|Title:||Design and dosimetric validation of an irradiation setup for preclinical research with x-rays and protons||Other Titles:||Entwicklung und dosimetrische Validierung eines Bestrahlungssetup für die präklinische Forschung mittels Photonen und Protonen||Language:||input.forms.value-pairs.iso-languages.en||Authors:||Langgartner, Lorenz||Qualification level:||Diploma||Advisor:||Georg, Dietmar||Issue Date:||2022||Citation:||
Langgartner, L. (2022). Design and dosimetric validation of an irradiation setup for preclinical research with x-rays and protons [Diploma Thesis, Technische Universität Wien]. reposiTUm. https://doi.org/10.34726/hss.2022.101942
|Number of Pages:||84||Qualification level:||Diploma||Abstract:||
Cancer poses one of the mayor challenges of modern medicine, with radiation therapy (RT) being one of the most common treatment options. Ion beam therapy (IBT) may be a favorable treatment modality for tumors in close proximity of organs at risk (OARs) and radiosensitive tissue, due to fundamentally different interaction processes as encountered in conventional photon therapy. Despite a rapidly growing number of IBT centers in recent years, our understanding of the fundamental radiobiological aspects for ion beams still lacks behind compared to conventional RT. Pre-clinical in vivo studies are a key tool to investigate open questions in this field and close the gap from in-vitro cell experiments to clinical implementation. Most commonly small animals, such as mice and rats, are used in pre-clinical irradiation experiments. Irradiation of small animals is however particularly challenging, as it necessitates highest positional and dosimetric accuracy. The Department of Radiation Oncology of the Medical University of Vienna currently makes efforts to implement the technological basis for image-guided irradiation of small animals with ion beams and X-rays at the MedAustron Ion Therapy Center (MedAustron) (Wiener Neustadt, Austria), a synchrotron-based IBT facility. As the facility was not designed for small animal irradiation, the available infrastructure has to be adapted and supplemented. Furthermore, a dedicated workflow, similar to the one for patient treatment, has to be developed and established for small animals. The purpose of this work was to contribute to the technological development for pre-clinical in vivo studies at the facility in the future. The first aspect was the design of a beam collimation system for the 200 keV X-ray irradiation unit (YXLON Maxishot, YXLON GmbH, Hamburg, Germany), which will be used for reference irradiation in the future. To overcome the device’s limitation of an uncollimated, 120 mm broad beam, a collimation setup was implemented to achieve variable diameters of 1–35 mm, while also providing accurate positioning, adequate beam characteristics and practicality. To investigate relevant beam parameters, dimensions and suitable materials, a prototype was built. Depth doses curves and lateral dose profiles (LDPs) were measured in air for the prototype using a microDiamond detector (PTW-Freiburg, Germany) and GafchromicTM EBT3 films (Ashland Inc., Wayne, NJ, USA). The encountered limitations of the prototype were taken into account for the construction of the final system, which was manufactured from brass. The key components of the final collimation system were a 130 × 130 × 20 mm3 primary collimator, combined with a 140 mm long tube, which reduced scatter radiation and served as a mount for interchangeable, 20 mm thick, cylindrical secondary collimators with an outer diameter of 46 mm. Depth dose curves and dose rates in air and water-equivalent material, as well as LDPs were measured for the final setup, using cylindrical secondary apertures of 5 mm, 10 mm and 15 mm diameter for a first dosimetric assessment with 200 keV X-rays. Dose rates were calculated at depths of 0–100 mm in air and 0–50 mm in water-equivalent material (with an additional 50 mm air gap). At a distance of 50 mm from the secondary collimator, which represents a potential arrangement for pre-clinical experiments, a dose rate of (2.300±0.003) Gy min−1 was found using a 15 mm aperture. Hence it was shown, that the setup provides suitable dose rates for irradiation of small animals under anesthesia. LDPs measured with EBT3 films directly mounted to the secondary collimator showed that no transmission occurs through the secondary collimator, however a transmission of 5 % at a distance of 50 mm from the central beam axis was revealed. The issue regarding transmission calls for further investigation regarding its origin, relevance and potential technical adaptions. Furthermore, LDPs at multiple depths of water-equivalent material were measured to evaluate the field in terms of full width at half maximum (FWHM), flatness and homogeneity index (HI) for multiple secondary apertures. A linear increase of the FWHM of 3.9 %/10mm was found at depths of 0–70 mm for all apertures together with maximum flatness values of 3.3 % at the surface and 5.1 % at a depth of 70 mm. Profiles measured at the surface exhibited increased dose fluctuations within the central region, which was also reflected by the HIs. The highest HI of 1.12 was found for the 5 mm aperture measured at the surface and reduced to 1.08 at 70 mm depth. The second aspect revolved around the design of a small field dosimetry phantom (SFDP), suitable for beam commissioning and verification measurements in X-ray as well as ion beams. A phantom holding 60 mm × 60 mm slabs of up to 150 mm water equivalent thickness, including additionally designed holders for the microDiamond detector and Advanced Markus Chamber (PTW-Freiburg, Germany), was developed. The SFDP was constructed using additive manufacturing technology. Dose verification measurements in proton beams were conducted as a proof of concept for the SFDP. Maximum relative deviations of 0.4 % and 1.2 % from the dose predicted by the treatment planning system (TPS) were found in the plateau region of a spread-out Bragg peak (SOBP) for the microDiamond detector and Advanced Markus Chamber, respectively. Thus, it was demonstrated that the SFDP is a viable alternative for commercially available water and water-equivalent phantoms. Additionally, the SFDP offers the possibility of dose measurements on the surface and within the first millimeters, which was previously not possible with the available solutions. Furthermore, the SFDP was used for the experimental setup regarding the final collimation system and proofed equally viable for X-ray beams. In conclusion, an X-ray collimation system and a small field dosimetry phantom were developed and manufactured. The X-ray collimation system provided suitable field sizes and beam characteristics for the irradiation of small animals. Further investigation regarding transmission through the collimator is necessary. The small field dosimetry phantom was proofed to be a viable option for beam commissioning and verification in X-ray and ion beams.
|Keywords:||Bestrahlung von Kleintieren; Ionentherapie; Röntgenstrahlung; Dosimetrie kleiner Felder
small animal irradiation; ion beam therapy; x-rays; small field dosimetry
|DOI:||10.34726/hss.2022.101942||Library ID:||AC16554864||Organisation:||E141 - Atominstitut||Publication Type:||Thesis
|Appears in Collections:||Thesis|
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checked on Jun 25, 2022
checked on Jun 25, 2022