The goal of Total Skin Electron Therapy (TSET) is to deliver a uniform dose to the patient's entire skin to a limited depth while sparing the body organs. Due to the laborious commissioning process, it is helpful to use the Monte Carlo (MC) method for the procedure and treatment planning. Calculations of dosimetric quantities were performed with EGSnrc/BEAM MC codes as well as with the fast MC code XVMC. The linac model for a 6 MeV Varian CL21EX accelerator was established by measuring the electron focal spot size with a slit camera. Using this measured focal spot value in our proposed divergent beam model, an improved result for large field profiles can be achieved. Measured PDDs and profiles under standard set-up and TSET conditions were compared to MC calculations. Overall, satisfactory results were obtained except for simulations with an additional scattering filter, suggesting the composition, density and dimensions of the filter need to be confirmed.

The goal of conformal radiation techniques is to improve local tumor control through dose escalation to target volumes while at the same time sparing surrounding healthy tissue. Accurate target volume delineation is essential in achieving this goal to avoid inadequate tumor coverage and/or irradiation of an unnecessary volume of healthy tissue. Respiratory motion is known to be the largest intra-fractional organ motion and the most significant source of uncertainty in treatment planning for chest lesions. A method to minimize effects of respiratory motion is to use four-dimensional (4D) radiotherapy. / A novel scanning procedure for 4D CT data acquisition is described in this work. Three single-slice helical scans are acquired simultaneously with the real-time tracking of several markers placed on a moving phantom. At the end of the three scans. CT data is binned into different respiratory phases according to the externally recorded respiratory signal and the scanned volume is reconstructed for several respiratory phases. The 4D CT images obtained show an overall improvement when compared to conventional CT images of a moving phantom.

<p> Accurate, cross-scanner assessment of in-vivo air density used to quantitatively assess amount and distribution of emphysema in COPD subjects has remained elusive. Hounsfield units (HU) within tracheal air can be considerably more positive than -1000 HU. With the advent of new dual-source scanners which employ dedicated scatter correction techniques, it is of interest to evaluate how the quantitative measures of lung density compare between dual-source and single-source scan modes. This study has sought to characterize in-vivo and phantom-based air metrics using dual-energy computed tomography technology where the nature of the technology has required adjustments to scatter correction. </p><p> Anesthetized ovine (N=6), swine (N=13: more human-like rib cage shape), lung phantom and a thoracic phantom were studied using a dual-source MDCT scanner (Siemens Definition Flash. Multiple dual-source dual-energy (DSDE) and single-source (SS) scans taken at different energy levels and scan settings were acquired for direct quantitative comparison. Density histograms were evaluated for the lung, tracheal, water and blood segments. Image data were obtained at 80, 100, 120, and 140 kVp in the SS mode (B35f kernel) and at 80, 100, 140, and 140-Sn (tin filtered) kVp in the DSDE mode (B35f and D30f kernels), in addition to variations in dose, rotation time, and pitch. To minimize the effect of cross-scatter, the phantom scans in the DSDE mode was obtained by reducing the tube current of one of the tubes to its minimum (near zero) value.</p><p> When using image data obtained in the DSDE mode, the median HU values in the tracheal regions of all animals and the phantom were consistently closer to -1000 HU regardless of reconstruction kernel (chapters 3 and 4). Similarly, HU values of water and blood were consistently closer to their nominal values of 0 HU and 55 HU respectively. When using image data obtained in the SS mode the air CT numbers demonstrated a consistent positive shift of up to 35 HU with respect to the nominal -1000 HU value. In vivo data demonstrated considerable variability in tracheal, influenced by local anatomy with SS mode scanning while tracheal air was more consistent with DSDE imaging. Scatter effects in the lung parenchyma differed from adjacent tracheal measures.</p><p> In summary, data suggest that enhanced scatter correction serves to provide more accurate CT lung density measures sought to quantitatively assess the presence and distribution of emphysema in COPD subjects. Data further suggest that CT images, acquired without adequate scatter correction, cannot be corrected by linear algorithms given the variability in tracheal air HU values and the independent scatter effects on lung parenchyma.</p>

