S2207
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
biological effectiveness (RBE) factor of 1.1 was used for all proton plans.
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Effects of variation in breath-hold reproducibility on proton and photon lung cancer stereotactic body radiotherapy Nils Olovsson 1,2 , Kenneth Wikström 1,3 , Anna Flejmer 1,4 , 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: Breath-hold can mitigate respiratory motion in lung cancer radiotherapy and could be especially beneficial for proton therapy which is more sensitive to geometrical perturbations than photon therapy. However, failure to accurately reproduce the tumor position during breath-hold could lessen this advantage. Further, there is a variation in the reported ability to reproduce the tumor breath-hold position. In this study the dosimetric effects of variation in reproducibility were investigated for 3D and 4D robust optimized proton and photon therapy. Material/Methods: Fourteen patients with early stage lung cancer were included. All had one breath-hold computed tomography image and reproducibility uncertainty was simulated with image deformations using values from the literature. One reproducibility uncertainty was assumed during treatment planning (Josipovic et al 2019) and this same uncertainty as well as a larger one (Hoffmann et al 2023) were both included in the evaluation of the treatment plans.Three proton, Proton3D-PS, Proton3D-BH, Proton4D-BH, and three photon, Photon3D-PS, Photon3D-BH, Photon4D-BH, therapy treatment plans were robustly optimized, Figure 1. First, the plans denoted 3D-PS only considered variations in patient positioning, using shifted positions in the robust optimization. Second, the plans denoted 3D-BH considered both patient positioning and breath-hold reproducibility with larger shifted positions in the robust optimization. Finally, the plans denoted 4D-BHaccounted for variations in patient positioning using shifted positions and the breath-hold reproducibility was consideredusing a set of deformed breath-hold images, simulating repeated breath-hold image acquisitions, and the inclusion of these images in the robust optimization.All six plans had the intention of delivering a dose in three fractions such that CTV D50% = 54 Gy. They were compared by sampling dose distributions according to modeled geometrical and radiological variations during simulated treatments in a probabilistic evaluation (Olovsson et al 2025). A constant relative
Results: Proton therapy resulted in reduced dose to the organs-of-interest but the spread in resulting dose to the target was higher than for the photon therapy plans. Higher reproducibility uncertainty affected the tumor dose ranges, Figure 2. However, the 90% probability of achieving similar median tumor dose remained stable across all plans.
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