S1899
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
4 Division of Oncology and Pathology, Department of Clinical Sciences, Lund University, Lund, Sweden
Purpose/Objective: Breast cancer is the most prevalent malignancy among women globally, with radiotherapy (RT) playing a vital role in local tumor control and survival.Despite technological advances such as deep inspiration breath-hold (DIBH), radiation-induced toxicities remain clinically significant. Ultra-high dose rate RT (UHDR-RT) has demonstrated the so-called “FLASH effect”, sparing normal tissue (NT) at high dose rates (>30-40 Gy/s) compared to conventional RT. However, clinical translation is limited, and the interplay between the dose-dependent FLASH-effect and hypofractionation- related toxicity is not fully understood. Given the low α / β -ratio of breast tumors and the common use of hypofractionated schedules, this study evaluates the dosimetric and practical impact of UHDR-RT in realistic breast cancer treatment scenarios, including its potential to expand DIBH applicability to a broader patient cohort. Material/Methods: Twenty breast cancer patients were selected from our institutional database. Based on previous published work, we developed a framework to assess UHDR-RT’s potential for NT sparing under intensified hypofractionated conditions (1-5 fractions) versus a reference 5.2Gy x 5 fraction schedule1. Dose distributions with higher dose-per-fraction were derived using the Linear-Quadratic model, defined to preserve tumor control. FLASH-modified dose distributions were simulated voxel-by-voxel using dose-modifying factors modelled from preclinical data. These plans were converted back to reference fractionation equivalents for radiobiological comparison. Standard α / β T- and α / β NT-ratios were applied. The FLASH-weighted plans were compared to the reference plans to quantify NT sparing via DVH reductions to OARs. To reflect realistic UHDR delivery, the FLASH-effect was modelled on field-specific dose distributions. Irradiation times were estimated per field, based on monitor units, assuming a dose-rate delivery of 30 Gy/s and 100 Gy/s, respectively Results: FLASH-modified UHDR treatment plans consistently reduced doses to all OARs compared to 5-fraction plans, suggesting that the FLASH-effect outweighed potential toxicity from intensified hypofractionation, even under realistic delivery conditions (fig 1). Modest NT sparing was also observed within the planning target volume and adjacent tissues with the 5-fraction regimen. Even under conservative assumptions (higher α / β T), these benefits were maintained. Estimated irradiation times at 30 Gy/s ranged from a median of 84 ms for 5-fraction to 216 ms for single- fraction regimens, decreasing to 25–65 ms at 100 Gy/s
Conclusion: Ultracompact electron accelerators are a promising innovation for radiation therapy. The developed treatment planning framework enables systematic analysis of the acceleration parameters influencing the electron energies and the resulting dose distributions to determine the optimal configuration for clinical implementation. The framework allows for the investigation of patient cases best suited for treatment with this novel technology, including the exploration of ablation therapy as a potential use case. References: [1] Matthias Fuchs, Ping Zhang, Kyle Jensen et al. Parametrically-excited laser-plasma acceleration, 15 October 2023, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs- 3421468/v1][2] https://smileipic.github.io/Smilei/[3] https://github.com/OpenTOPAS[4] https://github.com/e0404/pyRadPlanThis project is funded by the Carl Zeiss Stiftung Keywords: electron, internal radiation therapy, monte carlo Proffered Paper 2763 Exploring Ultra-High Dose Rate Radiotherapy for Breast Cancer: Dosimetric and Practical Considerations Filip Hörberger 1 , Kristoffer Petersson 2 , Sofie Ceberg 1 , Sven Bäck 3 , Gabriel Adrian 3,4 , Crister Ceberg 1 1 Medical Radiation Physics, Lund University, Lund, Sweden. 2 Department of Oncology, University of Oxford, Oxford, United Kingdom. 3 Radiation Physics, Department of Hematology, Oncology and Radiation Physics, Skåne University Hospital, Lund, Sweden.
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