Synthetic dyes, including thiazines, acridines, xanthenes, and a phenazine, were used as models for studying the photodynamic effect. In the presence of physiological reductants, the illuminated dyes produced superoxide (O$\sb2\sp-$), hydrogen peroxide (H$\sb2$O$\sb2$), and hydroxyl free radical (OH). Suitable reductants included NADH, glutathione, GMP, cysteine, tryptophan, and tyrosine. The production of OH$\cdot$ was dependent on chelated iron or on copper. Effective iron chelators included EDTA, DTPA, dipyridyl, and phenanthroline, and biologically significant compounds such as ATP, ADP, succinate, citrate, and DNA. DNA intercalation of the dyes did not prevent oxidization of NADH or the production of O$\sb2\sp-$ or OH$\cdot$. Hydroxyl radical scavengers competed effectively in the assays for the OH$\cdot$. While in the Haber-Weiss reaction superoxide reduces iron, which is oxidized by H$\sb2$O$\sb2$ to produce OH$\cdot$, excited state reduced dyes appeared capable of reducing the iron, so that SOD was only partially inhibitory of OH$\cdot$ production. Catalase prevented production of OH$\cdot$. Similar results were found for substrates and metals in an assay of single strand scission of DNA mediated by the dyes. Amounts of strand scission seen were dependent on concentrations of iron or dye. Cysteine, NADH, GTP, dGMP, tryptophan, and tyrosine were all able to provide electrons for the strand scission reaction. Furthermore, the single strand scission of DNA by the dyes was prevented by scavengers of O$\sb2\sp-$, H$\sb2$O$\sb2$ or OH$\cdot$ which are poor scavengers for singlet oxygen ($\sp1$O$\sb2$), and substitution of deuterium oxide for water, which exacerbates any damage produced by $\sp1$O$\sb2$, did not increase damage. The physiological reductant glutathione in E. coli was depleted by exposure to illuminated dye. Lethality of the dyes was reduced by enhanced levels of catalase or endonulcease IV provided by plasmid-coded genes, indicating that H$\sb2$O$\sb2$ is an important mediator of toxicity and that DNA is an important target, while OH$\cdot$ scavengers prevented kill, indicating that the OH$\cdot$ is also an important mediator of phototoxicity. Finally, DNA damage in vivo was reduced by an OH$\cdot$ scavenger. In conclusion, toxicity in the photodynamic effect is mediated by reduced oxygen species, particularly OH$\cdot$, and DNA damage probably underlies this toxicity.

Oblique-incidence, fiber-optic reflectometry is a simple and accurate method for measuring the absorption and reduced scattering coefficients, $\mu\sb{a}$ and $\mu\sb{s}\sp\prime ,$ of semi-infinite turbid media. Obliquely incident light produces a spatial distribution of diffuse reflectance that is not centered about the point of light entry. The shift in the center of diffuse reflectance is related to the medium's diffusion coefficient, D. We developed a fiber-optic probe to deliver light obliquely and sample the profile of diffuse reflectance. From a relative profile, we measure D, deduce the effective attenuation coefficient, $\mu\sb{eff},$ then calculate $\mu\sb{a}$ and $\mu\sb{s}\sp\prime .$ This method was verified with Monte Carlo simulations and tested on tissue phantoms. Measurements at 632.8 nm were accurate to within 5% for D and $\mu\sb{eff},$ resulting in 10% and 5% accuracy for $\mu\sb{a}$ and $\mu\sb{s}\sp\prime .$ In addition, $\mu\sb{a}$ and $\mu\sb{s}\sp\prime$ spectra were deduced from wavelength-resolved measurements of the diffuse reflectance from a white light source.

Cancer is a global problem, and many people in low-income countries do not have access to the treatment options, such as radiation therapy, that are available in wealthy countries. Where radiation therapy is available, it is often delivered using older Co-60 equipment that has not been updated to modern standards. Previous research has indicated that an updated Co-60 radiation therapy machine could deliver treatments that are equivalent to those performed with modern linear accelerators. Among the key features of these modern treatments is a tightly conformal dose distribution-- the radiation dose is shaped in three dimensions to closely match the tumour, with minimal irradiation of surrounding normal tissues. Very accurate alignment of the patient in the beam is therefore necessary to avoid missing the tumour, so all modern radiotherapy machines include imaging systems to verify the patient's position before treatment. Imaging with the treatment beam is relatively cost-effective, as it avoids the need for a second radiation source and the associated control systems. The dose rate from a Co-60 therapy source, though, is more than an order of magnitude too high to use for computed tomography (CT) imaging of a patient. Digital tomosynthesis (DT), a limited-arc imaging method that can be thought of as a hybrid of CT and conventional radiography, allows some of the three-dimensional selectivity of CT but with shorter imaging times and a five- to fifteen-fold reduction in dose. In the present work, a prototype Co-60 DT imaging system was developed and characterized. A class of clinically useful Co-60 DT protocols has been identified, based on the filtered backprojection algorithm originally designed for CT, with images acquired over a relatively small arc. Parts of the reconstruction algorithm must be modified for the DT case, and a way to reduce the beam intensity will be necessary to reduce the imaging dose to acceptable levels. Some additional study is required to determine whether improvements made to the DT imaging protocol translate to improvements in the accuracy of the image guidance process, but it is clear that Co-60 DT is feasible and will probably be practical for clinical use. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2012-01-30 12:56:56.075

