ESTRO 2025 - Abstract Book

S3253

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

ESTRO 2025

induced lung toxicity. Undetermined changes in respiratory motion during radiotherapy further complicate accurate tumor targeting. Identifying the relationship between tumor motion and patient-specific factors could facilitate the optimization of planning target volume (PTV) margins, enabling personalized treatment planning, and improved preservation of healthy lung tissue. Material/Methods: A cohort of 105 patients who underwent CyberKnife lung treatment with Synchrony fiducial tracking was selected. Tumor position data, recorded at 26 Hz, was extracted from the Modeler.log file for the entire treatment duration. Motion traces were segmented into sub-traces corresponding to beam-on intervals (from ERsiData.log), excluding periods when the correlation model was being prepared, such as at the start of treatment or after interruptions. Tumor motion amplitude in the superior-inferior (SI), left-right (LR), and anterior-posterior (AP) directions was quantified for each patient, and the method was validated using a quality assurance phantom. Patient characteristics, including demographics, histology, lesion diameter, lung lobe, lung volume, body mass index (BMI), pulmonary function, comorbidities, prior surgeries, radiotherapy, smoking/alcohol history, and performance status, were collected. Statistical analyses, including linear regression and analysis of variance (ANOVA), were performed to identify predictive variables. Results: Tumor location showed a strong, statistically significant correlation with motion (p = 1.86e-14), with tumors in the lower lobes exhibiting increased movement in the SI direction. While not reaching statistical significance (p > 0.05), boxplot visualizations suggested potential trends for several clinical factors. Specifically, prior lung surgery or radiotherapy, as well as higher alcohol intake, appeared to correlate with reduced motion in the LR direction, while both asthma and COPD were associated with reduced motion in the SI direction. Conclusion: Based on the results of this study, smaller margins are recommended for tumours in the upper lung lobes. For other patient factors, the observed effects are less pronounced, warranting further investigation before being applied in clinical practice. Proffered Paper Gain and pain of automatic beam gating in MR-guided radioablation of pancreatic tumours: Balancing dose coverage and treatment delivery efficiency Uffe Bernchou 1,2 , Rana Bahij 3 , Tine Schytte 3,2 , Carsten Brink 1,2 , Anders S Bertelsen 1 , Faisal Mahmood 1,2 1 Laboratory of Radiation Physics, Odense University Hospital, Odense, Denmark. 2 Department of Clinical Research, University of Southern Denmark, Odense, Denmark. 3 Department of Oncology, Odense University Hospital, Odense, Denmark Purpose/Objective: Respiration and drift motion can challenge the target dose coverage during MR-guided radioablation of pancreatic tumours. However, real-time target tracking and beam gating may mitigate such challenges. To manage respiration motion, irradiation can be automatically paused if part of the target moves outside a fixed gating window. For drift motion, baseline shift re-planning adjusts the MLC positions if the target moves outside a fixed drift window [1]. Both methods reduce the treatment delivery efficiency, i.e., the fraction of time the beam is on during a treatment session. This study aims to determine optimal gating and drift window sizes to balance the target dose coverage and treatment delivery efficiency in patients with pancreatic cancer. Keywords: CyberKnife, lung cancer, tumour amplitude 2373