S1684
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Conclusion: The prostate, rectum and bladder are clearly visible with 2D and 3D MR clinical scan protocols, including those for gating and motion management. The RUBY is suitable for demonstrating and testing MR-based workflows including the Unity MRgART process. References: 1.Poppinga, D. et al., 2020, “Evaluation of the RUBY modular QA phantom for planar and non-coplanar VMAT and stereotactic radiations”, J Appl Clin Med Phys Oct;21(10):69-79. doi: 10.1002/acm2.130062.Brodbek, L. et al., 2021, “Systematic end-to-end testing of multiple target treatments using the modular RUBY phantom”, Biomed Phys Eng Express, Dec 9;8(1). doi: 10.1088/2057-1976/ac3e373.Eulenstein, D. et al., 2024, “Adaptive Radiation Therapy; Applications of the RUBY Phantom for Adaptive Radiation Therapy”, PTW knowledge article D968.139.05/00 2024-04 Keywords: Adaptive RT, Dosimetry, Treatment plan QA Proffered Paper 4448 A new formalism for recombination correction of ionization chambers José Paz Martín 1 , Andreas Schüller 2 , Marvin Apel 2 , Araceli Gago Arias 1 , Diego González Castaño 3 , Juan Pardo Montero 4,5 , Faustino Gómez Rodríguez 1,3 1 Departamento de Física de Partículas, Universidade de Santiago, Santiago de Compostela, Spain. 2 Dosimetry for Radiotherapy, Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. 3 Laboratorio de Radiofísica, Universidade de Santiago, Santiago de Compostela, Spain. 4 Group of Medical Physics and Biomathematics, Instituto de Investigacion Sanitaria de Santiago (IDIS), Santiago de Compostela, Spain. 5 Department of Medical Physics, Complexo Hospitalario Universitario de Santiago de Compostela, Santiago de Compostela, Spain Purpose/Objective: The two-voltage method (TVM) is the recommended methodology by the TRS-398 and the TG-51 dosimetry protocols to evaluate the charge recombination losses due to volume recombination in ionization chambers (IC) exposed to clinical beams. This methodology is based on the Boag's model [1], neglecting the presence of free electrons and electric field perturbation inside the IC. It provides the user with a second-order parametrization that relates the recombination correction factor (ks) as a function of the collected charge ratios measured at two different bias voltages. In the recent years, with the emergence of FLASH radiotherapy, numerical simulations describing in detail the charge transport in ICs have
for QA of MR-guided adaptive RT (figure 1). The aim of this study is to evaluate the functionality of the RUBY phantom for assessing MRgART in a Unity 1.5 T MR- linac. Material/Methods: Prototype anatomical configuration inserts MR Prostate “A” and MR Prostate “B” were used, along with a film insert. A reference plan was made in the treatment planning system on a CT scan with Prostate ”A” anatomy. It was evaluated for CT Prostate “A”, and also retrospectively for MR Prostate “B”, the anatomy present during the treatment fraction. As expected, due to the changed anatomy, plan-adaptation was required. The reference plan was delivered, measured with ion chamber and film, and evaluated. To evaluate adaptive radiotherapy, a full adapt-to-shape online- optimization was performed, based on the T2 MR- scan. Volumes were recontoured and the adapted plan was optimized, delivered, measured and evaluated. Imaging was qualitatively assessed with 3D-T1, 3D-T2, B0-map, 2D-cine-BTFE and 2D-cine-T2 protocols used for patient imaging. Results: The reference plan had PTV V95% = 97.7%; when re- evaluated on the new anatomy it was 80.5%, confirming that plan adaptation was required. Ion chamber measurements of the reference plan on the Prostate “A” anatomy showed good agreement (target dose -0.7% (SD 0.7%)), as did the online-adapted plan measured on the Prostate “B” anatomy (target dose - 0.2% (SD 0.4%)). The sagittal film QA insert was functional; preliminary results with auto-positioning are shown in figure 2. In all 5 MR scan protocols, all volumes were clearly visible. The body of the phantom was not visible, as expected. The B0 map showed no significant artefacts.
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