterion for all geometric fields and pass rates 89.2-96.3% using a 2% criterion for modulated fields. The predicted PDD agreed with the measurements to within 1% beyond the first 5 mm of depth in water. The predicted beam profiles demonstrated 100% pass rate using 2%/2mm criterion between the prediction and water tank measurement.
Conclusions: An existing physics-based radiation transport model was extended to predict water-equivalent dose images in a commercial planar detector and volumetric dose distributions in a commercial scanning water tank. The extended predictive capability was validated with a variety of incident fields.
Scientific Session 3A:Radiation Dosimetry– Presentation 3
Characterization of discretely excited radial parallel plate transmission lines for use in a dielectric wall accelerator.
Morgan Maher, Chris Lund, Julien Bancheri, Jason Yuan, Andrew Currell, David Cooke, Jan Seuntjens
McGill University, University of Toronto Princess Margaret Cancer Centre
Purpose: Proton therapy is a highly effective treatment modality, and the dielectric wall accelerator may present a means of developing an affordable proton therapy device. This work computationally and experimentally studies the electromagnetic characteristics of discrete excitations, by means of nanosecond-scale voltage pulses, in radial parallel plate transmission lines (RPPTL) for electric field generation in a dielectric wall accelerator.
Methods: Simulations were used to study how number of excitation points impact the electric field generated at the beam pipe by RPPTL. An experimental setup was developed to study the impedance of the discrete excitation points, and two nano-second scale pulse generating circuits were examined as possible excitation sources for studying RPPTL .
Results:This work found that the number of excitation points used to excite a RPPTL impacts the field strength, field uniformity, and frequency spectrum of the pulses measured at the inner radius. Additionally, early work suggests that the impedance at the outer radius is impacted by the size of the contact point with the RPPTL thus alleviating concerns about low impedance and a prohibitively high power draw for radial lines.
Conclusions: This information will be used to inform upstream circuitry requirements, specifically with regards to the peak voltage and temporal profile of the injected pulses. Additional computational studies will expand on the work show here to study the impact of port size and the field behaviour as the maximum number of excitation points is achieved. Future experimental work will aim to confirm the impedance at the excitation points.
Scientific Session 3A:Radiation Dosimetry– Presentation 4
Application of TOPAS in Dose Calculations of Low Energy X-ray Therapy Machines.
Shahin Ahmadi, Thalat, Monajemi
Dalhousie University
TOPAS (TOol for PArticle Simulation) is a Monte Carlo tool initially designed for simulations of proton therapy machines. This study aims to test TOPAS’s applicability and accuracy in simulations of an Orthovoltage therapy machine. Specifically, this study used the Xstrahl 300 Orthovoltage machine as a subject for simulation, with evaluations conducted across various parameters and energy levels. The simulations involved calculating spectra, Half-Value Layers (HVLs), Percentage Depth Doses (PDDs), dose profiles, and backscatter factors for 100, 180, and 300 kVp energy levels. Furthermore, an assessment of the 3D dose distribution on a cranial CT was performed for a 5cm diameter cone. Where applicable, comparisons against measurements or other simulations are presented. The study results indicate a high level of agreement between TOPAS simulations and established calculations. Spectra calculations align well with SpekCalc calculations, demonstrating consistency, though some divergence is noted in fluorescent peaks. HVLs, PDDs, and dose profiles exhibit agreement with clinical measurements, underscoring the accuracy of TOPAS in simulating the behavior of the Orthovoltage therapy machine. Backscatter factors for four field sizes (1, 3, 5, and 10cm diameter) and all energies show agreement (difference <2%) with published data. The use of lead cutouts, visualization of dose distributions, and DVHs on clinical CT sets are accessible functionalities. TOPAS is a robust Monte Carlo simulation tool for an Orthovoltage treatment unit and could allow accessible visualization of 3D dose distributions in clinical plans.
Scientific Session 3A:Radiation Dosimetry– Presentation 5
Implementation of generalized 4-D Monte Carlo simulations in EGSnrc.
Reid Townson, Alex Demelo
National Research Council Canada, Carleton University
Purpose: In radiotherapy and medical imaging, motion of the radiation source, detector device and the radiation target are typically part of the treatment or imaging technique. To enable the Monte Carlo modelling of the effect of dynamic components, consideration of the time domain was incorporated into EGSnrc. This allows for arbitrary, continuous motion of geometries, chronological simulation observation, and synchronization of simulation elements.
