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Investigations on dynamic intensity modulated photon fields in radiation therapy Parsai, Homayon

Abstract

The objective of this thesis is to investigate the validity of a class of intensity modulated radiation beam delivery (IMRT) with the view towards its clinical implementation. Intensity modulated photon beam (IMB) delivery is a new radiation delivery technique in external beam radiation therapy. The technique is complex in planning, delivery and verification and frequently requires a large number of beams to achieve a desired dose distribution. Therefore, an important consideration in exploiting the advantages offered by this new technology is to study the degree of accuracy with which highly conformal dose distributions can be achieved. A methodology appropriate to dosimetry and quality assurance (QA) of dynamic fields is proposed in this thesis and the theoretical modeling was improved based on the results of this work. Dynamic fields utilizing the motion of multileaf collimators (MLCs) and backup diaphragms were used to produce 1 and 2D intensity maps arbitrary in orientation with respect to collimator axis. Dynamic wedge dose profiles produced by moving diaphragms are accurate to within ± 1% of those produced with conventional techniques. The accuracy of the omni wedge with arbitrary field orientations are found within ± 2° of calculations. A dosimetric verification technique used to monitor the dynamic beam delivery for IMRT plans is introduced. The design considerations and clinical evaluation of a QA phantom, facilitating the measurement of IMRT dose distribution and conversion of photon fluence to machine deliverable monitor units are described. Benchmark tests and clinical examples for IMRT dose verification techniques are carried out to demonstrate the accuracy of the technique, in-vitro and in-vivo dose measurements of dynamic IMB's were in good agreements with the calculation model. A theoretical error analysis on the influence of systematic and random field perturbations in highly conformal beam deliveries with emphasis on its dosimetric effects is also presented. It is shown that field inaccuracies in dynamic deliveries in the order of ±1.0 mm could lead to dose errors of 20% or more. Based on the findings of our study, we have proposed a QA procedure unique to the delivery of dynamic beams.

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