Year

2014

Degree Name

Master of Science - Research

Department

Centre for Medical Radiation Physics

Abstract

High dose Rate (HDR) brachytherapy utilises a HDR remote afterloader unit to deliver a high activity Ir-192 radioactive source directly into the tumour treatment volume. Since such high source activities are employed for this treatment type, special Quality Assurance (QA) measures need to be employed to ensure accurate radiation dose delivery as calculated by the treatment plan. Real time verification of the remote afterloader and pre-treatment patient specific treatment plan verification should be fundamental to a high dose rate (HDR ) brachytherapy quality assurance program. Such QA programs should have real time capabilities of accurately measuring static source dwell positions, precisely measure the time at each dwell location, display real time analysis of error in source positing/timing in comparison to the treatment plan, capable of evaluating transit velocity, track source movement and ideally, real time dosage evaluation and mapping of dose distributions for direct comparison to the treatment plan.

This thesis proposes the development of a new HDR QA smart system so named BrachyPix. The instrumentation and procedures associated with the development of this phantom is designed to address the requirements of an idealistic QA program for HDR brachytherapy. This instrument includes an innovative phantom specifically designed to provide a patient specific, pre-treatment confirmation that a treatment is being delivered according to patient plan. The thesis will focus upon characterisation of epitaxial diodes response to the Ir-192 HDR source for brachytherapy and will continue on to assess the feasibility of the Magic Plate (MP), an innovative CMRP two dimensional silicon detector array, for HDR source tracking and performance evaluation when combined with the BrachyPix phantom.

Tests for the angular dependence of the single epitaxial diode showed a 15±1% decrease in response when comparing face down to face up orientation of the detector. Depth dose experimental results showed excellent agreement with simulated dose calculation data from the treatment planning system, with subtle variation attributed to positional misalignment. As expected, source velocity was found to be dependent upon intra-dwell position spacing. For the standard clinical treatment step size of 2.5 mm, mean source transit velocity was found to be 15.2±0.6 cm/s. Source transit velocity was shown to be approximately consistent for dwell spacings >20mm with an average velocity of 31.2±1.8 cm/s. The MP was found to be highly capable of accurately tracking the iridium source by proficiently displaying a visual representation of a transversal reconstruction of source location for both a static and dynamic state. In addition to the visual representation capabilities, the system was proven to be able to numerically locate the local source position in three dimensions with a precision of 0.20 mm. These results give the sanction for continued development of a patient specific pre-treatment quality assurance program.

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