Abstract PS1-07-15: Ultra high dose rate radiation (FLASH) vs. conventional radiotherapy: normal tissue toxicity and long-term survival in cgas ko and wild-type mice

Abstract Background: Radiotherapy (RT) is central to cancer treatment, used either before or after surgery to reduce or eliminate remaining cancer cells. While effective, conventional radiotherapy (CONV. RT, ≤0.03 Gy/s) often causes significant side effects such as skin lesions, inflammation, and long-term tissue damage and this RT-induced toxicity further compromises quality of life. FLASH radiotherapy (FLASH RT, > 40 Gy/s) is an emerging approach, not yet used clinically, but gaining attention due to its “FLASH effect” the remarkable ability to spare normal tissues from radiation damage. RT-induced DNA damage activates the cGAS-STING pathway, triggering inflammation that can aid tumor clearance but also harm healthy tissue. Emerging studies suggest that FLASH may modulate this pathway differently than CONV RT. To better understand this, we studied the effects of FLASH and CONV RT on tissue toxicity and survival in mice with and without the cGAS gene, aiming to clarify how this pathway may influence the benefits of FLASH RT. Aim: To compare the normal tissue toxicity and healing response after FLASH and conv. RT in mice with and without the cGAS pathway, and to understand whether this pathway contributes to radiation-induced tissue damage. Methods: C57BL6 and cGAS KO mice (6-8 weeks) were irradiated unilaterally on the left chest with 30 Gy using CONV and FLASH RT. The mice were monitored regularly over several weeks for visible skin changes, including skin cracking, hair loss, and hair depigmentation development. Vetericyn was applied to help with any visible skin irritation. Observations focused on how quickly side effects appeared, how severe they were, and whether the skin started healing the mice. The skin toxicity was calculated by giving the score (1-6). Results: In the C57BL6 mice treated with CONV RT, skin cracking, hair loss, and hair depigmentation were observed within the first few weeks after radiation (Score 4). These effects were most noticeable early on but gradually improved (aside from hair depigmentation), with signs of healing by the end of the observation period. In the cGAS KO mice treated with CONV RT, side effects were milder mainly slight hair loss, and only a few mice showed very less skin changes (Score 2). In contrast, mice treated with FLASH RT showed no skin damage in C57BL6 as well as cGAS KO groups (Score 0). All mice stayed stable and healthy throughout the study, and none required euthanasia. These findings suggest that FLASH RT may mitigate normal tissue toxicity by avoiding full activation of the cGAS-STING pathway, similar to what is observed in cGAS-deficient mice, while still preserving tolerance and stability. Conclusion: CONV RT caused noticeable skin damage in C57BL6 mice, while cGAS KO mice showed milder side effects, suggesting that the cGAS pathway may contribute to early radiation-related inflammation and tissue damage. On the other hand, FLASH RT resulted in much lower or negligible skin toxicity in both mouse types and appears to avoid triggering the same damage. These results support the idea that FLASH RT is a promising, less toxic alternative to CONV RT for cancer treatment. Understanding how immune-related pathways like cGAS respond to radiation could help develop even better strategies for safer and more effective radiotherapy in the future. Citation Format: B. Verma, A. Mutahar, S. Melamenidis, S. Dutt, K. Casey, K. Horst, E. Graves, M. Clarke, B. Loo, F. Dirbas. Ultra high dose rate radiation (FLASH) vs. conventional radiotherapy: normal tissue toxicity and long-term survival in cgas ko and wild-type mice [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-07-15.