Abstract PS1-07-10: Tumor response to ultra high dose rate radiation (FLASH) vs. conventional radiotherapy in normal and cGAS-knockout mice

Abstract Background: Radiotherapy (RT) plays a major role in eradicating residual disease in primary breast cancer (BC) and in palliating metastatic BC. While conventional RT (≤0.03 Gy/s) is effective, it often causes significant normal tissue toxicity—such as skin inflammation—limiting treatment intensity and patient quality of life. FLASH RT, a newer approach delivering ultra-high dose rates (>40 Gy/s), has shown similar tumor control with reduced normal tissue damage (the “FLASH effect”). However, the mechanism by which FLASH spares normal tissue and supresses tumor growth remains poorly understood. Ionizing radiation activates the cGAS-STING pathway through DNA damage, triggering proinflammatory cytokine production and tissue inflammation. Recent reports suggest that FLASH RT also activates the cGAS-STING pathway. In this study, we explored the role of the cGAS-STING pathway in mediating the effects of FLASH and CONV RT on tumor eradication and tissue toxicity using cGAS KO mice, to better understand the contribution of the host cGAS pathway to the FLASH effect and tumor suppression. Aim: This study aims to systematically compare the therapeutic efficacy and normal tissue toxicity of FLASH vs CONV RT in a syngeneic BC mouse model and to elucidate the role of the cGAS pathway in mediating radiation-induced inflammation and tumor response. Methods: PYMT117 BC cells were orthotopically injected into the 3rd mammary fat pad of 6-8-week-old female C57BL/6 and cGAS KO mice. Once tumors reached ∼50 mm3, mice received a single 30 Gy dose of either FLASH or CONV RT. Tumor growth was monitored every other day with calipers, and mice were observed for skin toxicity (inflammation, cracking). Euthanasia was performed based on tumor burden or skin damage. Tumor volumes were analyzed using two-way ANOVA followed by Tukey’s post hoc test. Skin toxicity was graded (0-6). Each strain was analyzed separately, and p < 0.05 was considered significant. Results: Both FLASH and CONV RT significantly reduced tumor volume (p < 0.001, two-way ANOVA), with tumors becoming undetectable by day 14 post-irradiation compared to controls. Tumor recurrence began around day 25 in all treated groups. In C57BL6 mice, CONV RT showed a trend towards better tumor control with smaller recurrent tumors and some complete responses but caused severe skin toxicity, including inflammation and cracking, requiring euthanasia by day 50 (toxicity score: 6). FLASH RT, while slightly less effective in long-term tumor control, greatly reduced skin toxicity, with only mild lesions in 3 out of 10 mice (score: 4) in C57BL6 mice. Notably, cGAS KO mice showed no visible skin toxicity in either treatment group (score: 0). Conclusion: FLASH RT significantly reduces normal tissue toxicity compared to CONV RT, offering better tolerability with minimal skin damage. Although CONV RT showed stronger initial tumor control, its use was limited by severe skin toxicity. The absence of toxicity in cGAS KO mice suggests the cGAS pathway plays a key role in radiation-induced inflammation. These findings support FLASH RT as a promising, less toxic alternative and highlight the potential of targeting cGAS to improve radiotherapy outcomes in BC. Citation Format: B. Verma, A. Mutahar, S. Melamenidis, R. Verma, K. Casey, K. Horst, E. Graves, M. Clarke, B. Loo, F. Dirbas. Tumor response to ultra high dose rate radiation (FLASH) vs. conventional radiotherapy in normal and cGAS-knockout 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 PS1-07-10.