3D Verification of Dynamic and Breathing Adapted Radiotherapy using Polymer Gel Dosimetry

Abstract: Polymer gel dosimetry has been used since the 1990s, and several studies have shown that this detector system can be used for verification of static absorbed dose distributions in three dimensions (3D). Its unique properties, such as high resolution, normal tissue equivalence and independency of energy, field size and direction of the incident radiation, should also be advantageous for dosimetric verification of radiotherapy using today’s and tomorrow’s dynamic delivery techniques. However, unfavourable properties have also been reported, such as dose rate-, temperature-, oxygen contamination-, and cooling rate dependencies. It has been shown in this thesis that these shortcomings can be overcome by using a good practice strategy, and that results can be obtained with an uncertainty comparable to other detector systems.
Modern dynamic treatment techniques such as for example breathing adapted radiotherapy have created a need for dosimetry during motion, which poses new challenges. The purpose of this thesis was to investigate the performance of polymer gel dosimetry in such situations. For comparison, measurements using 1D, 2D and quasi-3D detector systems, as well as Monte Carlo simulations, were used to validate the results obtained using gel dosimetry.
The absorbed dose integrating property during fractionated irradiation delivery was investigated for two different polymer gel systems. A fractionation dependency was observed, especially pronounced for one of the systems. This effect was further investigated using compartment modelling. The results indicated that the dose response was approximately independent of the fractionation scheme, provided that the total absorbed dose was delivered during the same total delivery time. Under respiratory-like motion no influence of the dose rate variation related to motion in and out of the beam was observed. Full 3D absorbed dose verifications were also carried out for advanced delivery techniques involving simultaneous beam intensity modulation and gantry rotation around the patient, so called volumetric modulated arc therapy (VMAT). Using both gel measurements and Monte Carlo simulations it was successfully demonstrated that the VMAT plan was both accurately calculated and delivered as planned. Additionally, the performance of a tumour-tracking system during VMAT delivery was investigated. The dosimetric measurements, obtained using both gel and a bi-planar diode array, verified the improved dose conformity when enabling the target tracking system.
In this thesis the unique 3D properties of gel dosimetry were fully utilized, and the known uncertainties were minimized in every step of the procedure. It was shown that polymer gel is a useful tool for relative 3D dosimetry in dynamic and breathing adaptive radiotherapy.