Methods: The C++ class library egs++ was modified in EGSnrc to include a time index as a particle-specific quantity. A new feature was added that introduces motion for any geometry object, allowing for arbitrary time-dependent translations and rotations. Support was added for synchronized motion of particle sources, and recording particle tracks with time indices for 4-D visualization. The egs_view 3-D visualization tool was modified to include time evolution and inspection of tracks within time windows.
Results: The simulation results of cumulative quantities after simulation of dynamic motion were investigated for a range of cases. Tests of validity passed at the appropriate level. It’s worth noting that uniform generation of source particles over the time range can lead to statistical uncertainty considerations when the simulation includes variable-speed motion.
Conclusions: Generalized 4-D simulations including complex geometrical and source motion have been implemented in EGSnrc. This provides valuable new functionality for the modelling of modern radiotherapy, radiation imaging and related technologies.
Scientific Session 3A:Radiation Dosimetry – Presentation 6
Validation of A Novel Calorimeter Design for Synchrotron Produced X-ray Beams.
Malcolm McEwen, Islam, El Gamal, Jean, Dessureault
National Research Council Canada, Carleton University
Purpose: The performance of a novel aluminum-based calorimeter designed to measure the dose rate of a synchrotron produced X-ray beam has been independently validated through an air-kerma based comparison.
Methods: The air-kerma rate of 4 different monochromatic X-ray beams in the 80-140 keV range produced by the Canadian Light Source was evaluated. The beamline produces an X-ray beam with a non-uniform field size on the order of 1 mm and a peak dose rate of 1 Gy/s. Given the unique characteristics of such a beam, a purpose-built aluminum-in-vacuum calorimeter was developed. EGSnrc was used to convert the absorbed dose to the aluminum calorimeter to air-kerma. Measurements were then performed for the same beams using a PTW microDiamond (PTW 60019, Freiburg, Germany) detector with an air-kerma calibration coefficient directly traceable to Canadian primary standards.
Results: The microdiamond had an estimated uncertainty of 3.9 % (k=1) for the synchrotron air-kerma rate, whilst the aluminum calorimeter had an uncertainty of 0.9 % (k=1). The results showed agreement within the combined uncertainties for all beam energies.
Conclusion: These results serve as a preliminary validation of the novel calorimeter approach to determine the central axis air-kerma or absorbed dose rate of a synchrotron beam. The next step is a comparison with another national absorbed dose standard, likely at a different synchrotron facility. In the meantime, these measurements provide confidence in the output of the CLS BMIT beamline for pre-clinical and dosimetric investigations.
Scientific Session 3A:Radiation Dosimetry – Presentation 7
Reference dosimetry in 1.5T and 0.35T MR-linacs: experimental determination of magnetic field quality conversion factors.
Nathan Orlando, Jennie Crosby, Carri Glide-Hurst, Wesley Culberson, Brian Keller, Arman Sarfehnia
Odette Cancer Centre, Sunnybrook Health Sciences Centre, Carbone Cancer Center, University of Wisconsin-Madison
Purpose: The magnetic resonance (MR) field produced by MR-linac devices can influence charge collection in air-filled ionization chambers, necessitating the characterization of chamber response to enable accurate output calibration. The purpose of this work was to experimentally quantify the magnetic field quality conversion factor, kB,Q, for standard and MR-conditional ionization chamber models in 1.5T Elekta Unity and 0.35T ViewRay MRIdian MR-linacs.
Methods: Eleven standard and seven equivalent MR-conditional ionization chamber models were evaluated. A cross-calibration method was used to experimentally determine kB,Q by taking the ratio of absorbed dose measured with a reference Exradin A1SL chamber under reference conditions to corrected output measured with each test chamber at the same point of measurement. The dependence of ionization chamber orientation relative to the static magnetic field was assessed for the high-field 1.5T Elekta Unity system.
