Microdosimetry of electrons in liquid water using the low-energy models of Geant4

The biological effects of ionizing radiation at the cellular level are frequently studied using the well-known formalism of microdosimetry, which provides a quantitative description of the stochastic aspects of energy deposition in irradiated media. Energy deposition can be simulated using Monte Carlo codes, some adopting a computationally efficient condensed-history approach, while others follow a more detailed track-structure approach. In this work, we present the simulation of microdosimetry spectra and related quantities (frequency-mean and dose-mean lineal energies) for incident monoenergetic electrons (50 eV-10 keV) in spheres of liquid water with dimensions comparable to the size of biological targets: base pairs (2 nm diameter), nucleosomes (10 nm), chromatin fibres (30 nm) and chromosomes (300 nm). Simulations are performed using the condensed-history low-energy physics models ( Livermore and Penelope ) and the track-structure Geant4-DNA physics models, available in the Geant4 Monte Carlo simulation toolkit. The spectra are compared and the influence of simulation parameters and different physics models, with emphasis on recent developments, is discussed, underlining the suitability of Geant4-DNA models for microdosimetry simulations. It is further shown that with an appropriate choice of simulation parameters, condensed-history transport may yield reasonable results for sphere sizes as small as a few tens of a nanometer

Effects of a static magnetic field on biological samples

FTIR spectroscopy uses the absorbed light in an IR beam to determine the composition of a sample. This study was done using FTIR techniques to determine the damage done or alterations caused when a magnetic field was applied to a biological sample (cell cultures). The effects of magnetic fields on biological samples is an area that is not very well understood with little reliable data available. Various experiments investigating the influence of a magnetic field on cell growth, the chemical bonds in cells and the effects during irradiation were performed. Consistently it was seen that the largest changes to the cell were found in hydrogen bonds, most commonly in water. Though perhaps this may not normally create any significant biological impact when a biological sample is irradiated, as in radiotherapy, the chemical and physical structure of water is quite important

Assessment of Monte Carlo approaches to nanodosimetry

Though Nanodosimetry is a relatively new field it is rapidly developing into a potentially important tool in the fields of radiation protection and radiobiology. This thesis focused on an investigation into the use of Monte Carlo tools for Nanodosimetric based studies

A Path Forward: Empowering Teachers to Implement More Inclusive Special Education Frameworks in Ontario

This qualitative study employs a social-critical constructivist framework to investigate current Ontario teachers’ knowledge and opinions of inclusive education (IE) initiatives for special education in Ontario. Twenty-two participants were interviewed to gather data on their current understandings of IE policies and initiatives, as well as how they might re-define their current roles to actualize these IE policies across their broader school communities. Current IE empirical research that gathers similar attitudinal data represents teachers as uninformed, uncritical, and powerless; any recommendations that these studies present come from the researchers rather than the participants. Research findings also demonstrate that while teachers philosophically agree with IE, they question the extent to which they can successfully implement IE due to time constraints, limited resources, and inconsistent administrative support. This study’s constructivist framework demonstrates that participants have carefully reflected on IE within their instructional contexts. Participants are determined, engaged, knowledgeable, and conscientious professionals who are currently mobilizing to take on future IE implementation initiatives. Their critical perspectives helped to form the basis of this study’s recommendations. This study’s detailed policy analysis of Ontario Ministry of Education equity documents also reveals that these documents do not include teachers’ voices. This study specifically examines IE through a special education lens in order to achieve a deeper understanding of the knowledge gaps that significantly hamper teachers' abilities to successfully implement IE initiatives. To address these professional knowledge gaps, this study makes four significant recommendations. First, teachers should engage in professional learning that specifically focuses on Ontario IE policies. Second, job-embedded professional learning should serve as the departure point for all future IE implementation initiatives in Ontario. The third recommendation is that teachers must be given direct consultative opportunities to share their visions for future IE initiatives. Fourth, Ontario schools must move away from their traditional bell schedules to more flexible timetabling and staffing allocations. These recommendations are also significant because they are informed by participants’ lived experiences and have been proven successful in comparable North American education systems.Ph.D

Effect of a static magnetic field on nanodosimetric quantities in a DNA volume

Abstract Purpose: With the advent of magnetic resonance imaging (MRI)-guided radiation therapy it is becoming increasingly important to consider the potential influence of a magnetic field on ionising radiation. This paper aims to study the effect of a magnetic field on the track structure of radiation to determine if the biological effectiveness may be altered. Methods: Using the Geant4-DNA (GEometry ANd Tracking 4) Monte Carlo simulation toolkit, nanodosimetric track structure parameters were calculated for electrons, protons and alpha particles moving in transverse magnetic fields up to 10 Tesla. Applying the model proposed by Garty et al., the track structure parameters were used to derive the probability of producing a double-strand break (DSB). Results: For simulated primary particles of electrons (200 eV-10 keV), protons (300 keV-30 MeV) and alpha particles (1-9 MeV) the application of a magnetic field was shown to have no significant effect (within statistical uncertainty limits) on the parameters characterizing radiation track structure or the probability of producing a DSB. Conclusions: The null result found here implies that if the presence of a magnetic field were to induce a change in the biological effectiveness of radiation, the effect would likely not be due to a change in the track structure of the radiation

