Session: Dosimetry - Salle de bal et foyer

June 23, 2022 from 11:00am EDT to 12:00pm EDT

Scientific Session – Radiation Dosimetry
Thursday, June 23, 2022, 11:00-12:00

Scientific Session 1: Radiation Dosimetry – Presentation 1

Modelling an Orthovoltage X-ray Machine with EGSnrc

Dinindu Gunasekara, Lindsay Beaton-Green, Ruth Wilkins, Frederic Tessier
Carleton University, Health Canada, National Research Council Canada

Purpose: Biodosimetry techniques employ a dose calibration curve to convert biological damage induced by harmful ionizing radiation into an amount of dose received by an individual. At Health Canada, these curves are produced by irradiating biological samples with X-rays, but an exposure could consist of other types of radiation. Using a computational model, one could assess the effect of using a more realistic source of radiation. The goal of this study is to model the X-ray machine currently used to produce these curves and validate various approaches to creating the model against laboratory results. This serves as a first step towards modelling curves for different sources.

Methods: In the laboratory, samples are irradiated inside a phantom composed of water equivalent material using the XRAD-320 (Precision X-ray) cabinet X-ray machine. The computational model of this setup was created in EGSnrc, using the BEAMnrc and egs_chamber applications. Dose outputs from an ion chamber were measured in the laboratory for different scenarios and compared to those calculated in EGSnrc.

Results: X-ray spectra produced by each model were generated and compared. They were found to produce equivalent spectra, but the BEAMnrc model was more efficient, on account of the directional bremsstrahlung variance reduction technique. Validating both models for different phantom setups produced favorable results but failed when comparing doses output through different levels of filtration.

Conclusion: Both models are capable of reproducing lab results within uncertainty for the phantom comparison however, further work is required to investigate the discrepancy seen in the filtered spectra.


Scientific Session 1: Radiation Dosimetry – Presentation 2

Beam matching for small-field dosimetry applications using accelerator solenoid current and a miniature plastic scintillation detector.

Luc Gingras, Frédéric Beaulieu, Magali Besnier, Benjamin Côté, Danahé Leblanc, Luc Beaulieu, Louis Archambault
CHU de Québec - Université Laval, MedScint inc

Purpose: The goal of this work is to determine the optimal accelerator solenoid current (ASOL) that minimizes the spread of measured small field output factor (OF) values between a series of machines.

Methods: Small field OF measurements of 6 MV flattening filtered beams from 3 Varian TrueBeam linear accelerators were performed using a 1 mm diameter by 1 mm length plastic scintillation detector from MedScint HS-RP200 research platform. The measurements were acquired at 90 cm SSD and 10 cm depth for 4 field sizes (0.5x0.5 cm2 MLC defined, 0.6x0.6, 1x1 and 6x6 cm2 jaw defined) using an 11-points technique enabling precise detector centering and effective field size measurement. Series of measurements were taken at different ASOL values for each field size and machine combination.

Results: OFs normalized to the 6x6 cm2 field and corrected for field size differences between machines are presented following their ASOL dependency. The initial relative spread of corrected OF between machines were respectively 0.28%, 4.47% and 1.57% for 0.5, 0.6 and 1 cm field sizes. Manually optimizing ASOL values for each machine, in order to reduce the overall corrected OF spread, resulted in a minimized relative spread of 0.11%, 0.48% and 0.59% for respectively 0.5, 0.6 and 1 cm field sizes.

Conclusion: Small field dosimetry characteristics are highly sensitive to beam focal spot size. The possibility to optimize accelerator focusing coil current to reduce OF and penumbra width spread enables new avenues in beam matching of series of machines, especially for SRS and SBRT techniques.


Scientific Session 1: Radiation Dosimetry – Presentation 3

Skin dose investigations on a 0.5 T inline rotating biplanar linac-MR

Patricia Oliver, Eugene Yip, Michael Reynolds, Ben Burke, Gino Fallone, Brad Murray, David Sinn, Keith Wachowicz, Brad Warkentin, Shima Y Tari
Alberta Health Services & University of Alberta, MagnetTx

Purpose: Considering a rotating biplanar linac-MR, which has a 0.5 T magnetic field parallel to the beamline (“inline” arrangement), we investigate the effect of the magnetic field on skin dose using measurements and Monte Carlo (MC) simulations.

