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Monte Carlo Modelling and Measurements for Upright Radiotherapy

Year: 2025
Type: dissertation
Abstract: This thesis explores the development of upright radiotherapy, from its origins in the first external beamradiotherapy treatments to a modern solution. Unlike conventional supine radiotherapy, where a linearaccelerator within a gantry rotates around a patient in the supine position, upright radiotherapy rotatesa patient upright (perched or seated) with a fixed-beam linac. To explore the uncertainties associatedwith a conventional gantry (C-arm) linac due to rotation. Using a platform to eliminate gantry sag, therotational output dose uncertainty was shown to be 1 %. However, when rotating around a cylindricalphantom, this output uncertainty was less than 0.5 %. These mechanical uncertainties are not found in afixed-beam linac. Monte Carlo simulations were performed to predict dose distributions from the linachead. The EGSnrc and TOPAS toolkits were used to improve the designs of the primary collimator andthe entire linac head. An idealised shape for a primary collimator was found to be a conical frustum.This is an improvement over the conventional cylinder used in most radiotherapy linacs. This thesispresents a method for comparing shielding components, in collaboration with the engineering team fromLeo Cancer Care™, to optimise the design collimation for a fixed-beam linac. The linac head designwas simulated and adjusted to reduce (radiation) leakage. This resulted in a design which exceeds theregulatory requirement of 10-3 of the primary beam. Measurements and Monte Carlo TOPAS simulationswere performed on the multileaf collimator, including the leaf guide through which the primary beam wasattenuated. These simulations quantified the effect of the leaf guide in respect to the spectrum. Dosimetricmeasurements and Monte Carlo simulations were undertaken and validated with one another. Initialmeasurements on the multileaf collimator explored the attenuation through the bank, maximum interleafleakage, leakage due to abutting ends and further measurements of the leaf guide. These measurementswere undertaken with geometric challenges, yet the results could still be compared to industry standards.Measurements of the open field also highlighted the impact of the leaf guide. A physiological study wasalso performed on healthy participants to explore the differences in lung volumes and chest and abdomensurface measurements. This study demonstrated that vital capacity, expiratory reserve volume, peakexpiratory flow and forced expiration in one second (p < 0.05) were all larger in the upright position. Incontrast, inspiratory capacity was larger in the supine position (p < 0.05). These findings demonstratethe change in physiology due to changing patient position from the supine to the upright position and thepotential superior option for thoracic treatments.
Language: English
Subfield: Radiation
Open Access status: green
OpenAlex ID: W6961476305
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