ESTRO 2025 - Abstract Book

S3497

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTRO 2025

2326

Digital Poster Optimizing proton radiotherapy in thymic epithelial tumours through improved use of an Adaptive Aperture to trim the proton beam’s lateral penumbra Esther Kneepkens, Marije Velders, Judith van der Stoep, Maud Cobben, Maud van den Bosch, Nicole Hendrix, Giorgio Cartechini, Marta Bogowicz, Ilaria Rinaldi, Dirk de Ruysscher, Dianne Hartgerink, Judith van Loon, Stéphanie Peeters Department of Radiation Oncology, Maastro, GROW School for Oncology, Maastricht University Medical Center+, Maastricht, Netherlands Purpose/Objective: Thymic epithelial tumours (TET), are rare neoplasms arising from the epithelial cells of the thymus. At our center, patients receive proton therapy on the basis of the model-based approach for reduction in the estimated normal tissue complication probability (NTCP) for cardiac toxicity, pneumonitis and dysphagia. 1–3 This study aimed to refine proton plans by maximizing the use of the adaptive aperture (AA) of the Mevion Hyperscan 250i beam proton accelerator. The AA dynamically trims the lateral penumbra of the beam, typically limited to spots at the outer edges of each energy layer. Bogowicz et al. 4 proposed using additional AA shapes to trim spots further from the border, a technique called multi-collimated adaptive aperture (MCAA, schematically depicted in Figure 1). The use of MCAA enabled sharper, more conformal dose distributions in patients with brain tumors. Material/Methods: For a cohort of 20 recent clinical TET patients, two proton therapy plans were generated, one without and one with the use of MCAA. The prescription dose was 45 GyRBE in 25 fractions of 1.8 GyRBE (1.1 GyRBE = 1 Gy physical dose). Plans were robustly optimized with 5 mm setup and 3% range uncertainty in 21 scenarios, and evaluated on a voxelwise maximum and minimum dose distribution for these scenarios. For both plans, the same beam setup was used. The MCAA plan included up to three apertures per energy layer. A dosimetric comparison between the plans was done, focusing on mean dose to the heart, lungs and esophagus. Additionally, NTCP were compared between MCAA and clinical plans. Results: An example of a dose distribution for a plan without MCAA and with MCAA is depicted in Figure 1B. The use of MCAA led to median dose reductions of -0.8 GyRBE (-1.3 to -0.2 GyRBE) for lungs-GTV, -0.7 GyRBE (-1.5 to -0.2 GyRBE) for heart, and -0.5 GyRBE (-3.3 to 0 GyRBE) for esophagus (all p < 0.05; Figure 2A). The probability for radiation pneumonitis was reduced by a median of -0.5% (-3.8 to -0.1%), for cardiac toxicity by -0.8% (-5.3 to 0%), and dysphagia by -0.4% (-1.6 to -0.1%), all statistically significant (Figure 2B). 3/20 patients showed a >2% reduction in the probability of developing dysphagia and/or radiation pneumonitis.