<p> Mathematical modeling of tumor response to radiation therapy (RT) has great potential for designing therapy plans that are more personalized, more adaptive, and more reliable for outcome predictions. A preexisting model of tumor response to radiation therapy for cervical cancer has been shown to generate model parameters that correlate strongly with both tumor local control and disease-specific survival. This model is further developed through incorporation of another effect of RT not previously accounted for: the oxygen effect. An easily obtainable form of input data, hemoglobin level, enables simulation of the oxygen effect simultaneously with the other major model effects. For the Local Control (LC) patient group, the changes in the model parameters caused by incorporation of the oxygen effect are found to significantly improve the agreement of those parameters with actual patient data. For the Local Failure (LF) group and the overall patient group, the oxygen effect is incorporated without significant change to the agreement between the model-simulated output parameters and the actual patient data. Also, a strategy is presented for solving the main model equations to obtain analytic expressions for surviving cell fraction and regression volume ratio as functions of time.</p>

<p> Abstract not available.</p>

<p> Abstract not available.</p>

<p>Over the past five to ten years, public interest in nuclear energy, decommissioning, and waste management and stewardship has increased, leading to a renewed interest in radioecology (Kuhne 2012), or the study of the relationships between ionizing radiation and the environment (Whicker and Shultz 1982a). Several groups supporting collaborative radioecological research have recently been established, including the European Radioecology ALLIANCE in 2009 (Hinton et al. 2013), the Strategy for Allied Radioecology (STAR) network in 2011 (Kuhne 2012), and the National Center for Radioecology (NCoRE) in the United States in 2011 (Kuhne 2012). The earthquake, tsunami, and subsequent nuclear accident at Fukushima in March of 2011 further emphasized the importance of radioecology in providing timely and technically sound information (such as the transport and fate of radionuclides, potential doses and risks, etc.) for decision making in emergency response as well as in clean up and recovery (Kuhne 2012; Hinton et al. 2013) for both humans and their environment. Although the original and primary aims of the ICRP radiation protection recommendations have been to prevent deterministic effects and minimize stochastic effects to human beings from radiation exposure, the protection framework has recently been extended to include protecting the environment from harmful effects of radiation as well (ICRP 2007, 2008b, 2009). Radioecology is an interdisciplinary science that encompasses a wide array of topics, including, among others, radiation transport, effects, risk assessment, and remediation (Whicker and Shultz 1982a; Hinton et al. 2013). I consider two topics from different areas of radioecology in this dissertation: radionuclide uptake and dosimetry as well as an assessment of a technique for potential use in remediation. Part 1 outlines the development of empirical and computational models for prediction of activity concentration and subsequent radiation dose, respectively, in relevant rainbow trout (Oncorhynchus mykiss) organs for selected radionuclides. Radiation dose rates to biota are typically approximated utilizing dose conversion factors (DCF), which are values for absorbed dose rate per activity concentration in the body or organ (i.e. mGy d-1 per Bq g-1). The current methodology employed by both the International Commission on Radiological Protection (ICRP) and within the Environmental Risks from Ionizing Radiation in the Environment (ERICA) Integrated Approach for calculating dose conversion coefficients is to use Monte Carlo modeling of a homogenously distributed radionuclide within an ellipsoidal phantom chosen to represent a particular organism. However, more accurate estimates can be made based on specific absorbed fractions and activity concentrations. The first study in Part 1 examines the effects of lake tropic structure on the uptake of iodine-131 (131I) in rainbow trout and considers a simple computational model for the estimation of resulting radiation dose. Iodine-131 is a major component of the atmospheric releases following reactor accidents, and the passage of 131I through food chains from grass to human thyroids has been extensively studied. By comparison, the fate and effects of 131I deposition onto lakes and other aquatic systems has been less studied. In this study we reanalyze 1960s data from experimental releases of 131I into two small lakes and compare the effects of differences in lake trophic structures on 131I accumulation in fish. The largest concentrations in the thyroids of trout (Oncorhynchus mykiss) may occur from 8 to 32 days post initial release. DCFs for trout for whole body as well as thyroid were computed using Monte Carlo modeling with an anatomically-appropriate model of trout thyroid structure. Activity concentration data was used in conjunction with the calculated DCFs to estimate dose rates and ultimately determine cumulative radiation dose (Gy) to the thyroids after 32 days. The estimated cumulative thyroid doses at 32 days post-release ranged from 6 mGy to 18 mGy per 1 Bq mL-1 of initial 131I in the water, depending upon fish size. The subsequent studies in Part 1 seek to develop and compare different, increasingly detailed anatomical phantoms for O. mykiss for the purpose of estimating organ radiation dose and dose rates from 131I uptake and from molybdenum-99 (99Mo) uptake. Model comparison and refinement is important to the process of determining both dose rates and dose effects, and we develop and compare three models for O. mykiss: a simplistic geometry considering a single organ, a more specific geometry employing anatomically relevant organ size and