ESTRO 2025 - Abstract Book

S3307

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

ESTRO 2025

setup margin, excess absolute cancer risk (EAR) was calculated in seven organs with full, bell, and plateau models[1]. This allowed to determine at which setup margin the total EAR of each treatment protocol was equal to the total EAR of the reference. Results: In the calibration experiments, the difference between simulated and measured imaging doses was within 10%. Using these calibrated models, we found that, for most patients, cancer risk models, and treatment protocols, a 1 mm reduction in setup margin was sufficient to offset the additional risk from daily and weekly imaging, except when a standard-dose CT was acquired daily (Table 1). Reducing the margin primarily lowered carcinoma risk in the brain and, for some patients, in the oral cavity (Figure 1). For patient 4, however, a margin reduction of more than 2 mm was necessary for all protocols, as the non-linear dose-effect relationship in the oral cavity led to an increased cancer risk with margin reduction (Figure 1).

Conclusion: For head-and-neck proton therapy, only a 1 mm setup margin reduction was sufficient to compensate radiation induced cancer risk from increased imaging dose for most patients and imaging protocols. Our calculation framework is extendable to different treatment sites, modalities, and imaging protocols, supporting clinic-specific and patient-specific assessments, which is particularly important given the patient- and protocol-specific variability observed in this study.

Keywords: Radiation-induced cancer, imaging dose, robustness

References: [1] Schneider, U., Sumila, M., & Robotka, J. (2011). Site-specific dose-response relationships for cancer induction from the combined Japanese A-bomb and Hodgkin cohorts for doses relevant to radiotherapy. Theoretical Biology and Medical Modelling, 8 (1), 27. https://doi.org/10.1186/1742-4682-8-27