S2512
Radiobiology - Biology of novel irradiation techniques
ESTRO 2026
Conclusion: Under low-dose radiation exposure, FLASH-RT combined with MBRT showed synergistic antitumor effect with no obvious toxicity compared to CONV-RT. While upon high-dose radiation exposure, F-MBRT exhibited synergistically protective effect in skin. CombiningFLASH-RT and MBRT may represent a promising radiotherapy modality. References: 1. Schneider T, Fernandez-Palomo C, Bertho A, et al. Combining FLASH and spatially fractionated radiation therapy: The best of both worlds . Radiother Oncol. 2022,175:169-177.2. Bertho A, Iturri L, Prezado Y. Radiation-induced immune response in novel radiotherapy approaches FLASH and spatially fractionated radiotherapies . Int Rev Cell Mol Biol. 2023,376:37-68.3. Bertho A, Iturri L, Brisebard E, et al. Evaluation of the Role of the Immune System Response After Minibeam Radiation Therapy. Int J Radiat Oncol Biol Phys. 2023,115(2):426-439.4. Bonfrate A, Ronga MG, Patriarca A, et al. Monte Carlo modeling of a commercial machine and experimental setup for FLASH - minibeam irradiations with electrons . Medical Physics. 2024,52(2):1224-1234. Keywords: FLASH, minibeam radiotherapy, skin toxicity Combined FLASH and conventional radiotherapy: identification of dose thresholds and perspective for dual irradiation fractionation protocols Anouk Sesink 1,2 , Luca Soutter 3 , Walter-Reiner Geyer 3 , Till Tobias Böhlen 3 , Raphael Moeckli 3 , Claude Bailat 3 , Veljko Grilj 2 1 u1288, Institut Curie, Orsay, France. 2 Radiation- Oncology, CHUV, Lausanne, Switzerland. 3 Institut de Radiophysique, CHUV, Lausanne, Switzerland Proffered Paper 3889 Purpose/Objective: Maintaining dose conformity while benefiting from the FLASH sparing effect in future clinical protocols may prove challenging. One solution could be to combine ultra-high dose rate radiotherapy (FLASH-RT) with conventional dose rate radiotherapy (CONV-RT) within a treatment. Our research aims to systematically characterize the FLASH sparing effect within such combined radiation protocols and to identify dependence of this effect on the magnitude of the FLASH-RT dose with characterisation of a possible dose threshold. Material/Methods: The abdomen of C57BL/6 mice and the skin of BALB/c mice were irradiated using the IntraOp Mobetron and the Oriatron eRT6 irradiators, respectively. The single fraction hybrid irradiation involved a split dose
configuration consisting of a boost dose delivered either by FLASH-RT or CONV-RT followed by increasing doses of CONV-RT 1 minute later. Fractionated hybrid regimen consisted of three fractions with one or two fractions delivered by FLASH-RT and the remaining fractions delivered by CONV-RT. Abdominal and skin toxicity were assessed for each experimental configuration. Results: In the gastrointestinal toxicity model, a single fraction hybrid irradiation incorporating a FLASH boost demonstrated the preservation of the FLASH sparing effect for the boost dose of 14 Gy, whereas reducing the boost dose to 10 Gy resulted in the loss of this effect (Figure 1A). Similarly, single fraction hybrid irradiation of the skin revealed a significant sparing FLASH sparing effect for a boost dose of 20 Gy (Figure 1B). Again, reducing the boost dose negatively impacted the sparing effect. Fractionated hybrid regimen with one fraction delivered by CONV-RT (FLASH-FLASH-CONV) resulted in lower toxicity compared to delivering all three fractions by CONV-RT with a fraction dose modifying factor (DMF) of 1.19 (Figure 2). Replacement of another FLASH-RT fraction by CONV-RT led to a further reduction in DMF.
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