location, and voxel reconstruction of internal anatomy obtained from CT imaging (referred to as CSUTROUT). Dose Conversion Factors (DCFs) for whole body as well as selected organs of O. mykiss were computed using Monte Carlo modeling, and combined with the empirical models for predicting activity concentration, to estimate dose rates and ultimately determine cumulative radiation dose (?Gy) to selected organs after several half-lives of either 131I or 99Mo. The different computational models provided similar results, especially for organs that were both the source and target of radiation (less than 30% difference between estimated doses). Although CSUTROUT was the most anatomically realistic phantom, it required much more resource dedication to develop than did the stylized phantom for similar results. Additionally, the stylized phantom can be scaled to represent trout sizes whereas CSUTROUT cannot be. There may be instances where a detailed phantom such as CSUTROUT is appropriate, as it will provide the most accurate radiation dose and dose rate information, but generally, the stylized phantom appears to be the best choice for an ideal balance between accuracy and resource requirements. Part 2 considers the use of reflectance spectroscopy as a remediation tool through its potential to determine plant stress from metal contaminants. Reflectance spectroscopy is a rapid and non-destructive analytical technique that may be used for assessing plant stress and has potential applications for use in remediation. Changes in reflectance such as that due to metal stress may occur before damage is visible, and existing studies have shown that metal stress does cause changes in plant reflectance. The studies in Part 2 further investigate the potential use of reflectance spectroscopy as a method for assessing metal stress in plants. In the first study, Arabidopsis thaliana plants were treated twice weekly in a laboratory setting with varying levels (0 mM, 0.5 mM, or 5 mM) of cesium chloride (CsCl) solution, and reflectance spectra were collected every week for three weeks using an ASD FieldSpec Pro spectroradiometer with both a contact probe and a field of view probe at 36.8 and 66.7 cm above the plant. As metal stress is known to mimic drought stress, plants were harvested each week after spectra collection for determination of relative water content and chlorophyll content. A visual assessment of the plants was also conducted using point observations on a uniform grid of 81 points. Two-way ANOVAs were performed on selected vegetation indices (VI) to determine the significance of the effects of treatment level and length of treatment. Linear regression was used to relate the most appropriate vegetation indices to the aforementioned endpoints and to compare results provided by the three different spectra collection techniques. One-way ANOVAs were performed on selected VI at each time point to determine which, if any, indices offered a significant prediction of the overall extent of Cs toxicity. Of the 14 vegetation indices considered, the two most significant were the slope at the red edge position (SREP) and the ratio of reflectance at 950 nm to the reflectance at 750 nm (R950/R750). Contact probe readings and field of view readings differed significantly. Field of view measurements were generally consistent at each height. The second study investigated the potential use of reflectance spectroscopy as a method for assessing metal stress across four different species of plants, namely Arabidopsis thaliana, Helianthus annuus, Brassica napus var. rapa, and Zea mays. The purpose of this study was to determine whether a quantifiable relationship exists between reflectance spectra and lithium (Li) contamination in each species of plant considered, and if such a relationship exists similarly across species. Reflectance spectra were collected every week for three weeks using an ASD FieldSpec Pro Spectroradiometer with a contact probe and a field of view probe for plants treated twice weekly in a laboratory setting with 0 mM or 15 mM of lithium chloride (LiCl) solution. Plants were harvested each week immediately after spectra collection for determination of relative water content and chlorophyll content. Linear regression was used to relate the most appropriate vegetation indices (determined by the Pearson correlation coefficient) to the aforementioned endpoints and to compare results provided by the different spectra collection techniques. Two-way ANOVAs were performed on 12 selected vegetation indices (VI) for each species individually to determine the significance of the effects of treatment level and length of treatment on a species basis. Balanced ANOVAs were conducted across all species to determine significance of treatment, time, and species. LiCl effects and corresponding reflectance shifts were significant for A. thaliana, but Z. mays and H. annuus showed little response to LiCl at the treatment level considered in this study, with no significant differences in relative water content or chlorophyll content by treatment level. B. rapa reflectance spectra responded similarly to Li exposure as Z. mays, but B. rapa did have significant differences in relative water content by treatment level. All species demonstrated a potential stimulatory effect of LiCl, with at least one week of increased reflectance in the near-IR. Different VI proved to be the best predictor of endpoint values for each species, with only SIPI and the ratio of reflectance at 1390 nm to the reflectance at 1454 nm (R1390/R1454) common between species. The most significant VI considering all species together was SIPI, although A. thaliana effects dominate this result. VI determined separately by CP and FOV were occasionally well-related, but this relationship was inconsistent between species, further supporting the conclusion in the previous study that CP and FOV are not interchangeable. These techniques should either be used as compliments or independently, depending on the application.