Results: Compared to available Monte Carlo simulation and experimental data, our measured kB,Q values largely agreed to within uncertainty. kB,Q values measured for corresponding standard and MR-conditional ionization chamber models agreed to within uncertainty, demonstrating the suitability of these conventional models for use in MR-linacs. The magnetic field influence on charge collection was minimized with the chamber axis parallel or anti-parallel to the magnetic field.
Conclusions: This work provides critical experimental validation of Monte Carlo simulated magnetic field quality conversion factors. kB,Q values determined using our experimental method will serve as an important reference for MR-linac reference dosimetry protocols, and ultimately represent an important step towards accurate output calibration of MR-linac systems.
Scientific Session 3A:Radiation Dosimetry – Presentation 8
Characterization of a Micro-Ionization Chamber for Electron FLASH Dosimetry.
Jyothis George, Tania Karan, Peter Petric, Christopher Johnstone, Bryan Muir, Malcolm McEwen, James Renaud,, Cheryl Duzenli,
University of British Columbia, Vancouver Centre, National Research Council Canada
Purpose: To characterize the performance of the IBA CC01 micro-ionization chamber for use in electron FLASH dosimetry by benchmarking it against alanine dosimeters.
Methods: The CC01 micro-ionization chamber's performance was assessed for dose rate linearity, reproducibility, stem effect, percent depth dose, and dose-per-pulse dependencies. The ion chamber and alanine dosimeters were irradiated in closely matched configurations under ultra high dose rate conditions using a 10 MeV FLASH electron beam with a 9 MeV scattering foil at two different source-to-surface distances (SSDs). Pulse repetition frequency and chamber stem effect were investigated, and depth dose curves were obtained with corrections applied for effective point of measurement and density differences.
Results: The study found that the CC01 chamber exhibited excellent pulse repetition rate linearity, with an R2 value surpassing 0.9999. Reproducibility was confirmed through sequential beam delivery, with errors reduced by introducing a one-minute interval between irradiations. Analysis of the ion chamber stem effect revealed dose response varied by ≤ 0.44%. Discrepancies in dose-per-pulse measurements between the CC01 chamber and alanine dosimeters were observed. Correction factors were applied to align the chamber response with alanine measurements, ensuring improved accuracy and reliability in FLASH dosimetry applications.
Conclusions: With a well characterized correction matrix, we have shown that a CC01 ion chamber can be used to determine dose in real time during electron FLASH radiotherapy delivery. Benchmarked against dose-rate independent alanine dosimeters, the CC01 chamber shows potential for FLASH dosimetry.
Scientific Session 3A:Radiation Dosimetry – Presentation 9
Monte Carlo Simulations for Validation of Varian TrueBeam 4 MV FFF Research Beam.
Alexandru Badalan, Isabelle St-Martin, Ermias Gete, Tony Popescu
BC Cancer Vancouver
Purpose: The purpose of this research is to validate the phase space data for a 4MV flattening filter free (FFF) beam generated with Varian’s VirtuaLinac simulation engine by comparing with measurements.
Methods: TrueBeam 4MV FFF phase space was simulated using VirtuaLinac, a Monte Carlo linac head simulation package developed by Varian Medical Systems. VirtuaLinac is based on Geant4 and simulates the treatment head by incorporating a detailed model of the linac based on manufacturer’s drawings. The phase space from the simulation was recorded immediately upstream of the linac jaws and used as a virtual source in the subsequent simulation with BEAMnrc and DOSXYZnrc. BEAMnrc was used to simulate the linac head below the monitor chamber, and DOSXYZnrc was used for phantom dose calculations. Dose calculations in a water phantom were performed for fields ranging from 3x3 cm2 to 30x30 cm2. Percent depth doses (PDDs), transverse profiles and output factors were calculated and compared with ion chamber measurements for all fields measured.
Results: Monte Carlo simulations and measurements agreed within 1.5% for PDDs in the regions excluding the buildup, and within 2.0% for transverse profiles excluding the penumbra region. The agreement between calculated and measured output factors was within 0.6%. These results indicate that the 4 MV FFF phase space data is sufficiently accurate for use in dose calculation.
Conclusion: We have demonstrated the feasibility of using Varian’s cloud-based Monte Caro linac simulator VirtuaLinac with BEAMnrc and DOSXYZnrc. Good agreement was observed between Monte Carlo simulation and measurement for open fields.