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Verification of Intensity Modulated Radiotherapy

Cufflin, Rebecca Sian 2012. Verification of Intensity Modulated Radiotherapy. PhD Thesis, Cardiff University.
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The main aim of this work was to develop accurate and efficient methods for the verification of Intensity Modulated Radiotherapy (IMRT). IMRT is an advanced form of radiotherapy demanding extensive verification procedures to ensure treatments are delivered accurately. This requires comprehensive sampling of the complex dose distributions impacting on the tumour volume and radiationsensitive ‘organs at risk’. This work has focused on the use of electronic portal imaging devices (EPIDs) for verification purposes. Modern EPIDs are composed of a scintillator and an amorphous silicon detector panel with an array of photodiodes and thin film transistors. They are primarily used to verify the patient position during treatment by capturing transmission images, but they also have the potential to be used as efficient dose verification tools of high spatial resolution. Two complementary dose verification methods have been developed. One approach involves the calculation of portal dose using Monte Carlo (MC) methods. A MC model of the linear accelerator, in combination with the EPID, enables the dose to the detector to be predicted accurately and compared directly with acquired images. An alternative approach has also been developed. This utilises a clinical treatment planning system (TPS) to calculate the dose at the detector level, and convert this to predicted EPID intensity by application of a series of derived correction factors. Additionally, there have been numerous publications in the literature detailing problems in dosimetry caused by non-uniform backscatter to the imager from the model of detector support arm used in this work. Two novel methods to correct for this issue have been developed, a MC modelling solution and a matrix-based correction. These developed methods for IMRT dose verification have been applied both prior to and during treatment. When applied to pre-treatment verification, the MC solution is accurate to the 2%, 2 mm level (an average of 96% of points passing gamma criteria of 2%, 2 mm) and the TPS based method is accurate to the 3%, 3 mm level (an average of 98% of points passing gamma criteria of 3%, 3 mm). Both verification methods achieve acceptable verification results during treatment at the 5%, 5 mm level (average gamma pass rates of 97% and 96% being achieved for the MC and TPS based solutions respectively). Furthermore, in initial clinical studies, both techniques have identified dose delivery errors due to changes in patient position or patient anatomy.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: R Medicine > R Medicine (General)
T Technology > T Technology (General)
T Technology > TA Engineering (General). Civil engineering (General)
Uncontrolled Keywords: Radiotherapy; physics; verification; dosimetry; Monte Carlo
Last Modified: 19 Mar 2016 22:44

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