S1641
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
calculation of magnetic field correction factors for a plane-parallel ionization chamber. Med Phys. 2024;51(3):2293-2305.[3] Marot M, et al. Monte Carlo simulation for proton dosimetry in magnetic fields: Fano test and magnetic field correction factors for Farmer-type ionization chambers. Phys Med Biol. 2023;68(17):175013.[4] Blum I, et al. Fano cavity test and investigation of the response of the Roos chamber irradiated by proton beams in perpendicular magnetic fields up to 1 T. Phys Med Biol. 2024;69(8):085021. Keywords: dosimetry, MRiPT, proton therapy Low-diffusion viscous PDMS Fricke gel dosimeters Immacolata Vanore 1 , Fulvio Ratto 2 , Livia Marrazzo 1,3 , Lucia Cavigli 2 , Margherita Zani 3 , Cosimo Nardi 1 , Giacomo Insero 1 , Giovanni Romano 1 , Alberto Dalla Mora 4 , Laura Di Sieno 4 , Stefania Pallotta 1,3 1 Department of Experimental and Clinical Biomedical Sciences “Mario Serio”, University of Florence, Florence, Italy. 2 Italian National Research Council IFAC- CNR, Institute of Applied Physics, Sesto Fiorentino, Italy. 3 Medical Physics Unit, Azienda Ospedaliero- Universitaria Careggi, Florence, Italy. 4 Department of Physics, Politecnico di Milano, Milan, Italy Digital Poster 1710 Purpose/Objective: Fricke gels have always been attractive for the prospect of providing information on 3D dose distribution; however, ion-diffusion has limited its use over time. The purpose of this study is to develop and characterize a formulation of silicon Fricke gel dosimeters capable of providing a stable and reproducible MRI-signal over time. Material/Methods: The realised dosimeters are composed of a viscous semi-polymerized PoliDyMethilSiloxane solution, mixed with the standard Fricke solution [1] and 0.5 mM of Xylenol Orange.To calibrate the dosimeters, the material was injected in 4 mL vials, placed in a water- filled box whose lid was purposely perforated to accommodate 25 vials. This setting assures reproducibility in the vials' positioning. 1.5 T Magnetic Resonance Imaging (MRI) was employed for dose readout, exploiting the dependence of the relaxation rate (R1 = 1/T1) on the ferrous ion concentration.The calibration workflow consists of three main steps: MRI to scan the vials the day before the irradiation; vials’ irradiation with homogeneous dose; MRI two hours after the irradiation. To evaluate the dosimeter’s response to a heterogeneous dose distribution over time, a 6cm x 6 cm x 5mm sample was prepared. The sample was irradiated with a cross-shaped dose distribution, achieved intersecting two orthogonal rectangular fields of 3x1 cm2. The planned mean dose
at the cross-shaped PTV is 9 Gy.The first MRI was performed two hours after irradiation, while a second scan was repeated one week later under identical conditions to evaluate signal stability and reproducibility. Results: T1 maps were achieved through acquisition and re- elaboration of inversion recovery sequences at several inversion times. The calibration curve results in a second-degree polynomial trend in the investigated dose range of 0-20 Gy. Different vials were also irradiated with the same dose to verify inter-variability and reproducibility between the vials (Fig.1). T1-map, a slice of 5mm thickness, revealed a well-defined cross- shaped profile immediately after irradiation. Applying the calibration curve to the R1=1/T1 difference image, PTV's mean dose measured results (9±1) Gy, accordingly to the planned one.Remarkably, after one week, the vertical and horizontal normalized profiles remained consistent with the initial measurements, with a slight global signal attenuation <5% (Fig.2).
Conclusion: The developed Fricke gel dosimeter ensures reproducible and temporally stable dose
measurements, maintaining a coherent spatial dose pattern even one week after irradiation. This confirms the potential of this optimized system for 3D dosimetry applications where delayed readout is required, as in QA of complex RT treatments. References: [1] Gambarini, G., et al. "Characterization of Fricke-gel layers for absolute dose measurements in
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