ESTRO 2025 - Abstract Book

S2665

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Conclusion: Replacing direct measurements with simulated errors can yield inaccurate sensitivity and specificity estimates, especially for certain error types and for small and medium errors magnitudes. This highlights the importance of careful validation when employing inverse error methods.

Keywords: PSQA, EPID, sensitivity analysis

References: Esposito, Marco, et al. "A commissioning protocol for portal imaging-based radiotherapy in vivo dosimetry systems." Physics and Imaging in Radiation Oncology (2024): 100666. https://doi.org/10.1016/j.phro.2024.100666

4000

Digital Poster Development of an enhanced Fricke-based chemical dosimeter utilizing sorbitol and xylenol orange for signal amplification Natalie Hornik 1,2 , Michelle Meier 1,2 , Inessa Sakulov 1,2 , David Weishaar 1 , Larissa Derksen 1,3 , Robin Erdmann 1 , Ulrike Theiss 4,2,3 , Sebastian Adeberg 5,3,4 , Boris Keil 1,2 , Kai-Thomas Brinkmann 6,2 , Kilian-Simon Baumann 1,2,4 1 Institute of Medical Physics and Radiation Protection, University of Applied Sciences, Giessen, Germany. 2 LOEWE Research Cluster for Advanced Medical Physics in Imaging and Therapy (ADMIT), TH Mittelhessen University of Applied Sciences, Giessen, Germany. 3 Department of Radiotherapy and Radiation Oncology, Marburg Ion-Beam Therapy Center (MIT),Marburg University Hospital, Marburg, Germany. 4 Department of Radiotherapy and Radiation Oncology, Marburg University Hospital, Marburg, Germany. 5 Universitäres Centrum für Tumorerkrankungen (UCT) Frankfurt, University Cancer Center, Marburg, Germany. 6 Second Physics Institute, Justus Liebig University, Giessen, Germany Purpose/Objective: The Fricke solution, a chemical dosimeter to determine the absorbed radiation doses by tracking the oxidation of ferrous ions (Fe(II)) to ferric ions (Fe(III)) in an sulfuric acid solution, shows dose-dependent optical density changes at 304 nm measurable using a spectrometer. To address its limitations, especially the significant uncertainties in low-dose determinations, a modified dosimeter based on the Fricke solution was developed by incorporating sorbitol and xylenol orange (XO), as well as adjusting the pH value. The formation of the Fe(III)-XO complex is directly proportional to the quantity of chemical radicals generated by the ionizing radiation. In this study the performance of this optimized dosimeter is investigated in terms of dose-response and LET-dependency, with adjustable composition allowing targeted dose determination in specific regions of interest. Material/Methods: The introduction of sorbitol into the Fricke solution markedly enhances its chemical activity, increasing Fe(III) production. XO promotes the formation of the Fe(III)-XO complex in 25 mM sulfuric acid, producing a strong color change and shifting the optical density into the visible range with a peak absorbance at 560 nm. Dose-response curves (optical density vs dose) were determined using 80 MeV/u proton irradiation with a ferrous ammonium sulfate (FAS) to XO ratio of 2, while the sorbitol concentration remained constant at 100 µM. Additional curves with electrons, photons, and carbon ions were established. G-values (i.e. radical yield normalized to 100 eV of deposited energy) of Fe(III)-XO were calculated from the calibration curve slope to examine LET dependency, similar to the G value of Fe(III) in Fricke solution. Results: Figure 1 shows a ~ 50-fold increase in signal intensity in the low-dose range (up to 10 Gy) compared to the Fricke solution, when using a low concentration of FAS in the optimized dosimeter solution. However, early saturation occurs, as the ratio of 1:1 is achieved for XO and the Fe(III) ions produced by ionizing radiation in the dosimeter.