Two-dimensional transmission dosimetry in radiotherapy has been discussed in the literature for some time as being a potential method for in vivo dosimetry. However, it still remains to become a wide spread practice in radiotherapy clinics. This is most likely due to the variety in radiotherapy treatment sites and the challenges they would present in terms of detection and interpretation at the transmitted dose level. Thus, the full potential and limitations of applying transmission dosimetry in the presence of dosimetry errors still need to be demonstrated. This thesis is a theoretical evaluation of transmission dosimetry using the Pinnacle3 treatment planning system. The accuracy of predicting reliable and accurate absolute transmitted dose maps using the planning system dose algorithm for comparison with measured transmitted dose maps was initially investigated. The resolution in the dose calculations at the transmitted level was then evaluated for rectilinear and curved homogeneous phantoms and rectilinear inhomogeneous phantoms, followed by studies combining both surface curvature and heterogeneities using anthropomorphic phantoms. In order to perform transmitted dose calculations at clinically relevant beam focus-to-transmitted dose plane distances using clinical patient CT data it was first necessary to extend the CT volume. Finally, the thesis explored the efficacy of applying transmission dosimetry in the clinic by simulating realistic dosimetry errors in the planning system using patient treatment plans for a prostate, head and neck, and breast CRT (Conformal Radiotherapy) treatment. Any differences at the transmitted dose level were interpreted and quantified using the gamma formalism. To determine whether the transmitted dose alone was a sufficient indicator of the dosimetry errors, the magnitude in transmission dose differences were compared with those predicted at the midplane of the patient. Dose-Volume Histograms (DVHs) were also used to evaluate the clinical significance of the dose delivery errors on the target volume and surrounding healthy tissue structures. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1339807 / Thesis (Ph.D.) - University of Adelaide, School of Chemistry and Physics, 2008

The characterization and measurement of the spatial, temporal and energy emission of air-scattered photons, electrons and neutrons generated near 10 MV or greater accelerator-based bremsstrahlung photon sources is becoming important in many applications. The national and homeland security research community is interested in developing technologies which can detect illicit materials at substantial standoff distances in outdoor environments. These systems are referred to as "active" interrogation systems and are defined as inspection systems that take advantage of an externally applied "source" to perform traditional imaging of, or to stimulate characteristic emissions from, an inspected object. A key concern in the development of these systems is the ability to effectively predict the dose equivalents at long standoff distances from these sources in order to ascertain the operational radiation safety of said systems. Current computational radiation transport simulation tools have the ability to effectively model these systems; however, a paucity of experimental data exists in comparing the results of these simulations. A methodology to assess the radiation dose surrounding a high-energy bremsstrahlung-based accelerator system for national defense applications was developed. Fluence-to-dose conversion coefficients for the International Commission on Radiation Units and Measurements operational quantity ambient dose equivalent were calculated for photons and electrons up to 25 MeV utilizing the Los Alamos National Laboratory Monte Carlo N-Particle code, MCNP5 Version 1.51. Special consideration was given to the treatment of secondary charged particle equilibrium in all simulations. An extensive set of system simulations was performed to model a prototype high-energy bremsstrahlung-based accelerator system to obtain photon, electron and neutron fluence spectra. These fluence data were folded with the calculated ambient dose equivalent conversion coefficients as well as previously published effective dose conversion coefficients. A set of integral air-scatter measurements for accelerator-generated primary and secondary radiations (photon and neutron) were performed around the prototype system in order to provide a comparative data set from which to determine the total dose equivalent both in the beam (on-axis) and outside of the beam (off-axis).

In this dissertation work, a computer simulation model based on the Geant4 simulation package has been designed and developed to study the energy deposition and track structures of cosmic muons and their secondary electrons in tissue-like materials. The particle interactions in a cubic water volume were first simulated. To analyze the energy deposition and tracks in small structures, with the intention of studying the energy localization in nanometric structures such as DNA, the chamber was sliced in three dimentions. Validation studies have been performed by comparing the results with experimental, theoretical, and other simulation results to test the accuracy of the simulation model. A human body phantom in sea-level muon environment was modeled to measure the yearly dose to a human from cosmic muons. The yearly dose in this phantom is about 22 millirems. This is close to the accepted value for the yearly dose from cosmic radiation at sea level. Shielding cosmic muons with a concrete slab from 0 to 2 meters increased the dose received by the body. This dissertation presents an extensive study on the interactions of secondary electrons created by muons in water.

Thesis (M.Sc. (Res.))--University of Wollongong, 2009. / Typescript. Includes bibliographical references: leaf 69-71.

