Uncovering the Protective Neurologic Mechanisms of Hypofractionated FLASH Radiotherapy

Abstract

Implementation of ultra-high dose-rate FLASH radiotherapy (FLASH-RT) is rapidly gaining traction as a unique cancer treatment modality able to dramatically minimize normal tissue toxicity while maintaining anti-tumor efficacy compared to standard of care radiotherapy at conventional dose rate (CONV-RT). The resultant improvements in the therapeutic index have sparked intense investigations in pursuit of the underlying mechanisms. As a preamble to clinical translation, we exposed non-tumor bearing male and female mice to hypofractionated (3x10 Gy) whole brain FLASH- and CONV-RT to evaluate differential neurological responses using a comprehensive panel of functional and molecular outcomes over a 6-month follow up. In each instance, extensive and rigorous behavioral testing showed FLASH-RT to preserve cognitive indices of learning and memory that corresponded to a similar protection of synaptic plasticity as measured by long-term potentiation (LTP). These beneficial functional outcomes were not found after CONV-RT and were linked to a preservation of synaptic integrity at the molecular (synaptophysin) level and to reductions in neuroinflammation (CD68+ microglia) throughout specific brain regions known to be engaged by our selected cognitive tasks (hippocampus, medial prefrontal cortex). Ultra-structural changes in pre/post-synaptic bouton (Bassoon/Homer-1 puncta) within these same regions of the brain were not found to differ in response to dose rate. With this clinically relevant dosing regimen, we provide a mechanistic blueprint from synapse to cognition detailing how FLASH-RT reduces normal tissue complications in the irradiated brain.

Comparable survival in rats with intracranial glioblastoma irradiated with single-fraction conventional radiotherapy or FLASH radiotherapy

“…Other studies have shown a cognitive sparing effect of FLASH-RT in relation to CONV-RT (23)(24)(25). In future studies, it would be interesting also to explore the effects of fractionation and dose rate in relation to dose-response and cognitive side effects.…”

Electron FLASH radiotherapy in vivo studies. A systematic review

“…Interestingly, the same research group had previously explored the hypofractionated scheme in mouse animal models and published two articles in 2022 and 2023 ( 29 , 52 ). Male and female C57Bl/6 mice were divided into groups and exposed to hypofractionated whole-brain irradiation (2 × 10 Gy or 3 × 10 Gy with 48-hour intervals), either using FLASH-RT or CONV-RT, alongside unirradiated controls.…”

Structural plasticity of pyramidal cell neurons measured after FLASH and conventional dose-rate irradiation

“…Animals were prepared for immunohistochemistry as described previously (Allen et al 2022;Alaghband et al 2023a). Brie y, brains were sectioned coronally (30-35 µm thick) using a cryostat (Leica Microsystems) for super resolution microscopy and two 30 µm sections per animal containing the dorsal hippocampus were washed with PBS, before being permeabilized in a solution of 0.3% TX-100.…”