Developing Magnetic Resonance Imaging Biomarkers of Neuroinflammation, Cognitive Impairment, and Survival Outcomes for Radiotherapy-Induced Brain Injury in a Preclinical Mouse Model.

OBJECTIVE: Radiotherapy-induced brain injury (RIBI) is a chronic side effect that affects up to 90% of brain tumor survivors treated with radiotherapy. Here, we used multiparametric magnetic resonance imaging (MRI) to identify noninvasive and clinically translatable biomarkers of RIBI.

METHOD: 8-week-old female, immune competent BALB/c mice were stereotactically irradiated with a single dose of 80 Gy, at a dose rate of 1.7 Gy/minute. The irradiated mice were then monitored longitudinally with MRI, behavioral tests of learning and memory, and immunohistochemistry, in comparison to nonirradiated mice.

RESULTS: Three types of MRI biomarkers of RIBI were identified. A contrast-enhanced T 1 -weighted MRI biomarker was identified as being best suited to detect the onset of injury, by detecting changes in the blood-brain barrier (BBB) permeability. Maximum BBB permeability (18.95 ± 1.75) was detected with contrast-enhanced T 1 -weighted MRI at 1-month postirradiation in irradiated mice ( P  < 0.0001, n = 3). Interestingly, maximum neuroinflammation (24.14 ± 6.72) was also detected using IBA1 and CD68 immunohistochemistry at 1-month postirradiation in irradiated mice ( P  = 0.0041, n = 3). This simultaneous maximum BBB permeability and neuroinflammation detection also coincided with the detection of the onset of transient cognitive impairment, detected using the fear-conditioning behavioral test at 1-month postirradiation in irradiated mice compared to nonirradiated mice ( P  = 0.0017, n = 10). A T 2 -weighted MRI hyperintensity biomarker was also identified, and determined to be best suited to detect intermediate injury. Maximum T 2 -weighted MRI hyperintensity (3.97 ± 2.07) was detected at 2-month postirradiation in the irradiated mice compared to nonirradiated mice ( P  = 0.0368, n = 3). This T 2 -weighted MRI hyperintensity also correlated with maximum astrogliosis (9.92 ± 4.21), which was also detected at 2-month postirradiation using GFAP immunohistochemistry in the irradiated mice compared to nonirradiated mice ( P  = 0.0215, n = 3). Finally, T 2 -weighted and T 2 *-weighted MRI hypointensity biomarkers were identified as being best suited to detect late injury, from 4-month postirradiation. These biomarkers correlated with increased iron deposition from late vascular damage, which was validated with Perls' Prussian blue histology ( P  < 0.05, n = 3). These hypointense MRI biomarkers of late injury also preceded significant weight loss, severe cognitive impairment, and decreased survival in the irradiated mice compared to the nonirradiated mice.

CONCLUSIONS: Here, we identified 3 types of translational MRI biomarkers of RIBI that could enable the noninvasive longitudinal evaluation of potential RIBI prophylactic and therapeutic agents. These translational MRI biomarkers could also play a pivotal role in the management of RIBI in brain tumor survivors.