Dissertation (Ph.D.)--University of Toledo, 2009. / Typescript. "Submitted as partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics, special area of interest, Radiation Oncology Medical Physics." Bibliography: leaves 89-97.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Real-time feedback of absorbed dose at a point within a patient can help with radiological quality assurance and innovation. Two radiochromic materials from GafChromic MD-55 and EBT films have been investigated for applicability in real-time in vivo dosimetry of ionizing radiation. Both films were able to produce a real-time measurement of optical density from a small volume, allowing positioning onto a tip of an optical fibre in the future. The increase in optical density was linear with absorbed dose for MD-55, and non-linear for EBT. The non-linearity of EBT is associated with its increased sensitivity to ionizing radiation compared to MD-55, thus reaching optical saturation at a much lower dose. The radiochromic material in EBT film was also shown to polymerize and stabilize faster, decreasing dose rate dependence in real-time measurements in comparison to MD-55. The response of the two media was tested over 75 kVp – 18 MV range of x-ray beams. The optical density measured for EBT was constant within 3% throughout the entire range, while MD-55 exhibited a nearly 40% decrease at low energies. Both materials were also shown to be temperature sensitive, with the change in optical density generally decreasing when the temperature increased from ~22°C to ~37°C. This was accompanied by a shift in the peak absorbance wavelength. It was illustrated that some of this decrease can be corrected for by tracking the peak position and then multiplying the optical density by a correction factor based on the predicted temperature. Overall, the radiochromic material in GafChromic EBT film was found to be a better candidate for in vivo real-time dosimetry than the material in GafChromic MD-55. A novel mathematical model was proposed linking absorbance to physical parameters and processes of the radiochromic materials. The absorbance at every wavelength in the spectrum was represented as a sum of absorbances from multiple absorbers, where absorbance is characterized by its absorption coefficient, initiation constant, and polymerization constant. Preliminary fits of this model to experimental data assuming two absorbers suggested that there is a trade-off between EBT’s greater sensitivity and its dose linearity characteristics. This was confirmed by experimental results.

Real-time feedback of absorbed dose at a point within a patient can help with radiological quality assurance and innovation. Two radiochromic materials from GafChromic MD-55 and EBT films have been investigated for applicability in real-time in vivo dosimetry of ionizing radiation. Both films were able to produce a real-time measurement of optical density from a small volume, allowing positioning onto a tip of an optical fibre in the future. The increase in optical density was linear with absorbed dose for MD-55, and non-linear for EBT. The non-linearity of EBT is associated with its increased sensitivity to ionizing radiation compared to MD-55, thus reaching optical saturation at a much lower dose. The radiochromic material in EBT film was also shown to polymerize and stabilize faster, decreasing dose rate dependence in real-time measurements in comparison to MD-55. The response of the two media was tested over 75 kVp – 18 MV range of x-ray beams. The optical density measured for EBT was constant within 3% throughout the entire range, while MD-55 exhibited a nearly 40% decrease at low energies. Both materials were also shown to be temperature sensitive, with the change in optical density generally decreasing when the temperature increased from ~22°C to ~37°C. This was accompanied by a shift in the peak absorbance wavelength. It was illustrated that some of this decrease can be corrected for by tracking the peak position and then multiplying the optical density by a correction factor based on the predicted temperature. Overall, the radiochromic material in GafChromic EBT film was found to be a better candidate for in vivo real-time dosimetry than the material in GafChromic MD-55. A novel mathematical model was proposed linking absorbance to physical parameters and processes of the radiochromic materials. The absorbance at every wavelength in the spectrum was represented as a sum of absorbances from multiple absorbers, where absorbance is characterized by its absorption coefficient, initiation constant, and polymerization constant. Preliminary fits of this model to experimental data assuming two absorbers suggested that there is a trade-off between EBT’s greater sensitivity and its dose linearity characteristics. This was confirmed by experimental results.

<p>The natural &sup2 / &sup3 / ⁸U, &sup2 / &sup3 / &sup2 / Th and ⁴&deg / K radioactive content of vineyard soil was measured with an in-situ gamma-ray detector. The activity concentration measured with the in-situ detector are normalized using the laboratory-determined activity concentrations of several samples from the vineyard site. To determine the activity concentration of a particular soil sample, the gamma-ray photopeak detection efficiencies are required. In this work, the detection efficiencies were derived for each soil sample using gamma-ray photopeaks associated with the radionuclide of &sup2 / &sup3 / ⁸U, &sup2 / &sup3 / &sup2 / Th present in the sample, and the ⁴&deg / K, 1460.8 keV gamma-ray peak, from KCl salt.</p>

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