BrachyShade : real-time quality assurance for high dose rate brachytherapy

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High dose rate (HDR) brachytherapy is a popular form of radiotherapy in which radiation is delivered to the tumour via a small sealed radioactive source, which is moved through a sequence of positions in an array of catheters pre-implanted in the target area. Brachytherapy offers a key advantage over external beam radiotherapy, since the radiation dose is delivered directly to the diseased tissue while minimising the dose applied to healthy tissue in proximity to the target volume. The accuracy of source placement is critical to the success of HDR brachytherapy. Deviations between the planned source position and the actual position achieved during treatment, due to anatomical changes (e.g. due to swelling or post-imaging tumour growth) or imperfect catheter placement, can harm healthy tissue or under-irradiate diseased tissue. Therefore, a reliable, accurate, real-time 3D source tracking system would be extremely valuable for treatment quality assurance and would allow a treatment plan to be modified in real time if positioning errors are detected. HDR BrachyView is an in-body source tracking system designed to monitor the location of a HDR prostate brachytherapy source developed at the Centre of Medical Radiation Physics at the University of Wollongong, based on a tungsten pinhole camera with a silicon pixellated photon detector. Source position is estimated by back-projecting images of the source projected through the pinholes onto the imaging plane. Although HDR BrachyView has been shown to perform very well, it is challenging to manufacture, and suffers from a small systemic error in position estimation. BrachyShade proposes to replace the tungsten collimator with a series of small spherical or spheroidal tungsten occluders embedded in a plastic shell, suspended over the same pixellated detector (TimePix) used in the original HDR BrachyView. Instead of tracking bright projections of the source, the shadow of the source will be tracked, and by parametrically fitting an analytic model of the shadow map (where the model parameters are source position and intensity), the source position will be estimated. The proposed design significantly simplifies the manufacturing process, lowering the costs of manufacturing; it will also allow many more photons to arrive at the detector, enabling faster acquisition of a high quality position estimate. The achievable accuracy is comparable to HDR BrachyView, with a wider field of view achievable, depending on the specific configuration of tungsten occluders. This Thesis presents a set of Monte Carlo simulations of the system, performed in Geant4. A sophisticated analytic model of the shadow map has been derived, and an algorithm developed which estimates the source position by minimising the error between the output of the analytic model and the detected photon map. A post-processing stage eliminates the effects of Compton scatter, which are otherwise mathematically challenging to include in the analytic model. Exhaustive test results proving the accuracy of the algorithm are presented. A second analytic method for estimating source position via a hierarchical pattern-matching strategy is also described, and preliminary results presented.
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