Elucidating the neurological mechanism of the FLASH effect in juvenile mice exposed to hypofractionated radiotherapy

Abstract

Background Ultra-high dose-rate radiotherapy (FLASH-RT) affords improvements in the therapeutic index by minimizing normal tissue toxicities without compromising anti-tumor efficacy compared to conventional dose rate radiotherapy (CONV-RT). To investigate the translational potential of FLASH-RT to human pediatric medulloblastoma brain tumor, we used a radiosensitive juvenile mouse model to assess adverse long-term neurological outcomes. Methods Cohorts of three-week-old male and female C57Bl/6 mice exposed to hypofractionated (2×10 Gy, FLASH-RT or CONV-RT) whole brain irradiation and unirradiated controls underwent behavioral testing to ascertain cognitive status four months post-treatment. Animals were sacrificed 6 months post-irradiation and tissues analyzed for neurological and cerebrovascular decrements. Results The neurological impact of FLASH-RT was analyzed over a 6-month follow-up. FLASH-RT ameliorated neurocognitive decrements induced by CONV-RT and preserved synaptic plasticity and integrity at the electrophysiological (long-term potentiation), molecular (synaptophysin) and structural (Bassoon/Homer-1 bouton) levels in multiple brain regions. The benefits of FLASH-RT were also linked to reduced neuroinflammation (activated microglia) and a preservation of cerebrovascular structure, by maintaining aquaporin-4 levels and minimizing microglia colocalized to vessels. Conclusions Hypofractionated FLASH-RT affords significant and long-term normal tissue protection in the radiosensitive juvenile mouse brain when compared to CONV-RT. The capability of FLASH-RT to preserve critical cognitive outcomes and electrophysiological properties over 6-months is noteworthy and highlight its potential for resolving long-standing complications faced by pediatric brain tumor survivors. While care must be exercised before clinical translation is realized, present findings document the marked benefits of FLASH-RT that extend from synapse to cognition and the microvasculature.

A multi-institutional study to investigate the sparing effect after whole brain electron FLASH in mice: Reproducibility and temporal evolution of functional, electrophysiological, and neurogenic endpoints

“…CONV irradiation is known to induce impairments in learning and memory, effects that FLASH has been shown to minimize. (6,8,9,20).…”

“…To investigate the potential for FLASH to modulate early toxicities in the brain, we employed one of our most reliable late markers of the FLASH effect, namely LTP. When assessed at late times (> one month) CONV irradiated cohorts exhibit significant and persistent reductions in slope of the fEPSP, effects not evident after FLASH (8)(9)(10). While this finding was replicated after a single dose of 10 Gy at five months post-irradiation, no such change was found at two weeks post-irradiation.…”

“…The prolonged sparing of learning and memory deficits after FLASH suggests a preservation of synaptic elements involved in neurotransmission. In three recent studies, hypofractionated dosing regimens (2×10 Gy, 3×10 Gy) and standard of care fractionation (10×3 Gy) was shown to preserve LTP in FLASH cohorts when assessed months after irradiation, as opposed to the significant inhibition of LTP in the CONV cohorts (8)(9)(10).…”

“…Studies of whole-brain FLASH in mice have demonstrated sparing of long-term cognitive function evaluated with a multitude of neurobehavioral assessments, as opposed to the significant impairments found routinely in animals exposed to CONV (3)(4)(5)(6)(7)(8)(9). Behavioral outcomes correspond to similar sparing effects observed on electrophysiological assessments of long-term potentiation (LTP), protection of the cerebral vasculature and neuronal morphology along with reductions in neuroinflammation (5,6,(8)(9)(10).…”

Uncovering the Protective Neurologic Mechanisms of Hypofractionated FLASH Radiotherapy

“…5 ). Synaptophysin is a well-described integral membrane marker localized to presynaptic dendrites that has been used to measure synaptic plasticity ( 22 ) and neuronal architecture ( 23 ). Expression of the major synaptic vesicle protein synaptophysin was evaluated to quantify presynaptic vessel density in both the hippocampus and mPFC ( Fig.…”