Novel Developments and Applications of Dosimetry in Radiation Therapy Treatment

Abstract

Advanced treatment planning and treatment delivery techniques in radiation therapy have increased the need for comprehensive machine commissioning, quality assurance (QA), and patient-specific QA to ensure that treatments are delivered with precision and accuracy. In vivo dosimetry has an important role in verifying the treatment delivery and it is recommended for high dose irradiations, special procedures, and when implementing new treatment techniques. Most of the currently available in vivo radiation dosimeters can only provide point measurements or 2D superficial dose measurements, which are not accurate surrogates for the full 3D dose distribution. Ionizing radiation acoustic imaging (iRAI) is a novel 3D dosimetry tool, that has the potential for fast, non-invasive, per pulse dose measurement capability. This research focuses on detailed 3D dosimetry studies testing the feasibility of iRAI in ultra-high dose rate (FLASH) and conventional radiation therapy. A full simulation study was developed to test the potential of iRAI in FLASH-RT for the first time. This study demonstrated the feasibility of iRAI in beam characterization and localization and studied the effect of the linear accelerator (linac) operational parameters and their effect on iRAI imaging. As part of this work, a customized 2D array transducer was used to study the implementation of iRAI in conventional radiation therapy. Different treatment plan dose distributions were measured and verified with 3D gel dosimetry measurements. IRAI was capable of measuring shifts in the radiation fields within 0.3cm relative to gel results. Additionally, iRAI efficiently detected the radiation field sizes within 0.35cm. The repeatability and dosimetric evaluation of the acquired iRAI dose-related images were promising for relative 3D plan verification and monitoring. This dissertation also describes the full commissioning of a megavoltage research linac for small-field animal irradiation studies. An efficient and accurate full commissioning procedure has been developed and implemented using 1D, 2D, and 3D gel dosimeters. The characterized 3D gel dosimeters provide a full representation of 3D dose, and dosimeter misalignment corrections, and demonstrated high reproducibility with low interdosimeter variability. Gels have resulted in fast, full relative dosimetry commissioning and beam characterization for non-standard small radiation fields. Dosimetric characteristics have been measured with gels including, the linatron calibration factor, variability with time, beam divergence, dose profiles, percent depth dose curves, and relative output factors. Monte Carlo-based optimization and validation with the experimentally acquired beam characterization results have been implemented. The simulation of the linatron components and the optimization of its initial source has provided a full representation of the dose. The initial linac source parameters were investigated to be 9.8±0.2 MeV beam energy, with a 0.5o±0.1o angular distribution and 0.15±0.025 cm, 0.075±0.025 cm horizontal and vertical radial intensities respectively. The full phase-space files of the linatron static field and the different collimated small beam sizes have been scored. The phase-space files can be used as the source files for MC-based dosimetric pre-treatment validations, simulating the experimental setup, and work as a dosimetric planning and evaluation system for future small field animal-based treatments.

The results of this research will enable and enhance the implementation of dosimetry tools throughout the radiation therapy treatment process. IRAI can be implemented for deep-tissue in vivo dosimetry to enhance treatment monitoring and verification. Additionally, the 3D dosimetric capability of gel dosimetry has demonstrated a valuable role in improving the beam characterization and commissioning of non-standard radiation fields.