S1781
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
into RSP. These spectrally derived RSP were compared against two standards: 1. Reference RSP: Measured with a 200 MeV ProBeam system using a multi-layer ionisation chamber (IBA Giraffe). 2. Conventional RSP: Calculated from SECT using stoichiometric calibration. Robustness was tested by comparing the small removeable head phantom to the large full-body phantom and three I-value models (Bourque/Yang/Saito) and two energy settings (high/low) available in MMSim. The methodology was repeated in biological samples. Clinical plans, optimised on standard SECT, were recalculated using DLCT-derived RSP maps. This was done for 40 patients across 8 treatment sites (5/site). Dose differences were assessed and R80 range differences were calculated for dose profiles every 2 mm in beam direction. Results: DLCT demonstrated a marked improvement in accuracy, showing a nearly threefold reduction in error compared to SECT (Fig 1). Root-mean-square (RMS) RSP errors were consistently below 0.81% (e.g., 0.76% for Bourque) vs 2.22% for SECT. Accuracy was robust against phantom size (0.77% RMS head phantom vs. 0.81% full phantom) and insensitive to I-value models or energy settings. SECT-based dose calculations consistently underestimated penetration compared to DLCT-derived RSP images (Fig 2), causing substantial dose differences distal to the target, often exceeding 3 Gy. Median range differences varied by site, from 0.2 mm in head and neck to 5.0 mm in thorax.
profiles of Fig. 2). Ground-truth and high-resolution CT geometries deviated least from TOPAS in this respect; manual SPR override provided no overall improvement.
Conclusion: Proton PBS dose calculations with Eclipse’s pencil beam algorithm underestimated end-of-range and dose inhomogeneities behind a CranioFix implant compared to TOPAS Monte Carlo. CT-based implant representation increased end-of-range deviations to TOPAS compared to ground-truth, and manual SPR override worsened these discrepancies. These findings suggest that limitations of simplistic dose algorithms and CT representation are not mitigated by manual SPR correction for the case of small titanium objects. Keywords: Proton therapy, titanium implants, dose accuracy Digital Poster Highlight 4722 Clinical validation of a novel spectral CT for proton beam therapy Callum Gillies 1 , Joseph Meagher 1 , Esther Baer 1,2 1 Radiotherapy Department, University College London NHS Foundation Trust, London, United Kingdom. 2 Medical Physics and Biomedical Engineering, University College London, London, United Kingdom Purpose/Objective: To validate and commission a commercially available dual-layer spectral CT (DLCT) system for calculating relative stopping power (RSP) in proton therapy. Providing a more accurate alternative to conventional single-energy CT (SECT), whose fundamental limitations contribute to proton range uncertainties. Material/Methods: Spectral data was acquired from the Advanced Electron Density Phantom (SunNuclear) using a Philips DLCT scanner (CT7500). RSP maps were generated using a commercial platform (MMSim) converting electron density and effective atomic number maps
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