ESTRO 2025 - Abstract Book

S3958

Radiobiology - Normal tissue radiobiology

ESTRO 2025

the 0.1 Gy group), while CD8+ T cell numbers remained unchanged. The CD4+/CD8+ T cell ratio was also decreased in the 0.1 Gy group ( P <0.05).

Conclusion: Extremely low-dose hyperthermic neutron irradiation impairs immune function by suppressing CD4+ T cell mediated immunity, as evidenced by reductions in peripheral white blood cells, neutrophils, lymphocytes, and splenic T cells, particularly CD4+ T cells. Given that thermal neutron radiation may unavoidably affect surrounding normal tissues during BNCT, optimizing irradiation field design and enhancing radiation injury protection are imperative.

Keywords: BNCT, extremely low-dose

References: Fukuda H. Response of Normal Tissues to Boron Neutron Capture Therapy (BNCT) with 10 B-Borocaptate Sodium (BSH) and 10 B-Paraboronophenylalanine (BPA). Cells. 2021 Oct 26;10(11):2883. Herrera FG, Ronet C, Ochoa de Olza M et al. Low-Dose Radiotherapy Reverses Tumor Immune Desertification and Resistance to Immunotherapy. Cancer Discov. 2022 Jan;12(1):108-133.

4324

Digital Poster Recovery calculations for reirradiation: what are the implications of choosing one method over another?

William van den Berg, Chris Dean, Jackie Poxon, Niall MacDougall Radiotherapy, Barts Health NHS Trust, London, United Kingdom

Purpose/Objective: The number of patients returning for a new course of radiotherapy to a previously irradiated volume is increasing [1]. Recent guidelines have gone some way to harmonise reirradiation terminology [2] and some groups have attempted to set standards for reirradiation pathways in the clinic. Nonetheless, there is a notable paucity of clinical data on the underlying radiobiology, and many clinical reirradiation protocols are based on clinical judgement rather than high-level evidence. At Barts Health, as part of our wider reirradiation pathway restructuring, we have designed a simple radiobiology calculator which can be used for multiple organs-at-risk and for multiple previous courses of radiotherapy. Two distinct methods for calculating the effect of repair were identified in the literature [3] [4] but result in very different tolerance doses for new radiotherapy courses. In order to attempt to quantify the meaningful clinical effect of using one method over another, a retrospective analysis of patients returning for a course of reirradiation was carried out. This also served to validate the accuracy of the radiobiology calculator. Material/Methods: A total of 5 patients, with 6 PTVs and 12 critical OARs, who had received a new course of SABR to a previously irradiated volume were identified. The near-dose-max doses to the relevant organs-at-risk were entered into the radiobiology calculator using both calculation methods, and the resulting doses that the OARs could receive in the new course were compared. The difference between the two values was converted to a potential target compromise, using a typical SABR dose gradient of 10% of prescription dose per mm (in the same plane). Having assumed that each OAR received a dose equal to the new tolerance dose in the new course, the cumulative EQD2 doses to the OARs were also compared for each calculation method.