Olivia Moluchi

Topic

Optimized Polymer Gel Dosimetry for Kilovoltage X-ray Spatially-Fractionated Radiation Therapy

Department of Physics and Astronomy

Date & location

• Tuesday, August 11, 2026
• 12:00 P.M.
• Clearihue Building, Room A127

Examining Committee

Supervisory Committee

• Dr. Magdalena Bazalova-Carter, Department of Physics and Astronomy, University of Victoria (Supervisor)
• Dr. Mehran Goharian, Department of Physics and Astronomy, UVic (Member)

External Examiner

• Dr. Andrew Jirasek, Department of Computer Science, Mathematics, Physics and Statistics, University of British Columbia Okanagan

Chair of Oral Examination

• Dr. Dante Canil, School of Earth and Ocean Sciences, UVic

Abstract

Radiation therapy (RT) is a highly effective and widely used treatment modality for cancer management. Advancements in treatment delivery have enabled highly conformal dose distributions that closely match the 3D shape of the tumour, improving cancer cell kill while minimizing radiation exposure to surrounding healthy tissues. As these techniques continue to increase in complexity, accurate 3D dosimetric verification is essential to ensure safe and precise treatment delivery. Unlike conventional dosimeters, which are limited to point, planar, or reconstructed volumetric measurements, polymer gel dosimeters (PGDs) directly measure 3D dose distributions within an irradiated volume with high spatial resolution and radiological tissue equivalence. The goal of this thesis was to optimize a N-isopropylacrylamide (NIPAM)-based PGD for kilovoltage x-ray irradiations delivered using the low-cost dual-robot radiotherapy KOALA system and investigate its feasibility as a volumetric tool for RT applications.

First, the NIPAM-based PGD was optimized for kilovoltage x-ray irradiations through a sensitivity study. Six gel formulations with varying concentrations of NIPAM, N,N’-methylene-bis-acrylamide (BIS), and tetrakis(hydroxymethyl)phosphonium chloride (THPC) were investigated to increase the dose response of the PGD while maintaining optical clarity and cost-effectiveness. A formulation containing 7% NIPAM, 4% BIS, and 10 mM THPC provided the highest dose sensitivity, while higher BIS concentrations were limited by insolubility and crystallization. The optimized PGD demonstrated a non-linear dose response with saturation near 55 Gy for a 225 kVp beam of KOALA, showing limits of polymerization at higher doses.

The optimized PGD was then applied to the dosimetry of spatially-fractionated radiation therapy (SFRT) using kilovoltage x-ray irradiations. SFRT is a specialized form of RT characterized by the delivery of minibeams to create alternating high- and low-dose regions within the irradiated volume. Single- and multi-beam SFRT irradiations were delivered using a custom tungsten mini-slit collimator consisting of twelve 300-μm beams mounted on KOALA. The PGD was able to distinguish the peak and valley dose structures and demonstrated good agreement with experimental radiochromic film measurements and TOPAS Monte Carlo (MC) simulations in terms of the spatial distribution of the minibeam dose pattern. However, peak-to-valley dose ratios (PVDRs) were not uniform across the
SFRT dose profiles at 6 mm depth and ranged from 1.8 - 4.2, likely due to partial volume effects caused by the limited spatial resolution of the x-ray computed tomography (CT) scanner. The film and MC simulated PVDR values were 15.3 and 17.1 at the same depth, respectively, indicating greater contrast between peak and valley dose regions compared to the PGD measurements. This difference is attributable to the higher spatial resolution of the film readout system and the idealized nature of MC simulations.

The final component of this work was to investigate the use of PGDs for 3D isocenter verification using non-coplanar star shot irradiations. Six beams were delivered within a single PGD to evaluate the geometric accuracy of both KOALA and a Varian TrueBeam linear accelerator (LINAC). Two measurements performed on KOALA yielded an initial isocenter isocenter walkout radius of 3.5 mm, which improved to 1.6 mm following a geometric calibration adjustment of the delivery system. While the system performance was improved, the measured radius remained above the clinical tolerance of ± 1 mm. Film measurements performed on KOALA yielded a radius of 0.9 mm; however, the limited volumetric information provided by 2D film made accurate localization of certain beam trajectories challenging. The LINAC measurement demonstrated a radius of 0.6 mm, although increased lateral spread of high-energy secondary electrons resulted in a broader and more diffuse gel response.