3D dose imaging for arc therapy techniques by means of Fricke gel dosimetry and Monte Carlo simulations

MAURO VALENTE , GUSTAVO CASTELLANO AND CARLOS SOSA

Buenos Aires Argentina

Congreso; 12th International Congress of the International Radiation Protection Association (IRPA 12); 2008

International Radiation Protection Association

Abstract. Radiotherapy is one of the most effective techniques for tumour treatment and control. During the last years, significant developments were performed regarding both irradiation technology and techniques. However, accurate 3D dosimetric techniques are nowadays not commercially available. Due to their intrinsic characteristics, traditional dosimetric techniques like ionisation chambers, film dosimetry or TLDs do not offer proper continuous 3D dose mapping. The possibility of using ferrous sulphate (Fricke) dosimeters suitably fixed to a gel matrix, along with dedicated optical analysis methods, based on light transmission measurements for 3D absorbed dose imaging in tissue-equivalent materials, has become of great interest in radiotherapy. Since Gore et al. showed in 1984 that the oxidation of ferrous ions to ferric ions still happen even when fixing the ferrous sulphate solution to a gelatine matrix, important efforts have been dedicated to developing and improving real continuous 3D dosimetric systems based on Fricke solution. The purpose of this work is to investigate the capability and suitability of Fricke gel dosimetry for arc therapy irradiations, which may constitute a first step to attempt dedicated dosimetry for Intensity Modulated Arc Therapy. The dosimetric system is mainly composed by Fricke gel dosimeters, suitably shaped in form of thin layers and optically analysed by means of visible light transmission measurements, acquiring sample images just before and after irradiation by means of a commercial flatbed-like scanner. Image acquisition, conversion to matrices and further analysis are accomplished by means of dedicated software, which has been developed in order to include suitable algorithms for optical density difference calculation and corresponding absorbed dose conversion. Also, dedicated Monte Carlo (PENELOPE) subroutines have been adapted in order to achieve accurate simulation of arc therapy irradiation techniques. Applications to 90° arc irradiation by means of a cobalt 60 machine show qualitatively good overall agreement between Fricke gel dosimeters and Monte Carlo simulations. In addition, high quality dose imaging has been achieved by means of computer tomography-like 3D reconstruction algorithms. However, the proposed methods are still being developed and require significant further improvements.   KEYWORDS: External radiotherapy; Fricke gel dosimetry; Monte Carlo simulation; Arc therapy techniques.