Late radiation-induced brain injury is associated with functional cerebrovascular impairment without structural vessel loss in mice

Background: Radiation-induced brain injury (RIBI) is a major and often irreversible complication of cranial radiotherapy, affecting up to 90% of long-term survivors. Although cerebrovascular damage is a leading mechanistic hypothesis, whether chronic resting hypoperfusion occurs independently of structural vessel loss remains unknown. The aim of this study was to comprehensively characterize cerebrovascular, structural, and behavioral outcomes in a murine model of late RIBI to determine the primary drivers of vascular dysfunction. Methods: Adult C57BL/6 mice underwent 12 Gy whole-brain irradiation and were assessed one year later using laser speckle contrast imaging (LSCI), 7T MRI, immunohistochemistry, and behavioral testing. Results: Irradiated mice exhibited reduced pial flow velocity and flow area by LSCI, indicative of upstream cerebrovascular dysfunction, while bulk cortical tissue perfusion was preserved. This pial hypoperfusion was accompanied by cerebral atrophy, decreased T2* relaxation time and tissue iron deposition consistent with chronic microhemorrhage across multiple brain regions by MRI, blood-brain barrier disruption reflected by reduced ZO-1 and VE-cadherin expression, and increased anxiety-like behavior. Strikingly, capillary density, pericyte coverage, astrocyte endfeet, and NeuN-positive neuronal nuclei were all preserved. Conclusion: These findings demonstrate that functional vascular impairment - not structural vessel or neuronal loss - drives hypoperfusion and blood-brain barrier disruption in late RIBI. This dissociation between vascular dysfunction and rarefaction reframes the pathophysiology of RIBI and identifies endothelial dysfunction as a tractable therapeutic target for intervention.