Year

2019

Degree Name

Master of Philosophy

Department

School of Physics

Abstract

Volumetric Modulated Arc Therapy (VMAT) involves irradiating the tumour while simultaneously varying the dose rate, gantry speed and MLC apertures. The success of VMAT delivery depends on the accurate performance and synchronisation of its dynamic parameters. The aim of this research was to evaluate the use of high spatial and temporal resolution solid-state detectors (DUO and Octa) combined with a digital inclinometer as a machine-specific quality assurance (QA) device for VMAT. The QA tests were based on the guidelines published by the NCS Code of Practice Report 24.

The detector assembly was attached to the accessory tray and lodged into the designated slot while the inclinometer was mounted onto the linac head. All tests were performed on a Clinac 21iX and a Varian Truebeam linear accelerator. Measurements with the proposed system were simultaneously acquired and compared to machine log files.

The DUO detector’s response was characterised for flattened and unflattened megavoltage beams and evaluated in terms of output linearity and reproducibility at different dose rates. The DUO showed a linear response with accumulated dose and a reproducibility of ±0.5% at different dose rates. The dose rate and gantry speed were assessed as a function of gantry angle. Results agreed to within 1% in comparison to the machine log files in the constant gantry speed and dose rate sectors. The effect of inertia on the delivery was assessed under extreme modulations of dose rate and gantry speed and compared to machine log files data and EBT3 film. The detector/inclinometer system was able to detect discrepancies between plan and measurements due to the effect of inertia on the gantry. The proposed system also demonstrated sensitivity to delivery errors deliberately introduced in the spokes. Furthermore, the MLC leaf speed was evaluated using the Octa detector under static gantry conditions in directions parallel and orthogonal to gravity as well as under dynamic gantry conditions which incorporated simultaneous modulation of dose rate and gantry speed. The MLC leaf speeds measured with the Octa agreed with the nominal speeds and the machine log files to within 0.03 cm.s-1. The effect of gravity on the leaf motion was only observed when the leaves travelled at a speed that exceeded the maximum allowed as stated by the vendor. Results of the leaf speed tests under dynamic gantry conditions showed agreement with the machine log files with percentage differences that ranged from 0.91% to 5.71%. Based on the results of this research, the proposed system verified the capability in the accurate reconstruction of dose rate and gantry speed as a function of gantry angle as well as in the evaluation of the MLC leaf speed under static and dynamic gantry conditions and demonstrated its sensitivity to delivery errors. Agreement with the machine log files suggests the suitability of the proposed system as a commissioning and machine-specific QA device of VMAT.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.