<p>This diploma work was performed at the department for radiation physics at the Health University in Linköping. Its aim was to develop a method to retrospectively quantify the absorbed dose in coral. Coral is a material which suites well as a retrospective dosimeter because when exposed to radioactivity its induced free radicals are stable in time. The number of radicals is proportional to the accumulated dose so the dose can be calculated by quantifying the radicals. Therefore, coral can tell us something about the past environment and also be used in dating purposes.</p><p>A young, modern coral from the Red Sea was studied and also a fossil one which I was given from the Natural History Museum in Stockholm. It was estimated to be originated from the Tertiary period, i.e. 2-65 million years old.</p><p>To accurately calculate the corals’ accumulated doses I had to gain knowledge in a number of separate areas. The radicals were analyzed using electron spin resonance. This was possible because of the fact that free radicals contain unpaired electrons. Initially, the main goal was to study how to prepare the coral without neither mechanically inducing new radicals nor destroying the radiation induced radicals. When the preparation method was considered optimal the dose response was then crucial, i.e. how the number of radicals corresponded to radiation. To accurately quantify the absorbed doses the spectra needed to be modified and elaborated with signal analyses. By adding artificial irradiations to the samples the initial doses could be calculated. To estimate the age of a coral an assessment of the natural dose rate must be done. This was done by measure the disintegration activity of the samples. It was found that its main contributor was the ²³⁸U-series. The fossil’s minimum age was estimated to 3 million years whereas the ESR-noise made it impossible to calculate the dose in the modern coral. The detectable dose limit of the method was approximately 0.5 Gy, i.e. a minimum age of about 500 years. However, the modern coral was determined as an Elkhorn coral (family Acropora) which is a species with a fast growth rate. The coral’s size implicated its age to be about 100 years old. In addition, the method was put to the test by irradiating the young coral with an unknown dose. After modifying the method the absorbed dose was calculated to be 1.3 ± 0.3 Gy. The real dose was 1.2 Gy. Thus, the method exhibited large scales of uncertainties but it did nevertheless work sufficiently enough.</p>

This diploma work was performed at the department for radiation physics at the Health University in Linköping. Its aim was to develop a method to retrospectively quantify the absorbed dose in coral. Coral is a material which suites well as a retrospective dosimeter because when exposed to radioactivity its induced free radicals are stable in time. The number of radicals is proportional to the accumulated dose so the dose can be calculated by quantifying the radicals. Therefore, coral can tell us something about the past environment and also be used in dating purposes. A young, modern coral from the Red Sea was studied and also a fossil one which I was given from the Natural History Museum in Stockholm. It was estimated to be originated from the Tertiary period, i.e. 2-65 million years old. To accurately calculate the corals’ accumulated doses I had to gain knowledge in a number of separate areas. The radicals were analyzed using electron spin resonance. This was possible because of the fact that free radicals contain unpaired electrons. Initially, the main goal was to study how to prepare the coral without neither mechanically inducing new radicals nor destroying the radiation induced radicals. When the preparation method was considered optimal the dose response was then crucial, i.e. how the number of radicals corresponded to radiation. To accurately quantify the absorbed doses the spectra needed to be modified and elaborated with signal analyses. By adding artificial irradiations to the samples the initial doses could be calculated. To estimate the age of a coral an assessment of the natural dose rate must be done. This was done by measure the disintegration activity of the samples. It was found that its main contributor was the 238U-series. The fossil’s minimum age was estimated to 3 million years whereas the ESR-noise made it impossible to calculate the dose in the modern coral. The detectable dose limit of the method was approximately 0.5 Gy, i.e. a minimum age of about 500 years. However, the modern coral was determined as an Elkhorn coral (family Acropora) which is a species with a fast growth rate. The coral’s size implicated its age to be about 100 years old. In addition, the method was put to the test by irradiating the young coral with an unknown dose. After modifying the method the absorbed dose was calculated to be 1.3 ± 0.3 Gy. The real dose was 1.2 Gy. Thus, the method exhibited large scales of uncertainties but it did nevertheless work sufficiently enough.

Made available in DSpace on 2014-06-12T23:17:17Z (GMT). No. of bitstreams: 2 arquivo9157_1.pdf: 2640822 bytes, checksum: 5196f6de3238ff5559d618fb856ffadc (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2007 / Para estimar a dose absorvida pelo paciente em uma série de exames de raios-X diagnóstico, é necessário realizar simulações utilizando um modelo computacional de exposição. Tais modelos são compostos, fundamentalmente, por um simulador antropomórfico (fantoma) e um código Monte Carlo. O acoplamento de um fantoma de voxels a um código Monte Carlo é um processo complexo e quase sempre resulta na solução de um problema particular. Isto significa que é inviável a utilização destas ferramentas computacionais na rotina de clínicas e hospitais que realizam exames de raios-X, porque as simulações com modelo computacional de exposição demandam tempo, conhecimento do código utilizado e diversos ajustes a serem implementados de uma simulação para outra. Neste contexto, foi desenvolvido em C++ a GUI (Graphics User Interface) VoxelDose que cria arquivos de dados com o resultado da simulação de diversos exames e utiliza estes arquivos de dados para fornecer as informações dosimétricas. O arquivo de dados foi construído usando os fantomas de voxels MAX (Male Adult voXel) e FAX (Female Adult voXel), e o código Monte Carlo EGS4 (Electron Gamma Shower, versão 4). O software permite ao usuário criar os arquivos de dados, inserir novos exames, visualizar a região do exame e a posição da fonte, obter coeficientes de conversão e calcular dose. Os resultados dosimétricos e as imagens podem ser salvos ou impressos

