S2183
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
evaluation of VMAT (Elekta, Ltd) plans and compared its results with an existing IMPT(IBA, Ltd) 4DDR workflow1,2 . 20 lung cancer treatment plans (60GyRBE=1.1 ,DCTV98>95%) and characteristics indicated in (Table.1) were analyzed. 10/20 patients underwent treatment adaptation during IMPT treatment but no adaptation was performed for delivered back up VMAT treatments. 4DDR was applied consistently to ~ 5 verification v4DCTs per patient. Machine log files of robustly optimized IMPT plans and MLC log files of treatment back up VMAT PTV based optimized plans were used to generate 10 subplans synchronized with ANZAI breathing data for each v4DCT. Dose distributions across respiration phases were warped with deformable image registration, accumulated on the v4DCT reference phase CT(50%), and finally warped to planning v4DCT reference phase. DCTV2-DCTV98 and DCTV98 were compared between photons and protons to assess target coverage robustness relative to the planning target coverage DCTV98>95%. A Student t-test was used to analyze the data.
exhibited the smallest deviation (±3 Gy), with GI-OAR boxplots closest to zero difference. PBT indicated greater sensitivity to range and density changes, while VMAT displays some dosimetric changes. Overall, target coverage was maintained across modalities, with minor decreases in the MR-Linac observed because of re-optimisation and daily OAR proximity. Conclusion: Initial results suggest MR-Linac–based adaptive radiotherapy offers greater dosimetric stability for pancreatic SABR compared to VMAT and PBT for nearby GI-OARs sensitive to daily anatomical variation. While all modalities maintained target coverage, PBT showed largest sensitivity to GI-OAR fluctuations. Ongoing analysis with the full patient cohort will clarify clinical relevance of these dosimetric differences and quantifying the proximity and intra-fractional motion of OARs to the target to assess stratifying patients. References: [1] – Shouman MA et al. Stereotactic body radiotherapy for pancreatic cancer – a systematic review of prospective data. Clin Transl Radiat Oncol. 2024;45:100738.[2] – Uchinami Y et al. Evaluation of short-term gastrointestinal motion and its dosimetric impact in pancreatic SABR. Radiother Oncol. 2023;176:135-142. Keywords: Pancreas, MR-Linac, Proton Beam Therapy Proffered Paper 4361 Photon vs proton lung plan robustness assessment using patient-specific log file-4D CT and dose accumulation Zahra Ahmadi Ganjeh, Daniele Cannavo, Sabine Visser, Erik Korevaar, Meulman Gerolf, Robin Wijsman, Johannes A Langendijk, Stefan Both, Gabriel Guterres Marmitt Department of Radiation Oncology, University Medical Center Groningen, Groningen, Netherlands Purpose/Objective: Lung cancer radiotherapy robustness is influenced by respiratory motion and anatomical changes1. Photon therapy is hypothesized to offer better robustness than proton therapy, as the latter is impacted by range uncertainty. This study presents a novel framework for patient-specific treatment robustness assessment for photons versus protons, aiming to determine if our current clinical protocol enables similar treatment robustness of photon volumetric modulated arc therapy (VMAT) and intensity-modulated proton therapy (IMPT) plans for patients treated in free breathing. Material/Methods: We developed a Python-based workflow for patient- specific 4D dose reconstruction(4DDR) and robustness
Results: 4DDRIMPT had a slightly higher DCTV98 target coverage mean value(97.7% ± 3.2) than 4DDR VMAT (96.1% ± 1.6), however not statistically significant (p = 0.2, ANOVA p = 0.2).4DDRIMPT doses showed smaller deviations in DCTV2 - DCTV98 (3.0 Gy ) compared to 4DDR VMAT doses ( 3.9 Gy), (p = 0.1, ANOVA p = 0.1) (Fig. 1). Adaptation for 10/20 patients facilitated the consistent 4DDR IMPT target robustness .
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