Methods: Surface dose measurements in solid water are performed using radiochromic film and a parallel plate ion chamber. An MC model of our linac-MR is developed using BEAMnrc. Utilizing this BEAMnrc model as a radiation source, the EGSnrc user-code egs_chamber is used to calculate dose within computational phantoms. For an anthropomorphic computational phantom with a 2 mm-thick skin layer, three beam arrangements are investigated. We consider magnetic field on and off for measurements and simulations.

Results: Comparing MC simulations with radiochromic film measurements, for a 20x20 cm2 field, gamma pass rates (criteria: 2%, 2 mm) of 100% and 95% are obtained with magnetic field on and off, respectively. With field on, D0.2cc within the first 2 mm is 75% (normalized to Dmax), compared to 46% with field off. For the anthropomorphic phantom, MC-calculated skin D0.2cc values are increased by 9%, 9% and 3% when the magnetic field is on compared to when it is off, for medial-lateral tangents, anterior-posterior beams and a five-field arrangement, respectively. Ion chamber measurements demonstrate that the relative increase in surface dose due to the magnetic field decreases with increasing field size.

Conclusions: For the three beam arrangements considered, the magnetic field of our inline rotating biplanar linac-MR causes an increase in skin D0.2cc of 3% to 9%.


Scientific Session 1: Radiation Dosimetry – Presentation 4

Influence of accumulated dose on the sensitivity of a clinically-operable pyroelectric film calorimeter

James Renaud, Bryan Muir
National Research Council Canada

Purpose: The radiation hardness of low-Z materials commonly used in dosimeters has been questioned with the increasing prevalence of ultra-high dose-per-pulse (i.e., FLASH) beams. Pyroelectric calorimeters have been developed to provide a direct means of dose determination in FLASH beams with the benefit of electrometer readout. This work evaluates the sensitivity of a pyroelectric polyvinylidene fluoride (PVDF) film calorimeter as a function of accumulated dose up to 11.25 kGy.

Methods: When subject to variable temperature, pyroelectric materials operate as current sources with a signal magnitude that is proportional to the rate of temperature change, hence a calorimeter. Thermal calibration was performed by heating and cooling an aluminum phantom containing the PVDF with a halogen lamp and a Peltier element, respectively, while monitoring its temperature with a calibrated thermistor. The PVDF was then irradiated using an 8 MeV electron beam at a rate of 1.5 Gy/s in steps of 2.25 kGy. After each irradiation, the PVDF was recalibrated to evaluate the thermal sensitivity as a function of accumulated dose.

Results: Thermal calibration data spanning rates of temperature change from ±20 mK/s, or up to 8.5 Gy/s equivalent dose rate, was linearly correlated to the PVDF current. The sensitivity of the PVDF initially increased by 13% after the first 2.25 kGy delivery but stabilized to within 1.5% RMS with doses up to 11.25 kGy.

Conclusions: Other than an initial increase in sensitivity, which warrants further investigation, there is no evidence of further radiation damage at 11.25 kGy for the low-Z PVDF calorimeter.


Scientific Session 1: Radiation Dosimetry – Presentation 5

Monte-Carlo simulation of electron beam collimation using parallel magnetic fields

Jacob Groeneveld, Charles Kirkby
Jack Ady Cancer Centre and University of Calgary

Purpose: A coil in place of an electron applicator could produce a magnetic field parallel to an electron treatment beam, reduce lateral electron deflection and enable dynamic field shaping via the MLC. This work determines the magnetic field strength necessary for an MLC-collimated electron beam to match the dose penumbra produced by standard electron cutouts.

Methods: Monte Carlo software PRIMO generated an electron source model for a Varian Clinac 2100. The  electron source was validated against water tank measurements to within 3%/2mm. Phase spaces were tallied downstream of the MLC and used in subsequent PENELOPE simulations where uniform longitudinal magnetic fields were introduced. Dose-to-water was tallied for a 12 MeV electron beam with MLC-shaped field sizes of 1x1, 2x2, 4x4, and 10x10 cm2, and magnetic field strengths from 0, 0.1, 0.2, 0.3, 0.4, 0.5 and 1 T.

Results: Simulated percent-depth-dose and dose profiles passed gamma analysis when compared to water tank measurements. For smaller field sizes (1x1, 2x2, 4x4), a magnetic field strength between 0.4 T and 0.5 T generated a penumbra comparable with that of their respective electron cutouts. At larger field sizes (10x10), we observed a simulated output field that was rotated relative to a 0 T field. For all field sizes we observed a reduction in projected field size.