Cancer is a global problem, and many people in low-income countries do not have access to the treatment options, such as radiation therapy, that are available in wealthy countries. Where radiation therapy is available, it is often delivered using older Co-60 equipment that has not been updated to modern standards. Previous research has indicated that an updated Co-60 radiation therapy machine could deliver treatments that are equivalent to those performed with modern linear accelerators. Among the key features of these modern treatments is a tightly conformal dose distribution-- the radiation dose is shaped in three dimensions to closely match the tumour, with minimal irradiation of surrounding normal tissues. Very accurate alignment of the patient in the beam is therefore necessary to avoid missing the tumour, so all modern radiotherapy machines include imaging systems to verify the patient's position before treatment. Imaging with the treatment beam is relatively cost-effective, as it avoids the need for a second radiation source and the associated control systems. The dose rate from a Co-60 therapy source, though, is more than an order of magnitude too high to use for computed tomography (CT) imaging of a patient. Digital tomosynthesis (DT), a limited-arc imaging method that can be thought of as a hybrid of CT and conventional radiography, allows some of the three-dimensional selectivity of CT but with shorter imaging times and a five- to fifteen-fold reduction in dose. In the present work, a prototype Co-60 DT imaging system was developed and characterized. A class of clinically useful Co-60 DT protocols has been identified, based on the filtered backprojection algorithm originally designed for CT, with images acquired over a relatively small arc. Parts of the reconstruction algorithm must be modified for the DT case, and a way to reduce the beam intensity will be necessary to reduce the imaging dose to acceptable levels. Some additional study is required to determine whether improvements made to the DT imaging protocol translate to improvements in the accuracy of the image guidance process, but it is clear that Co-60 DT is feasible and will probably be practical for clinical use. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2012-01-30 12:56:56.075

The objective of this work is to use MOSFET dosimeters to accurately measure surface dose delivered during CT examinations in various scanning conditions. To achieve this, the behaviour of MOSFETs under kilovoltage x-ray irradiation first needed to be investigated. A dose-to-dose reproducibility of 4.5%, and a mean change in sensitivity response of 10.4% with accumulated dose were measured. A Monte Carlo model of the x-ray source of a PQ5000 CT simulator was built and validated in order to investigate the MOSFET response characteristics and perform dose calculations. An over-response of 10% was observed when the beam energy was decreased from 140 to 80 kVp, and a slight anisotropy of 8.5% from the mean value over 360º was observed. The dosimeters were calibrated on a solid water phantom using a method involving MC surface dose calculations. Good agreement was found between measurements and simulations of surface dose on a cylindrical PMMA phantom for a stationary tube technique, single axial scan and multiple contiguous axial scans, with generally less than 7.5% discrepancies. Film and MOSFET measurements were then performed for helical adult brain scan parameters using different pitch and collimator settings. The use of five MOSFETs combined in a linear array was found to be suitable to accurately measure surface dose in helical scans for almost all pitch and collimation combinations.

Since portal images are created by megavoltage, forward-directed bremsstrahlung beams, their image quality is inferior to that of images produced by kilovoltage beams. In this study, characteristics of orthogonal bremsstrahlung photons produced by megavoltage electron beams were studied and their suitability for radiotherapy imaging was evaluated. Orthogonal bremsstrahlung beams with kilovoltage effective energies can be obtained from megavoltage electrons striking low atomic number targets. A 10 MeV electron beam emerging out of the research port of a Varian Clinac-18 linac was made to strike carbon, aluminum and copper targets. Percentage depth dose and attenuation measurements of forward and orthogonal beams were performed, and experimental results were compared with Monte Carlo-calculated findings. Images of simple contrast objects taken using the orthogonal bremsstrahlung beams showed superior contrast levels in comparison to those produced by the forward beams.

Dynamic beam delivery (DBD), either in the step-and-shoot or dynamic mode, promises to be useful for developing conformal radiotherapy techniques. The introduction of DBD in the clinic requires stringent evaluation of the DBD system before any dynamic technique can be performed clinically. In this work, a series of quality assurance tests were performed to evaluate a DBD system (DBD Toolbox) on a linear accelerator. The results of the tests have shown that the DBD Toolbox is capable of precisely controlling jaw motion, collimator rotation, and gantry rotation for dynamic beam delivery to within a 1 mm tolerance for jaw motion and 0.5° tolerance for collimator or gantry rotation. In addition, a simple model for dynamic jaw dosimetry was evaluated. The quality assurance tests and model evaluation are presented in this thesis.