S1924
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Rome, Italy. 6 Rome I section, National Institute of Nuclear Physics, Rome, Italy
ribs and chestwall for the NSCLC case as a function of the FLASH sparing, demonstrating that significant sparing (FMFmin < 0.5) is needed to match the constraints (in red).
Purpose/Objective: Very high-energy electrons (VHEE, ~100–300 MeV) are nowadays being investigated as a potential alternative to conventional radiotherapy and particle therapy for the treatment of deep-seated tumours. The discovery of the FLASH effect, observed at ultra-high dose rates (UHDR, >40 Gy/s), has stimulated the interest in VHEE beams, but reaching the required dose and dose-rate conditions for FLASH remains a major challenge within conventional multi-field delivery schemes [1]. This work aims to quantitatively evaluate the potential advantages of a one-field-per-fraction (OFPF) hypofractionated approach, where each fraction is entirely delivered from a single beam orientation and the overall treatment is completed in a few fractions, maximising the achievable dose rate [2]. Material/Methods: We developed a Treatment Planning System (TPS) tailored for VHEE, exploiting FRED, a GPU-based Monte Carlo dose engine [3]. To account for hypofractionation-induced toxicity, we implemented an optimisation algorithm which directly uses the Biologically Effective Dose (BED). In parallel, a mathematical modelling of the FLASH effect [4] was incorporated to introduce a sparing factor whenever the dose and dose-rate thresholds are met. The clinical cases chosen to evaluate the technique potential are NSCLC (non-small-cell lung cancer) lesions treated with a stereotactic approach and vertebral metastasis. Figure 1 shows, for NSCLC, the optimised dose distributions for the VMAT treatment, the conventional VHEE delivery and the OFPF VHEE delivery (in this last case, BED is shown instead of physical dose, with 𝛼 / 𝛽 = 10 Gy for PTV and 𝛼 / 𝛽 = 3 Gy for OARs).
Conclusion: By not requiring technologically complex solutions like simultaneous irradiations from multiple directions, the proposed OFPF approach represents a straightforward and immediate strategy to reach the dose rates required to trigger the FLASH effect. The BED formalism has been used to account for the radiobiological impact of hypofractionation, to evaluate the minimum FLASH gain needed to obtain benefits from the OFPF approach with respect to conventional treatments. References: [1] Sesink et al. Decrease in dose per fraction impairs the FLASH sparing effect in murine intestine model. Accepted for publ. in Radiother. Oncol, doi:10.1101/2025.08.27.672569 [2] Mascia et al. Impact of Multiple Beams on the FLASH Effect in Soft Tissue and Skin in Mice. Int J Radiat Oncol Biol Phys. 2024;118(1):253-261. doi:10.1016/j.ijrobp.2023.07.024[3] Schiavi et al. Fred: a GPU-accelerated fast-Monte Carlo code for rapid treatment plan recalculation in ion beam therapy. Phys Med Biol, 62(18),7482–7504. 2017 doi:10.1088/1361-6560/aa8134[4] Böhlen et al. Normal Tissue Sparing by FLASH as a Function of Single- Fraction Dose: A Quantitative Analysis. Int J Radiat Oncol Biol Phys. 2022 1;114(5):1032-1044. doi:10.1016/j.ijrobp.2022.05.038 Keywords: VHEE, FLASH, biologically effective dose Digital Poster 3220 Down the road of performance evaluation of an O- ring’s CBCT imager line for dose calculation. Jinane Bakkali Tahiri, David Dechambre Radiotherapy, Cliniques Universitaire Saint-Luc, Brussels, Belgium
Results: The optimisation algorithm, integrating both the BED formalism and the FLASH effect, was implemented within the VHEE TPS. The comparison between conventional and FLASH treatments achieved with the OFPF approach will be discussed, with particular focus on quantifying the minimum level of FLASH sparing required to allow a clinical adoption of the OFPF paradigm. Figure 2 shows the maximum BED values in
Purpose/Objective: Since 2022, the Varian’s O-ring systems (Halcyon, Varian Medical Systems), originally using the iCBCT
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