DOSE IMAGING IN RADIOTHERAPY: COMPARISON OF POLYMER AND FRICKE DOSIMETERS

E.VANOSSI M. CARRARA G.GAMBARINI M.MARIANI M.VALENTE

Bologna, Italia

Congreso; SIRR (Società Italiana per le Ricerche sulla Radiazione); 2006

In recent radiotherapy techniques, aimed at achieving high dose release in volumes conformal to the tumours and low dose in healthy tissue around, the high non-uniformity of the dose spatial distribution makes imperative to perform reliable 3-D determinations. The high dose gradient of such radiation fields requires the development of particular dosimetry methods. The method proposed is based on a gel matrix with good tissue-equivalence in which suitable chemical dosimeters are added. The changes induced by the radiation in such gels are exploited to evaluate the spatial dose distribution. Such effects remain spatially localised and then this system constitutes a continuous dosimeter. The absorbed dose is not affected by the chemical dosimeter, always present in mM amount and whose role is that of making measurable the effect of the absorbed dose through the analysis of the variation resulting in certain physical parameters of the system such as the relaxation rates of hydrogen nuclei, measurable by nuclear magnetic resonance imaging (NMR), or the gel optical absorption in the visible light spectrum, measurable by spectrophotometry. Two kind of gel dosimeters were studied: i) a dosimeter in which the Fricke solution (a ferrous solution) was infused with Xylenol Orange (X.O.); ii) dosimeters with a monomer solution that can become a polymer after irradiation. In the first kind of dosimeters the ionising radiation causes oxidation of ferrous ions and the complex (Fe3+-XO) gives light absorption near 585 nm allowing optical imaging, while in the second kind the polymerisation due to the interaction with the radiation changes the opacity of the system. In order to obtain the spatial distribution of the dose, the dosimeters were prepared in form of layers with a thickness of 3 mm or less. In this way it is possible to place the layer in a phantom that can simulate the human tissue, or to put together more layers in order to obtain a dosimeter phantom. A CCD camera and an illuminator were used to analyse the dosimeters and to take images of transmittance and an apposite Matlab® software was developed. With this software it is possible to obtain images of the absorbed dose, profiles, isodose curves and 3D representations of the images by means of the analysis of the CCD camera images acquired before and after the irradiation[1]. The dosimeters were put in a polystyrene phantom and irradiated with the gamma rays from 60Co of a radiotherapy facility. The CCD images were utilised in order to achieve the depth-dose profiles and these profiles were compared to that calculated with the Monte Carlo code in the same conditions.