Conclusions: Preliminary results indicate that the introduction of a parallel magnetic field of about 0.4 T to an MLC-collimated beam can produce penumbra widths comparable to those of electron cutouts, however the work has identified additional challenges.


Scientific Session 1: Radiation Dosimetry – Presentation 6

In-house machined copper as an alternative to cerrobend for custom cutouts in electron radiation therapy

Cameron Hough, Phil McGeachy
Tom Baker Cancer Centre - University of Calgary

Purpose: Fabrication of lead-based cerrobend cutouts for electron radiotherapy requires dedicated equipment and stringent safety protocols, which may be avoided with alternative materials such as copper. However, material-dependent interactions with high-energy electrons may modify the delivered dose, and must be characterized.

Methods: Equivalent cutouts are fabricated from annealed cerrobend and machined copper. Central axis percent-depth-dose and lateral dose profiles at d_max are measured using a Varian linac (TrueBeam sTX) in a water tank (PTW Beamscan) with a diamond detector (PTW microDiamond). Data were acquired for each cutout, as well as 15×15cm^2 open-field reference exposures, for five electron energies ranging from 6 to 20MeV to quantify relative dosimetric modulations. 

Results: Relative to open-field, cerrobend induces negligible (<0.2%) dose modulation within the therapeutic range (0<d≤R_90), but significantly increases dose in the penumbra and tail regions (7.5% at 9MeV). Conversely, copper increases dose within the therapeutic range (1% at 16MeV), but decreases penumbral/tail dose relative to cerrobend (-5.3% at 9MeV). Profiles show similar target coverage (penumbra widths (x50-x​_90) within 0.1cm), but hotter horn doses for copper (106%, vs. 103% for cerrobend at 20MeV), attributed to sharp edges from high-precision machining, and may be mitigated by smoothing cutout edges.

Conclusions: Since electrons are used to target shallow lesions, the copper-induced dose-enhancement within the therapeutic range, and dose-reduction beyond, is therapeutically beneficial. Since machining copper has comparable demands on time and labour as fabricating cerrobend, results from this study support the use of in-house machined copper cutouts for electron radiotherapy for suitably equipped clinics.


Scientific Session 1: Radiation Dosimetry – Presentation 7

Clinical Reference Dosimetry on the Inline 0.5T Rotating Biplanar Linac-MR System

Eugene Yip, Shima Y Tari, Michael Reynolds, David Sinn, Brad Murray, Gino Fallone, Patricia Oliver
Univeristy of Alberta & Cross Cancer Institute, MagnetTx Oncology Solutions

Purpose: Commissioning is underway for the world’s first clinical inline 0.5T linac-MR (LMR) system.  Potential dosimetric advantages of an inline magnetic field include minimal distortion to ion chamber response and insensitivity to chamber orientation.  This work describes and validates a reference dose calibration procedure for the LMR’s 6FFF beam.

Methods: TPR20,10-based calibration procedure is performed.  Beam quality factor at the calibration condition (kQmsr) is determined by measuring TPR20,10 and converting to %dd10x using Kalach and Rogers’ equation.  Magnetic field effect on response, kB, is assumed to be unity.  TG51-derived kQmsr and assumed kB values for the Exradin A12 chamber are validated against Monte Carlo (MC) simulations (EGSnrc).  Dosimetry is performed at d = 10cm using TG51 equations and correction factors.  Independent external validation of the calibration procedure is performed with IROC using both TLDs and OSLDs. The calibration procedure is repeated with magnet ramped down, with setup unchanged, to assess magnetic field effects.

Results: TPR20,10 is measured to be 0.632, corresponding to %dd10x of 63.8% and kQmsr of 0.9964 for the A12, which was within MC simulation uncertainty (0.9961±0.0015).  The calibrated dose was found to be within 1% of expected dose by IROC (TLD and OSLD).  Repeating the dosimetry procedure with field ramped down resulted in a dose difference of -0.13%, confirming a minimal magnetic field effect, in agreement with MC simulations.

Conclusion: Using the described procedure, the 0.5T LMR can be calibrated accurately with the assumption of kB = 1 (No magnetic field effects on chamber response).