High Dose Rate Radiation does not Protect Normal Tissue in Mice Cardiac and Splenic Models of Lymphopenia and Gastrointestinal Mucosal Injury
Liu Y, Venkatesulu BP, Sharma A, Pollard-Larkin JM, Sadagopan R, Symons J, Neri S, Singh P, Tailor R, Lin S, and Krishnan S. High Dose Rate Radiation does not Protect Normal Tissue in Mice Cardiac and Splenic Models of Lymphopenia and Gastrointestinal Mucosal Injury. International Journal of Radiation Oncology Biology Physics 2020; 108(2):E24.
International Journal of Radiation Oncology Biology Physics
Background: High dose rate radiation, also known as FLASH, is delivery of radiation at rates that far exceed conventional radiation (35-100 Gy/s vs 1-4 Gy/min). This approach has shown promise in recent preclinical studies demonstrating that not only does it have equivalent therapeutic efficacy as conventional radiation, but it may also lead to reduced toxicity to certain normal tissues (Favaudon V, et al. Sci Transl Med 2014, Vozenin M, et al. Clin Cancer Res 2019, Montay-Gruel P, et al. Radiother Oncol 2017). However, our understanding of the types of normal tissue that have improved sparing by FLASH is incomplete.
Objectives: In this study, we first aimed to compare the tumoricidal efficacy of FLASH to conventional radiation. Additionally, we addressed whether FLASH can lead to reduced lymphopenia and/or gastrointestinal mucosal injury compared to conventional radiation.
Methods: FLASH radiation was delivered at 35 Gy/s. Tumoricidal efficacy was assessed in vitro using clonogenic assays with 2 murine pancreatic cell lines, KPC and Panc02, treated with 0-8 Gy of conventional or FLASH radiation. To address lymphopenia ex vivo, human peripheral blood mononuclear cells (PBMCs) were irradiated with 2 Gy of conventional or FLASH radiation and apoptosis and necrosis were assessed at 24h and 72h with flow cytometry. To assess lymphopenia in vivo, mice were exposed to conventional or FLASH radiation to the heart (2 Gy/day x 5 days or 10 Gy x 1) or spleen (1 Gy/day x 5 days or 5 Gy x 1) and levels of CD3, CD4, CD8, and CD19 cells were assessed at 3, 10, 17, and 24 days. Lastly, to assess GI mucosal toxicity in vivo, mice were exposed to a single 16Gy dose to the abdomen with conventional or FLASH radiation and survival was assessed.
Results: FLASH was approximately 30% more effective in killing mouse pancreatic cell lines compared to conventional radiation. FLASH did not lead to improved sparing of ex vivo human PBMCs compared to conventional radiation with a similar profile of early apoptotic, late apoptotic, and necrotic cells. Mice exposed to FLASH radiation to the heart or spleen had more severe depletion and slower recovery of lymphocytes compared to conventional radiation. Mice exposed to total abdominal radiation with FLASH had worse overall survival compared to mice treated with conventional radiation. All mice died within 7 days of FLASH compared to mice surviving up to 15 days after conventional radiation.
Conclusions: Our study confirms previous reports showing that FLASH is non-inferior to conventional radiation in tumoricidal efficacy and may actually be more effective. However, unlike previous studies, we found that FLASH had worse toxicity in the setting of lymphopenia and GI mucosa, which suggests that the advantages of FLASH may be tissue and/or dose dependent. The optimal regimen of FLASH and the mechanistic basis of its improved or worsened sparing of normal tissues remain important issues to be addressed.