ESTRO 2024 - Abstract Book

S5292

Radiobiology - Normal tissue radiobiology

ESTRO 2024

Involvement of senescence and macrophages in mouse lungs response to FLASH radiotherapy

Louize Brants 1 , Adrien Arrigo 1 , Verdi Vanreusel 1 , Ho Wa Lau 1 , Paul Meijnders 1,2 , Philip Poortmans 1,2 , Dirk Verellen 1,2 , Alessia Gasparini 1,2 , Pierre Montay-Gruel 1,2 1 University of Antwerp, Antwerp Research in Radiation Oncology (AReRO), Center for Oncological Research (CORE), Antwerp, Belgium. 2 Iridium Netwerk, Radiation Oncology Department, Antwerp, Belgium

Purpose/Objective:

Breast cancer is a highly prevalent disease in Europe, for which most patients receive radiotherapy. Nevertheless, 35% of the patients who receive thoracic RT develop radiation-induced lung injury, which may evolve into radiation-induced lung fibrosis in 16-28% of the cases, significantly impacting patients’ QoL. FLASH-RT is a new treatment modality based on the delivery of Ultra-high dose rate radiation beams. It has been described as capable of providing an identical tumor killing efficacy with less healthy tissue toxicities compared to conventional dose rate RT. Nevertheless, the molecular mechanism of this so-called “FLASH effect” is unknown. While previous studies have shown the contribution of senescent cells and M2 macrophages to the development of radiation induced lung fibrosis, we hypothesized that a differential senescence induction and macrophage polarization could be involved after FLASH-RT.

Material/Methods:

The investigation of the FLASH effect on healthy lung tissues was performed using healthy C57BL/6J mice exposed to a single 17.5 Gy dose to the whole thorax. Irradiations were performed using the ElectronFLASH linac (SIT, Sordina, Italy) at conventional dose rate (0.1 Gy/s) or ultra-high "FLASH" dose rate (> 10 6 Gy/s). Lung tissues were harvested 5 days, 13 days and 14 weeks post-irradiation. Lung fibrosis (Masson's trichrome, TGFß), senescence (SA-ßgal, p21) and immune infiltrates (CD45, F4/80) were quantified by histology analyses, and macrophage populations were quantified using flow-cytomertry techniques. RNAseq is currently being analyzed. All investigations were performed on early and late time points.

Results:

14 weeks post-RT, a reduced occurrence of subpleural fibrosis was observed in C57BL/6J healthy mice after FLASH RT compared to CONV-RT. These observations, along with a reduced amount of radiation-induced senescence confirm previously published results. Further investigation of cellular senescence showed that lungs exposed to FLASH-RT exhibited minimal presence of senescent macrophages along with less α-SMA expression compared to CONV-RT. Interestingly, senescent macrophages observed after FLASH-RT expressed less α-SMA, suggesting less macrophage-to-myofibrolast transition potentially explaining the reduced occurence of fibrosis. Flow cytometry analyses showed that contrary to conventional dose rate irradiation, FLASH-RT does not increase macrophage polarization or influence the balance between M1 and M2 macrophages 14 weeks post-RT. Nevertheless, a significant trend towards a decrease in macrophage population 5 days post-FLASH-RT was observed compared to non-irradiated and CONV-irradiated cohorts.

Conclusion:

In this study, we confirmed one more time that FLASH-RT induces less late normal tissue toxicity on healthy lungs, assessed by the occurence of lung fibrosis. Moreover, we showed that this quasi-absence of radiation-induced lung toxicity corroborates with fewer senescent and polarized macrophages observed after FLASH compared to