<p> The physics of the atomic nucleus and supernovas are fundamental to our very being. Indeed, supernovas provide the wind that disperses the nuclei of which we are composed, and the physics of nuclei is pivotal in supernova dynamics. During supernova core collapse, the extremely high temperatures and densities and low entropy favor large, neutron-rich nuclei at high excitation energy. My collaborators and I examine two weak interactions that occur in nuclei under these conditions. First, we study the production of neutrino pairs via de-excitation of hot nuclei. In de-exciting, the nucleus can emit a virtual Z⁰ boson that decays into a neutrino-antineutrino pair. We find this to be the dominant source of neutrino pairs of all flavors during collapse. Second, we use modern shell model computation techniques to revise the Brink-Axel hypothesis method of computing electron capture rates that was pioneered by Fuller, Fowler, and Newman. Our results show that the Brink-Axel hypothesis (which posits that the bulk of nuclear transition strength is distributed among transition energies independently of initial excitation energy) fails at low and moderate excitation, but that at high initial energies, the strength is largely independent of excitation. The failure of the Brink-Axel hypothesis manifests as the redistribution of strength to low and negative transition energies, which can have the effect of increasing the overall electron capture rate in the core.</p>

<p> The Lithium Tokamak Experiment (LTX) is a spherical tokamak designed to study the lowrecycling regime through the use of lithium-coated shells conformal to the last closed flux surface (LCFS). A lowered recycling rate is expected to flatten core <i>T</i>_e profiles, raise edge <i>T</i>_e, strongly affect <i>n</i>_e profiles, and enhance confinement.</p><p> To study these unique plasmas, a Thomson scattering diagnostic uses a &le; 20 J, 30 ns FWHM pulsed ruby laser to measure <i>T</i>_e and <i>n</i>_e at 11 radial points on the horizontal midplane, spaced from the magnetic axis to the outer edge at a single temporal point for each discharge. Scattered light is imaged through a spectrometer onto an intensified CCD. The diagnostic is absolutely calibrated using a precision light source and Raman scattering. Measurements of <i>n</i>_e are compared with line integrated density measurements from a microwave interferometer. Adequate signal to noise is obtained with ne &ge; 2 &times;10¹⁸ m^&ndash;3.</p><p> Thomson profiles of plasmas following evaporation of lithium onto room-temperature plasmafacing components (PFCs) are used in conjunction with magnetic equilibria as input for TRANSP modeling runs. Neoclassical calculations are used to determine <i> T</i>_i profiles, which have levels that agree with passive charge exchange recombination spectroscopy (CHERS) measurements. TRANSP results for confinement times and stored energies agree with diamagnetic loop measurements. Results of &chi;_e result in values as low as 7 m²/s near the core, which rise to around 100 m²/s near the edge. These are the first measurements of &chi;e in LTX, or its predecessor, the Current Drive Experiment-Upgrade (CDX-U), with lithium PFCs.</p>

