ESTRO 2025 - Abstract Book

S3241

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

1942

Mini-Oral Probabilistic evaluation and comparison of proton and photon lung SBRT subject to respiratory motion Nils Olovsson 1,2 , Kenneth Wikström 1,3 , Anna Flejmer 1,2,4 , Alexandru Dasu 1,2 1 Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden. 2 The Skandion Clinic, The Skandion Clinic, Uppsala, Sweden. 3 Department of Medical Physics, Uppsala University Hospital, Uppsala, Sweden. 4 Department of Oncology, Uppsala University Hospital, Uppsala, Sweden Purpose/Objective: Stereotactic body radiotherapy (SBRT) with photons has improved the outcome of patients with lung tumors. Proton therapy on the other hand, is more sensitive to geometric shifts and respiratory motion which has led to a slow clinical adoption of this treatment regime. This study compared proton SBRT and photon SBRT plans using probabilistic evaluation, taking patient setup uncertainties and respiratory motion into account. In addition, the effect of respiratory motion that may not be seen in the planning 4DCT was investigated based on cine-CT data [1]. Material/Methods: Proton and photon SBRT treatment plans were robustly optimized for 14 patients using a prescription of D 50% = 54 Gy (RBE) to the CTV to be delivered in three fractions. A constant RBE=1.1 was assumed for protons. Uncertainties in patient positioning and proton stopping power, as well as the respiratory motion in a 4DCT, were considered during treatment planning. All plans were robustly evaluated and normalized for harmonization in target coverage. Subsequently, a probabilistic evaluation of all plans was performed. Perturbations were sampled from the uncertainties used during planning and respiratory motion from either 4DCT or cine-CT was used in order to simulate treatment dose distributions, as illustrated in Figure 1. Since dose could not be computed directly using the cine-CTs, image registration and displacement vector field editing methods were used for the creation of evaluation images. Students t-tests were used to compare CTV D 50% of the proton with the photon plans, CTV D 50% computed using cine CT with 4DCT and the absolute lung volumes at dose between proton and photon plans.