Development of fibre-optic dosimetry system as a quality assurance tool for High Dose Rate Brachytherapy

Abstract

Brachytherapy is an advanced technique of cancer treatment, where a high dose rate radioactive source, typically 192Ir, is inserted within a tumour volume to destroy the spread of cancerous cells. The complexity of such advanced treatment options requires accurate quality assurance (QA) tools to verify the treatment plan and track the High Dose Rate (HDR) source position as clinically prescribed. The misplacement of a HDR 192Ir brachytherapy source by several millimetres within a treatment volume is not clinically tolerated, and must be addressed prior to conducting the actual treatment. For the safety of patients benefiting from this treatment, it is ideal if the planned position of the HDR 192Ir brachytherapy source position is verified in the targeted volume and tracked online throughout the entire treatment procedure. Sophisticated quality assurance tools are now widely implemented in clinical scenarios within treatment rooms to acquire online feedback of the treatment process to assure the treatment is going as clinically prescribed. The limitations of currently used QA tools for HDR brachytherapy have necessitated the development of innovative tools for performing accurate QA tests without the need for further measurement corrections. Fibre optic-based scintillation detectors have clinically demonstrated desirable properties over other conventional detectors, and show great potential to be reliably applied for dosimetric purposes in a variety of radiotherapy techniques The Centre for Medical Radiation Physics at the University of Wollongong, Australia has a long history and become distinguished for developing sophisticated real-time QA tools for various radiotherapy applications. The aim of this research project was to develop an innovative scintillating fibre optic system as real-time QA tool for HDR Brachytherapy. The in-house development of a photodiode amplifier-based data acquisition system (DAQ) designed for this purpose has been demonstrated. In addition, the proposed system has been clinically characterised and optimised for both green emitters scintillating fibres (BCF-60) and Thallium Doped-Caesium Iodide CsI (Tl) scintillation crystals. A triangulation method of applying multiple fibre probes using our developed system was capable of localizing source position within +/- 1.0 mm when using our CMRP amplifier, or to within +/- 0.6 mm when using an adapted Gigahertz Optik™ commercial amplifier. Hence, using our system for clinical HDR source localization routinely seems feasible. Based on pre-clinical investigations obtained, the feasibility of localizing a 192Ir HDR source was shown to have more reliability and accuracy when using a CsI (Tl) crystal due to its higher signal to noise ratio compared with polymer fibres. However, scintillating polymer detector fibres were very convenient, functional and reliable, and experimental results described herein indicate that the developed system is also eligible to be employed for real-time HDR source tracking. Polymer fibres as detectors are inexpensive, convenient and accurate, except at the very lowest end of the absorbed dose spectrum. Radiation damage to the polymer fibres was found to be minimal and fibres could have useful lifetimes of several years in clinical use.