A polymer gel prepared under normal levels of oxygen Basic studies and the potential for IMRT verifications

Abstract: "The technology for planning and delivering radiotherapy has in the last decade undergone some revolutionary development and it is now possible to perform complex treatments such as stereotactic treatments and treatments using dynamic wedges, multileaf collimator and intensity modulation. The more precise limits surrounding the PTV have made it of utmost interest to be able to verify calculated dose distributions in 3D. For this purpose gel dosimetry along with MRI has shown to be suitable. In this thesis a new type of gel was used and its basic characteristics evaluated. This polymerisation-based gel can be produced and stored under normal levels of oxygen. The potential for the gel dosimetry system was pointed out for 3D verification of IMRT. The components of the gel are water, gelatine, methacrylic acid, ascorbic acid and copper sulphate. The mixing procedure was performed in a fume cupboard and the time needed for the manufacturing was approximately 2 h. As a result of the radiation induced polymerisation the NMR relaxation rates for the water protons in the gel are increased with absorbed dose. Screw-top glass vials were used when evaluating dose response characteristics e.g. dynamic range, magnetic field dependence, and the use of different types of methacrylic acids. The vials were also used for absorbed dose calibration. For the IMRT verification a cylindrical shaped glass bottle was used. The dose distribution measured with the polymer gel was compared with the distribution calculated by the treatment planning system Helios, CadPlan. Isodoses were used for evaluation. For this new type of polymer gel the dynamic range was found to be greatly extended compared to former polymer gels. As for other polymer gels it was shown that the purity grade of the components was crucial for the dose response. Further, the slope and the intercept were found to be decreased for increased magnetic field strength. It was shown that the gel has great potential for verification of treatments like IMRT. In the comparison between the measured and the by the treatment planning system calculated relative absorbed dose map it was found that the absorbed dose isodoses agreed mostly within 2 mm. However, there is a need of improved image/matrix evaluation methods for the 3D verification of the treatment planning system calculated dose map, both spatial and regarding dose levels."