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UBC Theses and Dissertations

Combining Monte Carlo simulations and detector measurements for a novel quality assurance system of assembled brachytherapy eye plaques. Möehle, Swantje D.


Quality assurance of radiation therapy devices used to treat cancer is important to assure successful treatment and prevent over- or under-dosage. For brachytherapy eye plaques, the current standard procedure involves measuring the individual seed air-kerma strengths in a well chamber and visually inspecting the plaque. This leaves room for errors to occur between seed verification, plaque assembly, and implantation. In addition, the availability of manufacturer-assembled commercial eye plaques has increased, where independent, pre-implantation verification is limited to verifying the seed strength of additional loose seeds from the same seed lot. Currently, no commercially available system or technique exists for routine quality assurance of assembled eye plaques. This thesis investigates whether a flat panel image detector can be used in combination with Monte Carlo simulations, to develop a quality assurance system for assembled I-125 and Pd-103 brachytherapy eye plaques. The detector is used in conjunction with an acrylic plaque holder to record the 2D dose distribution of the plaque perpendicular to the plaque central axis. The system is simulated in Monte Carlo to generate a 3D dose distribution. Calibrating the detector relative to the Monte Carlo results allows comparisons between simulated and measured plaques, making it possible to verify the central dose rate of the plaque and to verify the 2D dose distribution in the detector plane using gamma index analysis. Results showed good agreement between the dose distributions and central strengths of detector measured and simulated results for a variety of seed strengths and seed arrangements, enabling the detection of dead or missing seeds. A procedure was established to compare the results to TG-43 dose calculations used in treatment planning. The method was tested on BC Cancer in-house design acrylic eye plaques and a commercial Eye Physics dummy plaque. The detector showed strong attenuation for incoming radiation with large angles of incidence, limiting the applicability of this method for individually collimating plaques. Overall, this study showed that a flat panel imaging detector and Monte Carlo simulations can be combined for routine quality assurance and verification of assembled eye plaques without individual seed collimation.

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