ESTRO 2024 - Abstract Book

S3214

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Purpose/Objective:

Magnetic resonance (MR) guidance has become a major field within radiation oncology, due to the advantages of magnetic resonance imaging (MRI), i.e., zero imaging dose and high soft-tissue contrast, functional imaging opportunities – compared to X-ray based imaging. In photon beam therapy, hybrid MR linear accelerator systems have been introduced into clinical practice for several years. Particle beam therapy (PT) allows to achieve a higher degree of conformality compared to advanced photon beam therapy and benefits from increased biological effectiveness; consequently, it could benefit from MR guidance as well. In PT, besides the charged secondary particles, also the primary particles are affected by the magnetic field. From a medical physics perspective, the establishment of reliable dosimetry methods in such a challenging environment is a prerequisite for further pre-clinical and clinical studies. In this work, we focused on the suitability of using thermoluminescence detectors (TLD-100) in magnetic fields up to 1 T for proton irradiation. From hybrid MR linear accelerators, it is already known, that magnetic fields can affect their dosimetric performance, which is most likely linked to air gaps around the TLDs and the associated electron return effect. Consequently, in addition to investigating different proton energies and magnetic field strengths, the impact of air gaps on the detector’s response was investigated in this study. Experiments were performed in a synchrotron-based facility. Magnetic fields of 0, 0.25, 0.5 and 1 T were created by employing a resistive electromagnet, which can be positioned such that the isocenter of the ion therapy beam line and the magnet iso center coincide. Homogeneous, mono-energetic energy 10x10 cm² fields were irradiated using 97.4 and 252.7 MeV scanned proton beams, respectively. TLD 100 pellets were positioned in a PMMA slab phantom at a depth of 1 cm and irradiated with physical doses of 1.8 Gy. Dosimetry was verified with and without magnetic fields using a ROOS electron chamber. No correction of TLD position due to the primary beam deflection was necessary for the experiment set-up. For 252.7 MeV protons, the effect of air gaps on the detector response was investigated using dedicated TLD holders with air gaps surrounding the pellet ranging from 0.1 to 0.5 mm. Material/Methods:

TLD read-out was performed using a Risö TL/OSL reader (DTU Physics, Denmark). To reduce TLD-100 variability, individual correction factors were determined based on their response to a reference 90SR calibration source.

Results:

No statistically significant changes in the detector response could be detected between any tested magnetic field strengths (0 to 1 T), for both 97.4 and 252.7 MeV proton beams. In addition, the evaluation yielded no statistically significant change in the detector response due to different air gaps with and without applied magnetic fields (see Figure 1).