Page 39 - Fisica In Medicina n° 1/2017

^Äëíê~Åí=cáëáÅá= =Optimization and characterization of a Fricke- gel dosimetry system for SBRT and SRS pre- treatment 3D dosimetry using MRI techniqueL.TROMBETTA1 *; G.M.LIOSI2; P.SALMOIRAGHI1; M.MARIANI2; F.LOCATELLI1; E.BOMBARDIERI11 Cliniche Humanitas Gavazzeni, Bergamo, Italy2 Department of Energy, Nuclear Engineering Division, Politecnico di Milano, Milano, ItalyIntroductionFricke gels chemical dosimeters (FGd) permit a 3D dose distribution assessment within the irradiated volume. They are based on the radiation-induced oxidation of Fe2+ ions form ferrous sulphate dispersed in a tissue equivalent gel matrix, into Fe3+ ions. The Fe3+ spatial concentration, linearly related to the absorbed dose, can be evaluated through MRI in T1-weighted sequences. Fe3+ ions diffusion process, with consequent spatial information loss, can be reduced by adding a chelating agent (Xylenol Orange – XO) to the recipe, even if this worsens dose sensitivity.This work has the aim of optimize and characterize a FG dosimetry system for patient pre-treatment QA with a special focus to SBRT and SRS plans, characterised by strong dose gradients in small treated volumes.Materials and MethodsOur FG dosimetry system was optimized in terms of Fe2+ ions and XO concentration, calibration method, MR acquisition (1.5T) and reconstruction parameters. We evaluated dose sensitivity, SNR and spatial diffusion of Fe3+ ions. The system was then characterized in terms of geometrical distortion (GD), image homogeneity, artefacts, image texture, dose accuracy and resolution, limit of detectability (LOD), dose rate dependence. Finally, the pre-treatment dosimetry of a mock SBRT plan was acquired and a relative planar profiles comparison with a standard dosimeter (Gafchromic EBT2) was performed. All FGd were studied in two gelling matrices (porcine gelatine and novel synthetic PVA).30