Polymer gel dosimetry applied to ¿ particles, electrons and 300 kv x-rays

Polymer gels were used with magnetic resonance imaging (MRI) to measure three-dimensional absorbed dose distributions for beta particles, electron and x-rays beams that are used in radiotherapy. The manufacturing processes and calibration procedures for two dosimeters (hypoxic PAG and normoxic MAGIC gels) were investigated. The response of both gels was energy independent over a range of electron and photon energies commonly used for radiotherapy. However, dose response of both gels was dependent on the temperature at the time of MR scanning, while MAGIC was also dependent on the temperature at the time of irradiation, which had not been previously reported. Results suggest that MAGIC gel is superior to PAG, since it is easier to manufacture and unaffected by oxygen diffusion through wall materials. The potential usefulness of both types of gel in different areas of radiotherapy was studied, including vascular brachytherapy. Results were compared with doses measured using radio- chromic film, confirming that dose distributions for vascular brachytherapy sources with a high dose gradient can be measured using PAG. However, because of the disadvantages of the gel manufacturing process and the need for access to a high-resolution scanner, it was concluded that radio-chromic film would be the method of choice for routine quality assurance in brachytherapy. PAG and MAGIC gels were also used for dosimetry across the junction of 6MV photon and 12MeV electron fields that are often used in radiotherapy. Different photon field configurations were studied, and dose profiles were measured. For each configuration either significant "hot" or "cold spots" were measured, with good agreement between the MAGIC and PAG and radio- chromic film. This work has confirmed the usefulness of gel dosimetry in radiotherapy in general, and in beta and electron dosimetry in particular. In addition, these studies have quantified the advantages of normoxic gels over the hypoxic PAG.