S1786
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
planning. In our Center, approximately 25% of patients undergoing proton therapy present with metallic implants, making this issue highly relevant for daily clinical practice.This study assessed the performance of five clinically available CT scanners in mitigating metal-induced uncertainties in pencil beam scanning proton therapy. Material/Methods: Five imaging modalities were investigated: single- energy CT (SECT), source-based dual-energy CT (DECT), detector-based spectral CT, photon-counting CT (PCCT), and mega-voltage CT (MVCT). Reconstructions employed standard protocols, metal artefact reduction (MAR), relative electron density (RED) and stopping power ratio (SPR) algorithms when available. Scans were acquired on a solid-water (RW3) phantom containing titanium and cobalt–chromium spinal rods. Geometric delineation accuracy was quantified using Dice similarity coefficients, Hausdorff distance and mean distance to agreement (DTA). Density assignment accuracy was evaluated through histograms of implant-region densities.Spread-out Bragg peaks (SOBPs) were robustly optimized in RayStation (RaySearch Laboratories) and range shifts (R80) induced by metallic implants were experimentally measured with EBT3 Gafchromic films. The most effective CT sequences were subsequently applied to complex geometries (dental posts in an anthropomorphic phantom, zirconium crowns, spinal rods and screws, hip prostheses). For materials denser than titanium, density overrides were implemented. Range deviations were quantified in RW3 and Alderson phantoms, and compared between Single-Field Uniform Dose (SFUD) and Intensity-Modulated Proton Therapy (IMPT) using three-beam configurations. Results: MVCT provided superior geometric accuracy (Dice: 0.89, DTA: 0.19mm, Haussdorff: 1.2mm), while SECT ensured precise density assignments (titanium: 4.34 g/cm 3 , RW3: 1.01 g/cm 3 ). DECT with iterative MAR and RED reconstructions achieved excellent dosimetric performance, comparable to SPR-based approaches. Gamma passing rates exceeded 98%, dose deviations remained below 1.1% and mean R80 differences were under 1.1 mm. For complex implants, R80 variations were within 2.6 mm for titanium dental posts, 5.6 mm for cobalt rods, and 3.6 mm for zirconium crowns. Hip prostheses demonstrated R80 errors of 0.8 mm for titanium stems and − 4.4 mm for cobalt heads. Analyses of spectral CT and PCCT are ongoing and will be reported.
Conclusion: Combining multiple CT modalities enhances geometric and dosimetric accuracy in proton therapy planning for patients with metallic implants, supporting improved clinical robustness in daily practice. Keywords: proton therapy, metal artefact mitigation
Digital Poster 4983
Simulation of ultra-high dose rate (UHDR) irradiations using a Monte Carlo-based dose engine Janne Frenz 1 , Celine Karle 1,2 , Thomas Tessonnier 3,4 , Amir Abdollahi 5,6 , Andrea Mairani 1,3 1 Clinical Cooperation Unit Translational Radiation Oncology, National Center for Tumor Diseases (NCT), Heidelberg University Hospital (UKHD) and German Cancer Research Center (DKFZ), Heidelberg, Germany. 2 Department of Physics and Astronomy, Heidelberg University, Heidelberg, Germany. 3 Heidelberg Ion- Beam Therapy Center (HIT), Department of Radiation Oncology, Heidelberg University Hospital (UKHD), Heidelberg, Germany. 4 Clinical Cooperation Unit Radiation Oncology, National Center for Tumor Diseases (NCT), Heidelberg University Hospital (UKHD) and German Cancer Research Center (DKFZ), Heidelberg, Germany. 5 German Cancer Research Center (DKFZ) and erman Cancer Consortium (DKTK), ore Center Heidelberg, Heidelberg, Germany. 6 Division of Molecular and Translational Radiation Oncology, Department of Radiation Oncology, Heidelberg Faculty of Medicine (MFHD) and Heidelberg University
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