Developing a Dosimetry Audit for Existing and Emerging Methods of Respiratory Motion Management in Radiation Therapy

Respiratory motion presents a significant challenge in external beam radiation therapy (RT), particularly for thoracic and abdominal treatment sites. Accurate dose delivery is critical for achieving local control of disease. Motion management refers to a diverse suite of techniques used to account for or mitigate the impact of respiration-induced anatomical shifts. Techniques vary in technical detail and complexity, resulting in a unique set of uncertainties impacting each step treatment chain (from simulation, planning, to image guidance and delivery) differently. Dosimetry audits serve as essential tools for assessing and improving the accuracy of RT, yet no national-scale audit for respiratory motion management currently exists in Australia and New Zealand. This is in part due to the lack of time-resolution in current commercial treatment planning systems, leading to the dynamic measurement problem: measurements conducted in a moving frame of reference cannot be directly compared to dose calculations in a static frame of reference. This thesis presents the design, development, and implementation of a national-scale end-to-end dosimetry audit tailored to respiratory motion management. The audit was developed with the Australian Clinical Dosimetry Service (ACDS) and is intended to establish minimum standards of accuracy for treatments involving motion management. The research was conducted in three stages: (1) defining the scope of the audit based on clinical needs, (2) designing and validating a novel phantom and measurement system, and (3) evaluating the audit’s performance through experimental studies and field trials. A national survey of RT facilities informed the audit’s design, revealing that passive motion management and breath-hold gating are the most commonly used techniques. The audit phantom was designed to simulate both lung and liver treatment sites, accommodate various motion management strategies, and enable simultaneous acquisition of motion-coupled and motion-decoupled radiochromic film measurements. A key challenge addressed in this work is the dynamic measurement problem. This thesis proposes and validates two complementary solutions: (1) region-of-interest (ROI) gamma analysis in motion-coupled (moving) films, and (2) while the motion-decoupled (static) films enable an assessment of the extra-target dose distributions. Bespoke dose-to-medium correction factors (kmed) were derived from Monte Carlo simulations to convert the measured film signal to the quantity reported by the TPS (dose-to-medium), and experimentally validated. Page 1 Alexander G Bur ton The audit’s sensitivity and specificity were quantified through controlled experiments involving known planning and delivery errors. The proposed ACDS scoring methods demonstrated superior error classification compared to existing standards, enabling robust detection of clinically relevant discrepancies. Field trials conducted at ten RT facilities across Australia confirmed the audit’s compatibility with diverse hardware and software systems, and preliminary dosimetry results showed sensitivity to errors across multiple motion management techniques. This thesis concludes that the ACDS motion management audit is a world-leading initiative that addresses a critical gap in RT quality assurance. It offers a modular framework for benchmarking motion management practices, with the potential to improve treatment accuracy and patient outcomes by establishing minimum standards for accuracy, which have not previously been defined in a clinically meaningful context. Future work will focus on greater efficiency and compatibility through design improvements, expanding the audit’s scope to emerging techniques, and refining assessment criteria.<p></p>