ESTRO 2025 - Abstract Book

S3930

Radiobiology - Normal tissue radiobiology

ESTRO 2025

Radiation-induced damage was assessed in female CDF1 mice following irradiation with conventional dose rates (0.162 Gy/s, CONV arm) or UHDR (mean±sd 247±10 Gy/s, FLASH arm). The right hindleg of unanaesthetised mice was irradiated using a single dose (24.5-53.1 Gy), one daily dose for four consecutive days (33.7-89.7 Gy total dose) or two daily doses with a 6-hour interval for four consecutive days (54.2-90.4 Gy total dose) with 4-8 mice per dose (Table I). Irradiation was delivered with a 16 MeV electron beam using a FLASH-enabled Varian TrueBeam accelerator. Acute skin toxicity was quantified by hair loss, moist desquamation and toe separation and was monitored daily from 9 to 25 days post-treatment.

Results: The comparison of CONV to FLASH irradiated mice indicated a tissue-sparing effect of FLASH in all instances (Figure 1). Single-dose irradiations provided a mean FLASH protection ratio of 1.42 (Figure 1A). In contrast, 4 fractions provided a lower mean FLASH protection ratio of 1.20 (Figure 1B). Data collection on 8 fractions is ongoing and will be finished in time for presentation at the ESTRO conference. Preliminary data on 8 fractions indicates an even further reduced acute skin-sparing FLASH effect.

Conclusion: Fractionation reduces the sparing effect seen in single fraction FLASH studies. However, a tissue sparing effect of 20 % is still present at 4 fractions. Thus, the FLASH tissue-sparing is increasingly reduced but not eliminated in the investigated fractionation schemes. Keywords: FLASH irradiation, fractionation, acute toxicity