Tissue equivalence of diamond for heavy charged particles

A dedicated Geant4 study was developed to determine a correction factor (C) to convert the energy deposition response in diamond to water for heavy charged ions, with atomic number (Z) greater than 2 with energies typical of Galactic Cosmic Rays. The energy deposition response within an ideal diamond based microdosimeter was modelled and converted into a microdosimetric spectrum. The simulation was then repeated, substituting diamond with water. It was shown that by applying the correction factor, the energy deposition and microdosimetric response in diamond could be matched to that of water. The correction factor was determined to be C = 0.32 to 0.33. This study has shown a weak dependence of the correction factor C with respect to the Z of the projectile. The correction factor remains applicable for converting microdosimetric spectra in diamond to water for Galactic Cosmic Rays. This result is extremely encouraging and indicative of the applicability of diamond for use in radioprotection applications in space environments

Investigation of track structure and condensed history physics models for applications in radiation dosimetry on a micro and nano scale in Geant4

Monte Carlo methods apply various physical models, either condensed history (CH) or track structure (TS), to simulate the passage of radiation through matter. Both CH and TS models continue to be applied to radiation dosimetry investigations on a micro and nano scale. However, as there has been no systematic comparison of the use of these models for such applications there can be no quantification of the uncertainty that is being introduced by the choice of physics model. A comparison of CH and TS models available in Geant4, along with a quantification of the differences in calculated quantities on a micro and nano scale, has been undertaken in this study. A sphere of liquid water was simulated, with an incident beam of monoenergetic electrons with kinetic energy between 50 eV and 10 keV. The energy deposition (typical of microdosimetry) and number of ionisations (typical of nanodosimetry), per incident particle, were recorded in a water sphere with diameter varying between 1 nm and 1 m. The simulations were repeated using the following physics packages: Livermore (CH), Penelope (CH) and Geant4-DNA (TS). Results indicated that substantial differences were present between calculated physical quantities, depending on the physics model, target diameter and ratio of the target diameter and mean track length of the incident electron. In the case of the smallest targets, the calculated energy deposition was higher when using the CH models, while the number of ionisations was typically underestimated. In larger targets the energy deposition was in good agreement for all physics models, however the number of ionisations was significantly underestimated by the CH approach, in some cases by almost two orders of magnitude. Regarding CH models, the parameter that had the greatest impact on the results was found to be the threshold of production of secondary particles; when this was minimised the CH and TS results showed the best agreement

Advances in modelling gold nanoparticle radiosensitization using new Geant4-DNA physics models

Gold nanoparticles have demonstrated significant radiosensitization of cancer treatment with x-ray radiotherapy. To understand the mechanisms at the basis of nanoparticle radiosensitization, Monte Carlo simulations are used to investigate the dose enhancement, given a certain nanoparticle concentration and distribution in the biological medium. Earlier studies have ordinarily used condensed history physics models to predict nanoscale dose enhancement with nanoparticles. This study uses Geant4-DNA complemented with novel track structure physics models to accurately describe electron interactions in gold and to calculate the dose surrounding gold nanoparticle structures at nanoscale level. The computed dose in silico due to a clinical kilovoltage beam and the presence of gold nanoparticles was related to in vitro brain cancer cell survival using the local effect model. The comparison of the simulation results with radiobiological experimental measurements shows that Geant4-DNA and local effect model can be used to predict cell survival in silico in the case of x-ray kilovoltage beams

Functional imaging equivalence and proof of concept for image-guided adaptive radiotherapy with fixed gantry and rotating couch

Purpose: The purpose of this article is to present the first imaging experiments to demonstrate the functional equivalence between a conventional rotational gantry and a fixed-beam imaging geometry, and the feasibility of an iterative image-reconstruction technique under gravitational deformation. Methods and materials: Experiments were performed using an Elekta Axesse with Agility MLC and XVI, a custom-built rotating phantom stage, a Catphan QA phantom, and a porcine heart. For the imaging equivalence, a conventional cone beam computed tomography (CBCT) of the Catphan was acquired, as well as a set of 660 x-ray projections with a static gantry and rotating Catphan. Both datasets were reconstructed with the Feldkamp-Davis-Kress (FDK) algorithm, and the resultant volumetric images were compared using standard metrics. For imaging under gravitational deformation, a conventional CBCT of the Catphan and a set of 660 x-ray projections with a static gantry and rotating Catphan were also acquired with a porcine heart. The conventional CBCT was reconstructed using FDK. The projections that were acquired with the heart rotating were sorted into angular bins and reconstructed with prior image constrained compressed sensing using a deformation-blurred FDK prior. Deformation was quantified with B-spline transformation-based deformable image registration. Results: For imaging equivalence, the difference between the two Catphan images was consistent with Poisson noise. For imaging under gravitational deformation, the conventional CBCT porcine heart image (ground truth at 0 degrees) matched the static gantry, rotating heart reconstruction with a mean magnitude of <3 mm and maximum magnitude of <5 mm of the deformation vector field. The mean deformation of the rotating heart was 3.0 to 8.9 mm, up to 16.1 mm maximum deformation. Deformation was mainly observed in the direction of gravity. Conclusions: We have demonstrated imaging equivalence in cone beam CT reconstructions between rigid phantom images acquired with a conventional rotating gantry and with a fixed-gantry and rotating phantom. We have presented a method for image reconstruction under a fixed-beam imaging geometry using a deformable phantom