Abstract Introduction: Glioblastoma is a lethal brain tumor with poor response to current therapies, which include surgery, chemotherapy, and conventional radiation therapy (CONV-RT). Although, CONV-RT (0.01Gy/second) to brain tumors stimulate tumor antigen release, it also recruits immunosuppressive myeloid-derived suppressor cells and is associated with neurotoxicity. Ultrahigh-dose-rate or FLASH-RT, which delivers CONV-RT doses over a significantly shorter period (more than 40Gy/second) maintains tumor control, reduces normal tissue injury and is less immunosuppressive compared to CONV-RT across multiple cancer types. In our study, we compared the effects of FLASH-RT to CONV RT in syngeneic mouse GL261 glioblastoma-bearing mice. We hypothesized that FLASH-RT would be equally or more effective than CONV-RT for tumor control and result in less immunosuppression within the tumor and systemically. Methods: We stereotactically implanted 2 × 105 mouse GL261 cells into the right forebrain of C57BL/6 mice. Five days after tumor initiation, tumor bearing mice were treated with Sham-RT (control), CONV-RT (mean dose rate 0.373 Gy/s), or FLASH-RT (mean dose rate 3.6x106 Gy/s). Brain tumor tissue and peripheral blood were collected on days 5 and 12 after treatment. To assess for changes in the tumor microenvironment after CONV-RT or FLASH-RT, we performed 10x Xenium spatial transcriptomics analysis (stRNA-seq; brain tumor, n = 6 mice per group), Lunaphore COMET multiplexed immunofluorescence assay (brain tumor, n = 6 mice per group), and flow cytometry (peripheral mononuclear cells, n = 6 mice per group). We also evaluated survival outcomes following treatment (n = 8 mice per group). Results: FLASH-RT significantly improved overall survival rate of GL261 bearing mice compared to CONV-RT (P 0.05) and Sham-RT (P 0.001). FLASH-RT treatment markedly increased intratumoral CD8+ T-cell infiltration compared with CONV-RT (P 0.01) and Sham-RT (P 0.01). We also found that compared to FLASH-RT, CONV-RT caused a significant decrease in circulating PD-1+CD8+ T-cells (P 0.01), a potent antigen reactive cytotoxic T-cell population previously identified in human patients with glioblastoma. Conclusion: FLASH-RT is associated with better tumor control in mouse GL261 glioblastoma, increased intratumoral CD8+ T-cell infiltration, and preserves circulating antigen reactive PD1+CD8+ T-cells. These results indicate that FLASH-RT may synergize better with immune checkpoint inhibitors to re-invigorate anti-tumor T-cell responses against glioblastoma. Citation Format: Yanxia Ma, Nhat Nguyen, Xuehong Gui, Edgardo Aguilar, Luke Connell, Denae Neill, Emil Schüler, Chibawanye I. Ene. FLASH radiotherapy improves survival in mouse glioblastoma and spares circulating antigen reactive CD8+T-cells abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5265.
Ma et al. (Fri,) studied this question.