UC Irvine

Purpose: FLASH radiotherapy's enhanced normal tissue protective effect without compromising cancer treatment efficacy has stimulated investigations exploring its mechanisms. To this date, mitochondrial endpoints after FLASH radiotherapy have not been explored. This master’s thesis aims to narrate the in vivo accounts of the normal tissue-sparing FLASH effect observed across organ systems and report the findings in ATP production and mitochondrial dynamics in the murine brain as possible explanations for the FLASH effect.

Methods: Two cohorts received 10 Gy whole-brain FLASH and conventional dose rate irradiations at two different facilities. Stanford FLASH dose rate per pulse was 5.3 × 10^5 Gy/s Gy/s, with mean dose rate of 225 Gy/s. CHUV FLASH dose rate was 5.6 × 10^6 Gy/s, where a single 1.8 microsecond pulse was used to administer the total dose. The conventional dose rate was 0.1 Gy/s at both locations. At 4 months post-irradiation, ATP concentration and OPA-1 mitochondrial fusion protein levels were measured from the hippocampus and frontal cortex tissues and compared across the treatment groups via one-way ANOVA and Pearson's correlation analyses.

Results: The hippocampus tissues from Stanford FLASH- and CHUV conventional-irradiated groups had significantly different associations between long and short OPA-1 isoforms compared to the control (p=0.005 and p=0.012, respectively). Although the strengths of associations in the hippocampal tissues were weaker than their cortex counterparts across the cohorts, only the Stanford CONV and FLASH irradiated groups showed significant and marginally significant differences between the brain regions (p=0.0496 and 0.057, respectively). These differences indicate that the mitochondrial dynamics may be dysregulated in the hippocampus at 4 months post-irradiation. The individual levels of ATP, L-OPA1, S-OPA1, and the ratios of isoforms did not significantly differ across the treatment groups.

Conclusion: Despite the low sample size (n≤12), there may be evidence for dysregulated mitochondrial dynamics in the hippocampus after CONV and FLASH irradiations at a late time point. Further study should confirm the radiation-induced effect on mitochondrial physiology through more refined tests.