An Investigation into the Applications of Polymer Gel Dosimetry in Radiotherapy

The verification of complex dose distributions produced using novel techniques in radiotherapy requires measurements in three dimensions with high spatial resolution. Recent developments in polymer gel dosimetry employing MRI suggest this may be the best available method. In this work the properties of a polymer gel (BANG) based on acrylamide and bis-acrylamide were investigated. The gel was found to be tissue equivalent and its response to absorbed dose reproducible to within ± 4% and linear up to 10-12 Gy. The linearity can be increased by using a higher weight fraction of bis-acrylamide. The response of the gel was also found to be independent of energy and doserate, but dependent on oxygen contamination and gel temperature during MR imaging. The potential usefulness of BANG gel in different areas of radiotherapy was determined. In brachytherapy a comparison of the relative doserate distributions between gel, calculation and TLDs agreed to within ± 5%. The overall precision in measurements of absorbed dose was estimated to be ±5%. The measured dose distribution from complex brachytherapy agreed well with the distribution produced by the Helax-TMS planning system. For external beam irradiation the depth doses measured in the gel showed good agreement with ion chamber measurements to within ± 3%. The dose distributions produced in the gel from multi-field irradiations were in agreement to within ± 4% at the 50% isodose level. The gel was also used to verify the absorbed dose distribution produced by intensity modulated beam techniques using compensators. The results of the gel measurements agreed well with film dosimetry. BANG gel was also found to be an excellent candidate for dynamic measurements and measurements in areas with restricted access. A novel technique for measurements in boron neutron capture therapy using the gel was also investigated. It was concluded that gel can be used to verify complex dose distributions