PETSIM, a Monte Carlo simulation program of Positron Emission Tomography (PET) systems, was improved in terms of accuracy and efficiency. First, the accuracy, the speed and the ease of use of PETSIM were improved by using tabulated values of the Compton scattering and photoelectric absorption partial interaction attenuation coefficients for all common biological, collimator and detector materials. These were generated from chemical formula, or physical composition, and density of the absorbing medium. / Furthermore, simulations of PET systems waste considerable time generating events which will never be detected. For events in which the original photons are usually directed towards the detectors, the efficiency of the simulations was improved by giving the photons additional chances of being detected. For simulation programs which cascade the simulation process into source, collimation, and detection phases such as PETSIM, the additional detections resulted in an improvement in the simulation precision without requiring larger files of events from the source/phantom phase of the simulation. This also reduced the simulation time since fewer positron annihilations were needed to achieve a given statistical precision. This was shown to be a useful improvement over conventional Monte Carlo simulations of PET systems.

Dose predictions by Monte-Carlo (MC) techniques could alleviate the measurement load required in linac commissioning and clinical radiotherapy practice, where small or irregular electron fields are routinely encountered. In particular, this study focused on the MC calculation of cutout factors for clinical electron beams. A MC model for a Varian linac CL2300C/D was built and validated for all electron energies and applicators. A MC user code for simulation of irregular cutouts was then developed and validated. Supported by a home-developed graphical user interface, it determines in situ cutout factors and depth dose curves for arbitrarily shaped electron fields and collects phase space data. Overall, the agreement between simulations and measurements was excellent for fields larger than 2 cm. / The MC model was also used to calculate dose distributions with the fast MC code XVMC in CT images of phantoms of clinical interest. These dose distributions were compared to dose calculations performed by the pencil-beam algorithm-based treatment planning system CadPlan and verified against measurements. Good agreement between calculations and measurements was achieved with both systems for phantoms containing 1-dimensional heterogeneities, provided a minimal quality of the CT images. In phantoms with 3-dimensional heterogeneities however, CadPlan appeared unable to predict the dose accurately, whereas MC provided with a more satisfactory dose distribution, despite some local discrepancies.

Liquid ionization chambers have characteristics that can remedy some of the drawbacks of air-filled ionization chamber dosimetry: large sensitive volumes, fluence perturbations, and energy dependence. However, high ion recombination rates can be a significant problem in liquid chambers. In this work, we have investigated properties of a new liquid chamber, called the GLIC-03 (Guarded Liquid Ionization Chamber), including chamber stability, reproducibility, and establishing recombination corrections. The response varied by less than 1% over 10 hours, and was reproducible within 1.5% of the mean over different liquid fills. Recombination corrections were established, and were small for low dose rates and high voltages. The establishment of these characteristics allowed us to compare measurements of the GLIC-03 in a region of charged particle disequilibrium to those made with a diamond detector. Results show the GLIC-03's suitability as a high resolution detector.

To optimize delivery of radiation therapy treatments, accurate knowledge of absorbed dose in the clinical beam is essential. In this work we investigated issues related to the use of different types of ionization chambers for accurate radiation dosimetry. We determined values of Pwall at 60Co for plane-parallel chambers and used them to derive the conversion factors kecal k'R 50 or kQ for a 20 MeV electron beam. Our results agreed within 0.4% with kQ values given in the IAEA TRS-398 protocol, but differences of up to 1.8% were found between our kecalk' R50 values and those in the AAPM TG-51 protocol. We also investigated the behavior of two liquid-filled ionization chambers in megavoltage photon beams, examining stability, ion recombination and beam quality dependence. Methods for determining and correcting for recombination were investigated. Preliminary results show that the energy dependence of a liquid-filled chamber can typically be limited to less than 1% for megavoltage photon beams.