Characterization and Clinical Application of Normoxic Polymer Gel in Radiation Therapy Dosimetry

Abstract: Cancer is one of the main causes of death in the world, particularly in developing countries and is considered as a major public health problem. Radiation therapy has been an important modality for treatment of cancer for many decades. It can be used as a sole treatment modality or in combination with for example surgery, chemotherapy, hormone therapy and/or immune therapy. It has been calculated that 52% of all cancer patients, at least once during their illness, would benefit from external beam radiation therapy. The purpose of radiation therapy is to deliver an accurate and homogenous absorbed dose of radiation to the tumour sufficient to either cure the patient or to shrink the tumour for pain relief. To reduce the probability of complications the absorbed dose to surrounding normal tissue should be minimized. For a successful outcome of the treatment it is therefore crucial to deliver geometrically and dosimetrically precise absorbed dose distributions. Polymer gel dosimetry has successfully been used in several studies for evaluation of the dosimetry in clinical situations and geometries. Measured absorbed dose distributions can be evaluated in three dimensions with high spatial resolution. For absorbed dose verification of clinical high energy photon beams, other great advantages of polymer gel dosimeters include negligible dependence on beam quality and near radiological soft tissue equivalence. The complete treatment chain, which involves CT-scanning, treatment planning and absorbed dose delivery, can be verified. A complete volume can be simultaneously evaluated and the gel-filled phantom can be anthropomorphically shaped. Nevertheless, gel dosimetry is still at times considered to be a research project and not a fully established clinical dosimetry system. In this thesis a ?good practice strategy? on how to use normoxic polymer gel dosimetry for the measurement of absorbed dose in the clinical radiation therapy setting was suggested. The recommendations were based on estimation and evaluation of the uncertainties in normoxic polymer gel dosimetry, performed in accordance with an accepted standard. To achieve the above and to facilitate clinical use of the dosimeter an image processing software was designed and developed to be used for all steps in polymer gel dosimetry. Furthermore, the absorbed dose integrating property was investigated for three different normoxic polymer gel dosimeters as part of the overall uncertainty evaluation. For all three systems the absorbed dose response was found to be dependent on the fractionation scheme. The feasibility of using normoxic polymer gel dosimetry for complete relative absorbed dose verification was investigated. The clinical applications of polymer gel dosimetry together with a point-based dosimeter for normalization of relative absorbed dose distributions were evaluated and were found to be of great value. Further, a low-density normoxic polymer gel dosimeter that can be used to simulate lung tissue was developed. Using the proposed "good practice strategy" the combined standard uncertainty for gel dosimetry was found to be comparable to corresponding figures for other detector systems.