32 A FLASH of Hope - Preventing Recurrent Glioblastoma

Abstract Introduction Glioblastoma (GBM) remains the most aggressive and lethal primary brain tumour, with a 5-year survival rate of only ∼13% and a recurrence rate approaching 90% within two years of diagnosis. Current treatment options include surgery followed by radiotherapy and chemotherapy, but these are limited by radioresistance and the inability to eliminate residual infiltrative cells. There is an urgent need for more effective, kinder treatment strategies that prevent recurrence. This project has, so far, explored two promising therapeutic avenues: ultra-high dose-rate FLASH radiotherapy and metabolic inhibition, with the long term goal of combining both modalities to target recurrent GBM. Methods Patient-derived GBM primary and recurrent spheroid models of the infiltrative margin (GIN98R, GIN38, GIN44) were exposed to either conventional dose-rate radiotherapy or FLASH dose-rate radiotherapy at doses between 0-20Gy. Changes in spheroid area were quantified to assess tumour control. Separately, the metabolic inhibitor 3-bromopyruvate (3-BP) was applied to GBM spheroids to evaluate its impact on tumour cell viability. Statistical analysis was performed to determine treatment efficacy across conditions. Results FLASH radiotherapy demonstrated enhanced control of GBM spheroid growth compared with conventional radiotherapy, particularly at doses ≥10Gy, and was notably effective against the GIN98R cell line, which has shown evidence of radioresistance. Independently, 3‑BP significantly reduced tumour viability in GIN38 and GIN44 spheroids. This metabolic inhibition exploits the altered energy pathways on which GBM cells rely, offering a therapeutic vulnerability. Conclusions Our findings indicate that residual GBM cells can be targeted through two distinct but complementary mechanisms: energy metabolism inhibition and ultra-high dose-rate FLASH irradiation. These results lay the groundwork for a future combined modality strategy, with the potential for a synergistic and less toxic therapeutic option to prevent GBM recurrence.