ESTRO 2025 - Abstract Book

S3912

Radiobiology - Normal tissue radiobiology

ESTRO 2025

Conclusion: Tenascin C was effective in detecting cardiac damage in rats. It was also observed that a dose of 5mg/kg melatonin could be used to prevent RT-induced cardiac damage in rats.

Keywords: melatonin, tenascin C, cardiotoxicity

914

Digital Poster Discordance in the induction of the FLASH effect between synchrotron-based proton and linac-based electron ultra-high dose rate irradiation Kevin Liu 1 , Uwe Titt 1 , Nolan Esplen 1 , Luke Connell 1 , Elise Konradsson 1 , Ming Yang 1 , Xiaochun Wang 1 , Takeshi Takaoka 1 , Ziyi Li 2 , Albert Koong 3 , Devarati Mitra 3 , Radhe Mohan 1 , Billy Loo 4 , Steven Lin 3 , Emil Schueler 1 1 Radiation Physics, MD Anderson, Houston, USA. 2 Biostatistics, MD Anderson, Houston, USA. 3 Radiation Oncology, MD Anderson, Houston, USA. 4 Radiation Oncology, Stanford, Palo Alto, USA

Purpose/Objective: The objective of this study is to evaluate the extent of radiation induced gastrointestinal

(GI) toxicity in mice receiving abdominal irradiations using protons at FLASH and CONV dose-rates utilizing both a shoot-though (S-T) and spread-out Bragg peak (SOBP) beam. The biological response to proton irradiations in the GI under different dose rate conditions was compared with electrons under similar physical beam conditions.

Material/Methods: Abdominal irradiations (12-14 Gy single fraction) were performed on female C57BL/6 mice

using CONV and FLASH proton beams for both the S-T and SOBP configurations of a synchrotron. The proton beam had an energy of 87 MeV, field size of 2 cm diameter and dose-rates of 0.2 or 0.3 Gy/s (CONV) and 140 or 240 Gy/s (FLASH) for S-T and SOBP, respectively. The GI toxicity was quantified through the regenerating crypt assay from tissue harvested at 84 hours post-irradiation and actuarial survival over 50 days. The response following proton irradiations were compared against electrons where abdominal irradiations were performed using the IntraOp Mobetron with 9 MeV electrons under CONV (0.2 Gy/s) and FLASH (185-225 Gy/s) conditions using a 4 x 4 cm 2 field size. Results: Mice treated with FLASH protons exhibited significantly fewer regenerating crypts than mice treated with CONV protons at delivered doses of 12 and 13 Gy. When comparing mice that were treated under identical dose and similar dose rate conditions, there was no significant difference in the number of regenerating crypts between mice treated with either S-T or SOBP indicating no beam delivery technique dependence in acute GI response. Between mice treated with protons and electrons, there was no significant difference in GI toxicity for mice treated under CONV conditions. However, under FLASH conditions, mice treated with electrons exhibited significantly higher number of regenerating crypts than those treated with protons (p<0.001). Conclusion: FLASH proton irradiation was found to induce higher tissue toxicity compared to CONV proton irradiation. When comparing protons and electrons, both modalities led to similar levels of toxicity under CONV conditions whereas FLASH electron irradiation exhibited a significant reduction in toxicity compared to CONV electron irradiation, whereas FLASH proton irradiation showed a marked increase in toxicity. This study demonstrates that normal tissue toxicity and the conditions to induce the FLASH effect may depend on parameters that extend beyond mean dose rate to also include radiation type.