The image quality and dosimetric benefits of a novel 2.5 MV sintered diamond target beam on a modern radiation therapy linear accelerator

Abstract

Targeted dose delivery is achieved in radiotherapy treatments using image guidance for accurate patient positioning, and specialized collimators to shape the radiation field to the beam’s-eye-view of the treatment volume while shielding healthy tissue. Image guidance plays an important role in this workflow to inform patient setup before and/or during treatment. In the stereotactic treatment scheme, which involves high dose delivery in one or a few treatments, dose conformity to the treatment volume is of heightened concern, necessitating sharp dose fall-off immediately beyond the prescription isodose surface.

The purpose of this thesis is two-fold; to investigate the potential improved image quality in beam’s-eye-view MV image guidance and the dose fall-off in stereotactic treatments using a novel 2.5 MV low-Z target beam on a modern linear accelerator. Due to the decreased photoelectric absorption of low-energy photons within the target, low-Z beams contain higher proportions of low-energy photons compared to a high-Z target beam. In this work, a commercial 2.5 MV beam was modified to replace the conventional copper target with sintered diamond. It was hypothesized that this modification would produce a beam with a higher yield of low-energy photons that would contribute improved contrast characteristics for imaging and sharper dose fall-off for stereotactic treatments.

This thesis is composed of three manuscripts that examine the image quality characteristics of the novel beam and the dosimetric characteristics in stereotactic treatments. The first manuscript, ‘Investigation of planar image quality for a novel 2.5 MV diamond target beam from a radiotherapy linear accelerator’, evaluates the planar image quality of the 2.5 MV low-Z target beam compared to the commercial 2.5 and 6 MV beams. This work demonstrates improved planar image quality compared to the commercial beams. The second manuscript, ‘Improving image quality and reducing dose with 2.5 MV diamond target volume-of-interest cone beam CT’, investigates the image quality and dose characteristics of the 2.5 MV low-Z target beam in both full field of view and volume of interest cone beam CT. This study reports on improved image quality in both acquisition modes and the resultant dose-sparing from collimation in volume of interest acquisitions. The third manuscript, ‘Investigation of a novel 2.5 MV sintered diamond target beam for intracranial linac-based stereotactic treatments’, evaluates the 2.5 MV low-Z target beam in stereotactic treatments of trigeminal neuralgia and ocular melanoma. This study describes a dosimetric advantage in terms of dose fall-off and sparing of normal tissue compared to a conventional 6 MV beam.

The manuscripts contained in this thesis highlight the design and installation of the sintered diamond target, the methods of evaluating the novel beam in both imaging and stereotactic treatment applications, and the combined image quality and dosimetric advantages which could be realized through clinical implementation.