ESTUDO DA INFLUÊNCIA DE PARTÍCULAS DE OURO NA DOSE ...

Absorbed dose enhancement caused by gold particles in polymer gelsL.C. Afonso, F. Schöfer and C. HoeschenInstitute of Radiation Protection, Helmholtz Zentrum, Munich, GermanyAbstract— Previous studies showed that the presence ofhigh-Z materials adjacent to soft tissues, when submitted toirradiation, enhances the absorbed dose in these tissues. Thiseffect is due to the outscattering of photoelectrons from thehigh-Z materials. The aim of the present work was to measurethe absorbed dose enhancement caused by the presence ofdifferent concentrations of gold particles in polymeric gels.Keywords— polymer gel, three-dimensional dosimetry, NMR,gold particles.I. INTRODUCTIONIt is known from previous studies that the presence ofhigh-Z materials adjacent to soft tissues, when submitted toirradiation, enhances the absorbed dose in these soft tissues.This is due to the outscattering of photoelectrons from thehigh-Z materials. [1, 2]The concern about the effect of dose enhancement aroseafter discovering that oral cavity radiation therapy treatmentson patients that had golden tooth replacements resultedin an increased damage of the soft tissues surroundingthese gold replacements. It was found that the absorbeddose could reach an enhancement of two orders of magnitudein the microscope vicinity of gold replacements, whenexposing the patient to diagnostic X-ray examinations. [1]In experiments with human lymphocytes covered with athin gold film and then irradiated with diagnostic X-raybeams, it was found that the dose absorbed by the cells wasincreased by a factor of 45.4. The number of chromosomicaberrations was also in accordance to this dose enhancement.[2]Thereby, the aim of the present work was to measure theabsorbed dose enhancement caused by the presence of differentconcentrations of gold particles in polymeric gels,which the main characteristic is the soft tissue equivalence.The polymer gel dosimetry technique allows the absorbeddose to be measured with high three-dimensional resolution,which is essential to analyze the enhancement distributiondue to photoelectrons outscattered by the gold particles.A. Polymer gelII. DOSIMETRIC METHODPolymeric Systems began to be studied in relation totheir dosimetric capacity in the fifties, by evaluating theradiation effects in poly-methyl-methacrylate [3]; andstudying radio-induced polymerization in liquids [4]. Later,works combined these properties as the first polymer geldeveloped by Maryanski et al [5, 6], which had formulationthat was composed by acrylamide, bis-acrylamide and nitrousoxide diluted in an agarose aqueous matrix. The polymerizationreaction in this gel was based on the crosslinkingof acrylamide monomers.Further gel formulations were developed by changing thegel matrix (agarose to gelatin), the monomer (acrylamide tomethacrylic acid, for example), etc. However, the workingprinciple of any polymer gel dosimeter is based on the radiation-inducedpolymerization of the monomers present inthe gel. When the gel is irradiated, the water radiolysis isinduced. The polymerization process is mainly initiated byfree radical reactions. The polymerization degree dependson the number of free radicals created by the incident radiation,which depends on the absorbed dose; therefore, originatingspatial dose resolution. [7, 8]The polymer gel used for this work is known as MAGIC(Methacrylic Ascorbic in Gelatin Initiated by Copper),foremost developed by Fong et al [9]. Its formulation iscomposed by methacrylic acid, ascorbic acid, gelatin andcopper sulfate. As the gel polymerization process is initiatedby free radical reactions, and molecular oxygen is an efficient“scavenger” of free radicals, its presence inhibits gelpolymerization. Therefore, the ascorbic acid, in a processinitiated by the copper sulfate, is responsible for capturingoxygen in the gel solution.B. Nuclear Magnetic ResonanceNuclear Magnetic Resonance (NMR) is a technique thatallows determining the properties of a substance by correlatingthe absorbed energy and the frequency, in the radiofrequencyrange of the electromagnetic spectrum. The techniqueuses the transitions between the rotational energyO. Dössel and W.C. Schlegel (Eds.): WC 2009, IFMBE Proceedings 25/III, pp. 414–415, 2009.www.springerlink.com