S1629
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
radiation reaching the foetal region, and2. Beams directed in the inferior-superior direction require the linac head to approach the patient’s body, resulting in increased leakage dose reaching the foetal region. References: 1. Noyama T, et al., J Cancer Ther 11(11): 738-744 (2020). doi: 10.1016/j.ctro.2019.10.0022. Takahashi W, et al., Clin Transl Radiat Oncol 20:9-12 (2020). doi: 10.1016/j.ctro.2019.10.0023. Crowe S B, et al., Australas Phys Eng Sci Med 37(3): 475-482 (2014). doi:10.1007/s13246-014-0274-94. Peet S C, et al., Med Dosim 46(4): 342-346 (2021). doi:10.1016/j.meddos.2021.03.0085. Kairn T, et al., IFMBE Proc 51: 557-560 (2015). doi:10.1007/978-3-319- 19387-8_1366. Peet S C, et al., Phys Eng Sci Med 45(2): 613-621 (2022). doi:10.1007/s13246-022-01131-57. Kairn, T., et al., IFMBE Proc 68/3: 549-551 (2019). doi:10.1007/978-981-10-9023-3_100 Keywords: radiation protection, treatment planning, pregnant Digital Poster 742 Absolute dose measurements in carbon ion beam therapy in 3D using nanoclay Fricke gels Karolin Milewski 1,2 , José Vedelago 3,1 , Armin Runz 4,5 , Stephan Brons 6 , Tanja Platt 7 , Petr Bulanov 7 , Alexander Neuholz 6 , Christian Karger 1,5 1 Department of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany. 2 Faculty of Physics and Astronomy, Heidelberg University, Heidelberg, Germany. 3 Department of Radiation Oncology, Heidelberg University Hospital (UKHD), Heidelberg, Germany. 4 Department of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ)German Cancer Research Center (DKFZ), Heidelberg, Germany. 5 National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany. 6 Heidelberg Ion Beam Therapy Center (HIT), Department of Radiation Oncology, Heidelberg University Hospital (UKHD), Heidelberg, Germany. 7 Department of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
spatial resolution dose measurements of clinical carbon ion treatment plans in 3D. Material/Methods: A protocol for absolute 3D dosimetry of carbon ion beams with NC-Fricke gel was developed. It includes gel production, irradiation and MR readout of the T1- relaxation rate with a saturation recovery (SR) and inversion recovery imaging sequence (FLASH-IR) with a resolution of 1 mm x 1 mm x 5 mm [2]. A correlation between the applied dose and change in R1=1/T1 was established by the readout of samples irradiated with monoenergetic carbon ion beams of different entrance surface doses. The reference doses were determined using base data of the treatment planning system (TPS) and normalisation with a PinPoint Ionisation Chamber [3]. As proof of concept for absolute dosimetry, a 3D high-LET carbon ion treatment plan, optimised for a uniform physical dose of 600 Gy in a cubic volume of 8 cm3, was delivered to a cylindrical gel-filled phantom with an outer diameter of 101 mm via pencil beam scanning. The determined dose image was registered to the treatment planning CT based on the phantom geometry and compared to the dose calculated by the clinical TPS using the root mean square error (RMSE) relative to the maximum planned dose. Results: The recorded dose calibration curves prove an LET- independent correlation between R1 and the applied dose in the NC-Fricke gel with R2 = 0.995, supporting previous findings. The calibration curves have been applied for the absolute dose measurement of the carbon ion treatment plan. A pixelwise statistical comparison of slices of the measured dose against the planned dose in coronal, sagittal and transversal plane (Table 1) confirms satisfactory concordance of the detector with the TPS for LET values up to 253.4 keV/um with a maximum RMSE of 3.6 %.
Purpose/Objective: Current dosimeters for ion beams either lack
continuous spatial resolution or mis-respond at high linear energy transfer (LET). Fricke gel in combination with magnetic resonance (MR) readout allows for high- resolution dosimetry in 3D and, through the addition of so-called nanoclay particles, has been proven to respond to dose LET-independently [1]. The goal of this study is to investigate the feasibility of applying the nanoclay Fricke (NC-Fricke) gel dosimeter for high-
Conclusion: NC-Fricke gel is well-suited for independent high- resolution 3D dosimetry of multi-energetic carbon ion treatment plans. The low sensitivity of the gel remains a limitation to overcome. Nevertheless, our findings enable the use of NC-Fricke gel for patient-specific quality assurance in ion beam facilities.
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