<p>Silicon diodes compared to other types of dosimeters have several attractive properties, such as an excellent spatial resolution, a high sensitivity, and clinically practical to use. These properties make silicon diodes a preferred dosimeter for relative dosimetry for several types of measurements in small field dosimetry, e.g., stereotactic treatments and intensity modulated radiotherapy (IMRT). Silicon diodes are, however, limited by an energy dependent response variation in photon beams, resulting in that the diode readout per dose to the phantom medium varies with photon spectral changes, thereby introducing a significant uncertainty in the measured data. The traditional solution for the energy dependent over-response caused by low-energy photons is to use diodes with a shielding filter of high atomic number. These shielded diodes, however, show an incorrect readout for small fields due to electrons scattered from the shielding (Griessbach <em>et al</em>. 2005). In regions with degraded lateral electron equilibrium (LEE) shielded diodes over-respond due to an increased degree of LEE, as a consequence of the high density shielding (Lee <em>et al</em>. 2002).</p><p>In this work a prototype software that corrects for the energy dependent response of a silicon diode is developed and validated for small field sizes. The developed software is based on the novel concept of Monte Carlo (MC) simulated fluence pencil beam kernels to calculate spectra (Eklund and Ahnesjö 2008), and the spectra based silicon diode response model proposed by Eklund and Ahnesjö (2009). The software was also extended to include correction of ionization chambers, for the energy dependent Spencer-Attix water/air stopping power ratio (<em>s</em>_w,air). The calculated <em>s</em>_w,air are shown to be in excellent agreement with published values to better than 0.1% for most values, the maximum deviation being 0.3%.</p><p>Measured relative depth doses, relative profiles, and output factors in water, for small square field sizes, for 6 MV and 15 MV clinical photon beams are presented in this work. The results show that the unshielded Scanditronix-Wellhöfer EFD^3G silicon diode response, corrected by the developed software, is in excellent agreement with reference ionization chamber measurements (corrected for change in <em>s</em>_w,air), the maximum deviation being 0.4%.</p><p>Measurements with two types of shielded diodes, namely Scanditronix-Wellhöfer PFD silicon diodes (FP1990 and FP2730), are also included in this work. The shielded diodes are shown to have an over-response as large as 2-3.5% for field sizes smaller than 5 cm x 5 cm. The presented results also suggest a difference in accuracy as large as 0.5-1% between the two types of shielded diodes, where the spectral composition at the measurement position dictates which type of diode is more accurate.</p><p>The fast correction of silicon diodes provided by the developed software is more accurate than shielded diodes for small field sizes, and can in radiotherapeutic clinical practice increase the dosimetric accuracy of silicon diodes.</p>

Silicon diodes compared to other types of dosimeters have several attractive properties, such as an excellent spatial resolution, a high sensitivity, and clinically practical to use. These properties make silicon diodes a preferred dosimeter for relative dosimetry for several types of measurements in small field dosimetry, e.g., stereotactic treatments and intensity modulated radiotherapy (IMRT). Silicon diodes are, however, limited by an energy dependent response variation in photon beams, resulting in that the diode readout per dose to the phantom medium varies with photon spectral changes, thereby introducing a significant uncertainty in the measured data. The traditional solution for the energy dependent over-response caused by low-energy photons is to use diodes with a shielding filter of high atomic number. These shielded diodes, however, show an incorrect readout for small fields due to electrons scattered from the shielding (Griessbach et al. 2005). In regions with degraded lateral electron equilibrium (LEE) shielded diodes over-respond due to an increased degree of LEE, as a consequence of the high density shielding (Lee et al. 2002). In this work a prototype software that corrects for the energy dependent response of a silicon diode is developed and validated for small field sizes. The developed software is based on the novel concept of Monte Carlo (MC) simulated fluence pencil beam kernels to calculate spectra (Eklund and Ahnesjö 2008), and the spectra based silicon diode response model proposed by Eklund and Ahnesjö (2009). The software was also extended to include correction of ionization chambers, for the energy dependent Spencer-Attix water/air stopping power ratio (sw,air). The calculated sw,air are shown to be in excellent agreement with published values to better than 0.1% for most values, the maximum deviation being 0.3%. Measured relative depth doses, relative profiles, and output factors in water, for small square field sizes, for 6 MV and 15 MV clinical photon beams are presented in this work. The results show that the unshielded Scanditronix-Wellhöfer EFD3G silicon diode response, corrected by the developed software, is in excellent agreement with reference ionization chamber measurements (corrected for change in sw,air), the maximum deviation being 0.4%. Measurements with two types of shielded diodes, namely Scanditronix-Wellhöfer PFD silicon diodes (FP1990 and FP2730), are also included in this work. The shielded diodes are shown to have an over-response as large as 2-3.5% for field sizes smaller than 5 cm x 5 cm. The presented results also suggest a difference in accuracy as large as 0.5-1% between the two types of shielded diodes, where the spectral composition at the measurement position dictates which type of diode is more accurate. The fast correction of silicon diodes provided by the developed software is more accurate than shielded diodes for small field sizes, and can in radiotherapeutic clinical practice increase the dosimetric accuracy of silicon diodes.