S1771
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
range uncertainties[1]. In standard proton therapy, this can be managed through robust optimization, but in lattice therapy such methods lead to unwanted blurring of the dose pattern. We propose a new robust optimization approach that maintains peaks and valleys under positioning uncertainties by allowing the lattice pattern to move within the GTV. Material/Methods: To achieve a translationally invariant dose pattern, we simulated patient images with 5 mm translations in inferior-superior, anterior-posterior, and right-left directions. The vertex contours were kept stationary relative to the isocenter, while other contours moved with the patient (Figure 1A). Robust optimization was performed using 6 shifted images, the nominal image, and three range errors (0%, ±3%), yielding 21 scenarios.The new method was applied to two patients previously treated with lattice in the abdominal region using conformal VMAT. Plans were optimized to treat multiple vertex spheres within the GTV: Patient 1: 8 spheres (Ø=1.2cm), 14 Gy; Patient 2: 5 spheres (Ø=0.9cm), 16 Gy. Three different robustness strategies were evaluated: new robustness, standard robustness, and no robustness, as well as four plan types created on an IBA ProteusPlus machine: proton arc (PAT), shoot-through enhanced proton arc (PAT+ST), shoot-through-only arcs (ST-PAT) and VMAT. Shoot-through layers were included for their potential to sharpen the penumbra and lower sensitivity to uncertainties[2].Plans were evaluated for D95 of the vertices, peak-to-valley dose ratio (PVDR), the percentage of the lattice volume receiving <5Gy and <3Gy, and mean dose to external, kidneys, and bowel. The lattice volume was defined as the hull around the vertices[3], and PVDR as vertex D95 divided by the mean dose in the lowest 50% of the lattice volume. Robustness evaluation was performed over all 21 scenarios. Results: Figure 2 shows that the new robust optimization maintains vertex doses under uncertainties while reducing dose to the patient and lattice volumes compared to standard robustness and VMAT plans. Without robust optimization the plans exhibit a large spread, especially in vertex D95. Shoot-through layers sharpen the lattice pattern at the expense of slight increased external, left kidney, and bowel doses.
around it was less than ±3mm. The positioning accuracy and co-localization of MRI and beam coordinates was within ±1mm. The overall setup uncertainty (2 σ ) was quantified as 6mm.In the E2E test, the measured dose inside the phantom was in excellent agreement with the treatment planning system calculation (deviation<0.5%). Furthermore, the radiochromic film measurement showed good spatial agreement (deviation<2mm). The PSQA tests passed with absolute dose deviations <0.5% and 100% gamma pass rates (Fig.2).
Fig.2: Example results from QA tests. Conclusion: The developed daily workflow and QA program ensure the safety and treatment accuracy of the MRiPT application. This was confirmed by the satisfactory dosimetric results of the end-to-end test. In conclusion, the technical requirements for accurate dose delivery within the intended clinical application of MRiPT are now fulfilled. Keywords: MRI-guided, proton therapy, end-to-end test Poster Discussion 4252 Evaluation of novel robustness strategies in lattice proton arc therapy employing translationally invariant dose patterns and shoot-through layers Erik Engwall 1 , Albin Fredriksson 1 , Johan Sundström 1 , Gian Guyer 2 , Rémy Kijn 3 , David Patin 2 , Agnes Angerud 1 1 Research and Development, RaySearch Laboratories, Stockholm, Sweden. 2 Institute of Radiation Physics (IRA), Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland. 3 Department of Oncology, Radio-Oncology Service, Lausanne University Hospital and Lausanne University, Lausanne, Switzerland Purpose/Objective: Proton therapy could produce superior lattice dose patterns in spatially fractionated radiotherapy by exploiting the low entrance dose and high Bragg peak dose. However, sharp proton beams delivered to small vertices are highly sensitive to patient positioning and
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