Almurayshid, MMS; (2016) Evaluation of photography of a plastic scintillator for quality assurance in radiation therapy. Doctoral thesis , UCL (University College London).

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Abstract

Quality assurance is a vital part of modern radiation therapy. This thesis deals with the development of a detector system for the quality assurance (QA) of modern external beam radiation therapy. The system consists of a plastic scintillator, a commercial camera and a computer. Different available organic scintillators were initially evaluated to select the most suitable scintillator for our design. Subsequently, many optical artefacts in our prototype design were evaluated and possible correction methods were presented to reduce the impact of the optical artefacts. The basic characteristics of the system (e.g. the reproducibility and response to changes of dose) were assessed in a series of low energy x-rays and high energy proton irradiations. Photographs of the scintillation light distributions were acquired using the detector system for low and high energy photons, electrons and protons and compared with the depth-dose curves measured with an ionisation chamber. During proton irradiation, there was a reduction in the light intensity in the Bragg peak region because the protons‘ high linear energy transfer (LET) leads to quenching where less light is produced than expected. We developed an approach which used Birks equation to correct for the quenching using the Monte Carlo code, Geant4. LET was modelled in Geant4 and was combined with the measured scintillation light to calculate Birks constant. We then used the derived value of Birks constant to correct the measured scintillation light distribution for quenching using Geant4. The results show that the light output increased linearly with the x-rays and proton dose with a correlation coefficient greater than 0.99. The system is stable and provides reproducible results to within 1% in all type of radiation. Good agreements were obtained between the scintillation and the ionisation chamber depth dose curves for both photon and electron beams if depth-scaling factor was considered for the depth dose for electrons. However, energy dependence was seen with low energy x-rays due to the mechanism of interaction at these energies depending on the material's mean atomic number. For protons, no energy and dose rate dependencies were observed for the dose rates and energies used in this work. The results show that Geant4 simulation offered an effective way to correct for quenching for any desired energy. The quenched simulated scintillation results are in good agreement with the measured scintillation results and with the variation in the position of the Bragg peak is less than 0.7%. The results show that the system has the advantage of providing 2D visualisation of individual radiation fields and responded linearly to dose for low energy x-ray beam (50-100 kV) but suffers from energy dependency. The detector system provides acceptable depth dose curves for high energy photons and electron beams but could be enhanced if the optical artefact is corrected for. In addition, we developed an effective way to correct for quenching during proton irradiation. The technique provides a convenient method for rapid, convenient, routine quality assurance for clinical proton beams.

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