Development and Validation of a Dosimetry Setup for LiF:Mg,Ti Thermoluminescent Detectors in X-ray and Ion Beam Irradiation

Abstract

Thermoluminescent dosimeters (TLDs) are of great interest in radiation therapy and radiation protection due to their compact size and tissue equivalence. The aim of this MSc thesis was to investigate the application possibilities of these detectors in ion beam therapy and reference irradiation (i.e., kV x-rays). LiF:Mg,Ti TLDs were used for this purpose, which were available as pellets with a diameter of 4.5 mm and a thickness of 0.9 mm (type MTS-N, manufactured by Radcard s. c., Cracow, Poland). First, individual correction factors (CFs) were calculated for each TLD using beta irradiation. CFs were applied to correct for the varying responses between individual TLDs, due to the variation in their crystal structure. The respective measurements, as well as all the further TLD-readouts, were performed using a Risø TLD reader with an integrated beta irradiation source. The CFs were then applied to the reference beam qualities (kV x-rays, proton and carbon ion beams) used in the calibration of the TLDs. Furthermore, a TLD holder in the form of a dedicated ionization chamber was designed with Autodesk Fusion 2.0.21487 (Autodesk, Inc., San Rafael, CA, United States) and 3D printed from acrylonitrile butadiene styrene (ABS) during this thesis. This prototype in combination with the ionization chamber (PTW 30013 Farmer) was used for the dose-calibration of TLDs in a water-equivalent phantom under x-ray and ion beam irradiation. For calibration in x-rays, dose values between 0.1 Gy and 10 Gy with 70 kV and 200 kV tube voltage were used, while the experimental setup aligned with the recommendations of the IAEA TRS-398 (Rev. 1) guidelines [1], as accurately as possible. The calibration factor obtained from the 200 kV calibration was then used in an in-vivo kV x-ray irradiation setting for a spinal cord transplantation experiment conducted with mice at the AKH (General Hospital, Vienna). Finally, linear energy transfer (LET) dependencies were analyzed in mono-energetic and spread-out Bragg peak (SOBP) proton and carbon ion beams. The TLD holder in combination with the ionization chamber was previously used for the dose-calibration of TLDs in the water-equivalent phantom with the experimental setup adapted as closely as possible to the IAEA TRS-398 (Rev. 1) guidelines [1]. For this calibration, dose values of 1 Gy, 2 Gy and 5 Gy with energies of 179.2 MeV for proton beams and 346.6 MeV per nucleon for carbon ion beams were used. The calibration factors obtained during this calibration with mono-energetic beams were then used in two different modulations per particle type at varying positions within SOBPs with a physical dose (D50) of 2 Gy, to analyze the LET dependencies of TLDs along SOBPs with a constant dose. The TLDs exhibited linear behavior in response within the dose range considered in x-ray and ion beam calibrations. Combined with further assessments of the repeatability of the TLD response and validations of the calibration factor obtained in 200 kV x-ray irradiation indicated that the calibration method used provides potentially reliable results. Dose measurements in kV x-rays with calibrated TLDs in combination with the use of CFs were comparable to the measurements with the reference detector (PTW 30013 Farmer), with the exception of a higher uncertainty in the order of one magnitude. With this result and assuming the TLD measurements during the in-vivo experiment were correct, the experimental setup for small animal whole body irradiation was identified as requiring recalibration by means of TLD verification. However, dose determinations using calibrated TLDs in ion beams were less reliable because the TLD response strongly decreased with increasing LET values. The finding in those LET dependencies when using calibration factors obtained with the calibration method developed during this thesis was an approximately linear behavior of the decrease in the TLD response between 4 keV/μm and 100 keV/μm, which could potentially be used to determine specific LET values in SOBP ion beams, provided the dose is known. Therefore, further research on the LET dependencies of the TLD response is strongly recommended.