IC3DDose 2010 – 6th International Conference on 3D Radiation DosimetryDose enhancement caused by gold foils on polymer gelsL.C. Afonso; M. Greiter; F. Schoefer; C. HoeschenHelmholtz Center Munich, Ingolstaedter Landstrasse 1, 85764 Neuherberg, Germanyluciana.afonso@helmholtz-muenchen.deAbstract. The presence of high-Z materials adjacent to soft tissues, when submitted toirradiation, locally enhances the absorbed dose in these tissues. This effect is due to theoutscattering of photoelectrons from the high-Z materials. The aim of the present work was tomeasure the absorbed dose enhancement caused by gold foils on polymeric gel.1. IntroductionPrevious studies showed that the presence of high-Z materials adjacent to soft tissues, when submittedto irradiation, enhances the absorbed dose in these soft tissues. Such effects happen due to theoutscattering of photoelectrons from the high-Z materials. [1, 2, 3]The concern about the effect of dose enhancement arose after discovering that oral cavity radiationtherapy treatments on patients that had golden tooth replacements resulted in an increased damage ofthe soft tissues surrounding these gold replacements. It was found that the absorbed dose could reachan enhancement of two orders of magnitude in the microscopic vicinity of gold replacements, whenexposing the patient to diagnostic X-ray examinations. [1]In experiments with human lymphocytes covered with a thin gold film and then irradiated withdiagnostic X-ray beams, it was found that the dose absorbed by the cells was increased by a factor of45.4. The number of chromosomic aberrations was also in accordance to this dose enhancement. [2]Thereby, the aim of the present work was to measure the absorbed dose enhancement caused by thepresence of gold foils on the surface of polymeric gel samples, whose main characteristic is the softtissue equivalence. The polymer gel dosimetry associated to the Nuclear Magnetic Resonancetechnique allows the absorbed dose to be measured with high three-dimensional resolution, providingthe spatial distribution of the dose enhancement caused by the photoelectrons outscattered from thegold foil.2. Dosimetric MethodIn order to resolve local dose distributions in soft tissue equivalent material close to high-Z materialspolymer gel dosimetry was used.Polymeric systems were first studied in relation to their dosimetric capacity in the fifties, byevaluating the radiation effects in poly-methyl-methacrylate [4]; and studying radioinducedpolymerization in liquids [5]. Later works combined these properties as the first polymer geldeveloped by Maryanski et al [6, 7], which used a formulation that was composed by acrylamide, bisacrylamideand nitrous oxide diluted in an agarose aqueous matrix.457
IC3DDose 2010 – 6th International Conference on 3D Radiation DosimetryFurther gel formulations were developed by changing the gel matrix (agarose to gelatin), themonomer (acrylamide to methacrylic acid, for example), etc. However, the working principle of anypolymer gel dosimetry is based on the radiation-induced polymerization of the monomers present inthe gel. When the gel is irradiated, water radiolysis is induced. The polymerization process is mainlyinitiated by free radical reactions. The polymerization degree depends on the number of free radicalscreated by the incident radiation, which depends on the absorbed dose; therefore, originating spatialdose resolution. [8, 9]The polymer gel used for this work is known as MAGIC (Methacrylic Ascorbic in Gelatin Initiatedby Copper), foremost developed by Fong et al [10]. Its formulation is composed by methacrylic acid,ascorbic acid, gelatin and copper sulfate. As the gel polymerization process is initiated by free radicalreactions, and molecular oxygen is an efficient “scavenger” of free radicals, its presence inhibits gelpolymerization. The ascorbic acid, in a process initiated by the copper sulfate, is responsible forcapturing oxygen in the gel solution.3. Methodology3.1. Polymer Gel PreparationThe polymer gel samples used in this study were produced using deionized water, gelatin (swine skin,300 bloom, Sigma Aldrich), methacrylic acid (99%, Sigma Aldrich), copper (II) sulfate penta-hydrate,and ascorbic acid (extra pure, Merck).At room temperature, 140 g of gelatin were added to 700 ml of water. After the water has beenabsorbed by the gelatin, the mixture was heated to 55 ºC in a water bath and was let to liquefy forapproximately 1 hour. The ascorbic acid (0.3522 g) was added; then, copper (II) sulfate (0.025 g); andfinally the methacrylic acid. After each reagent has been added, the gel was mixtured thoroughly. Themixture was kept in the water bath for more approximately 1 hour. Afterwards the gel was poured into12 glasses (50 ml, 5 cm of diameter), which were previously immersed in an oxygen scavengersolution (water, ascorbic acid and copper (II) sulfate) for 6 hours. The filled glasses were covered withcling film (14 ± 2 µm) and were positioned upside down in an oxygen scavenger solution, in order toprevent oxygen penetration through the cling film into the polymer gel.3.2. IrradiationThe polymer gel samples were irradiated 24 hours after production. A Philips Industrial X-rays (nowYxlon International) system with a MCN 323 tube was used to generate x-rays at 200 kV, 15 mAfiltered with 0.96 mm Cu and 4.05 mm Al. A PTW ionization chamber (type 23361, 30 cm 3 sensitivevolume) coupled with a PTW Unidos electrometer was used for the dose measurement.Gold foils (thickness about 100 nm) were placed on the cling film surface of each sample, coveringhalf of the surface area (Figure 1). The polymer gel samples were individually positioned with thecling film surface perpendicular to the beam at the reference point at 70 cm from the x-ray focal spot(Figure 2). Irradiation doses, measured as air kerma at the reference point, were 0, 1, 2, 3, 4, and 5 Gy,attained at a constant dose rate of 4.52 mGy/s (± 1.5 % combined uncertainty; including positioning,calibration factors, instrumental stability, and corrections for air density). Two samples per dose wereused.5 cmAu foilClingfilmGlass withpolymer gelFigure 1: Schematic representation of a sample.458
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AUTARQUIA ASSOCIADA À UNIVERSIDADE
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AGRADECIMENTOSÀ Dra. Linda V. E. C
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STUDY OF THE INFLUENCE OF GOLD PART
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5.1.3 Dependência do Dosímetro em
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A resposta dos tecidos às radiaç
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1978), indicando que pode haver um
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delta (δ). Feixes contínuos de el
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da energia do fóton. Quando intera
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Figura 3.1 - Esquema de interação
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material atravessado. A Figura 3.2
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esultados experimentais, complement
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Já o processo de transição não
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K tr(Eq. 3.24)Outra grandeza impor
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comum. Este tipo de gel ficou conhe
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Estes radicais peróxido reagem rap
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submetidos ao mesmo campo magnétic
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t T1M zM01 e(Eq. 3.45) em que T
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3.4.4 Avaliação dos Dosímetros d
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3.5 Método de Monte CarloO método
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Tabela 4.1 - Especificações dos f
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5 METODOLOGIA E RESULTADOSNeste cap
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Os parâmetros TE (tempo de eco, in
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Figura 5.3 - Ajuste monoexponencial
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Tubo de raios XFiltrosCâmara monit
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IA = 1, 4 e 5 dias.Figura 5.8 - PI
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Figura 5.9 - Relação entre R 2 -R
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As amostras foram avaliadas 1 dia a
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As amostras foram avaliadas 1 dia a
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polimérico deveriam seguir uma dis
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Coeficiente angularFigura 5.16 - Re
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