P18. Using a dosimetric gel to assess the spatial dose distribution around a brachytherapy 125I seed

IntroductionBrachytherapy 125I ophthalmic seeds are used to treat eye tumors. The overall absorbed dose distribution within the eye is the sum of the spatial distributions, in terms of absorbed dose to water, of each used seed. The spatial absorbed dose to water distribution of any seed is obtained by calibrating its relative distribution, as published by the AAPM [1], by its absolute dose value [1] ;  [2]. This preliminary study assesses, using a dosimetric gel, the relative spatial absorbed dose distribution around a brachytherapy 125I seed, then compares it to the one calculated here using the MCNPX code [3] and to that published by the AAPM. This work is aimed at validating the use of this gel for low energy photons (u003c50 keV) as encountered in contact radiotherapy.MethodsThe used dosimetric gel is developed by Jean-Perrin CRLCC in Clermont Ferrand and Paul Sabatier University in Toulouse. Its relaxation speed (R2) is read using Magnetic Resonance Imaging (MRI) and the R2 variations depend directly on the absorbed dose in the gel. Moreover, since the chemical species, produced under irradiation and responsible for the R2 variations, do not diffuse during several hours, the gel spatial resolution is millimetric. The gel sensibility threshold is about 1 Gy. Fig. 1.A shows the experimental setup. A waterproof cylindrical phantom, with a Kapton® tube closed at the bottom side and positioned at its center, is filled with liquid gel. Once jellified at low temperature, the gel phantom is immersed in a water tank, adapted to the AAPM published conditions. An 125I seed (BEBIG, I125.S16, 20 mCi) positioned at the Kapton® tube’s end irradiated the dosimetric gel for 70 min. Once the seed removed, the gel phantom was read by MRI. In the MCNPX model of the experimental setup (Fig. 1.B), a cylindrical mesh tally in the gel was defined to calculate the deposited energy per unit volume.ResultsThe relative absorbed dose distributions, obtained by MCNPX, for water and gel, agree to that published by the AAPM. The limited irradiation time, governed by the diffusion of the paramagnetic agent in the gel, resulted in satisfying results only near the source (up to about 5 mm) where the delivered doses were bigger than the gel sensibility threshold. Moreover, the removal of the 125I seed and tube disturbed the gel signal at the source interface.ConclusionThe dosimetric gel requires a radiation source of a higher dose rate, as delivered by the INTRABEAM®, to more robustly validate its use in the domain of low-energy photons. Figure optionsDownload full-size imageDownload high-quality image (99 K)Download as PowerPoint slide