Purpose To quantify the effectiveness of anti-VEGF antibodies (bevacizumab and B20-4.1.1) as mitigators of radiation-induced, CNS (brain) necrosis in a mouse model. Experimental Design Cohorts of mice were irradiated with single-fraction 50- or 60-Gy doses of radiation targeted to the left hemisphere (brain) using the Leksell Perfexion Gamma Knife. The onset and progression of radiation necrosis were monitored longitudinally by in vivo, small-animal MRI, beginning four weeks post-irradiation. MRI-derived necrotic volumes for antibody (Ab)-treated and untreated mice were compared. MRI results were supported by correlative histology. Results Hematoxylin and eosin stained sections of brains from irradiated, non-Ab-treated mice confirmed profound tissue damage, including regions of fibrinoid vascular necrosis, vascular telangiectasia, hemorrhage, loss of neurons, and edema. Treatment with the murine anti-VEGF antibody B20-4.1.1 mitigated radiation-induced changes in an extraordinary, highly statistically-significant manner. The development of radiation necrosis in mice under treatment with bevacizumab (a humanized anti-VEGF antibody) was intermediate between that for B20-4.1.1-treated and non-Ab-treated animals. MRI findings were validated by histologic assessment, which confirmed that anti-VEGF-antibody treatment dramatically reduced late-onset necrosis in irradiated brain. Conclusions The single-hemispheric-irradiation mouse model, with longitudinal MRI monitoring, provides a powerful platform for studying the onset and progression of radiation necrosis and for developing and testing new therapies. The observation that anti-VEGF antibodies are effective mitigants of necrosis in our mouse model will enable a wide variety of studies aimed at dose optimization and timing and mechanism of action with direct relevance to ongoing clinical trials of bevacizumab as a treatment for radiation necrosis.

PurposeTo develop a Gamma Knife-based mouse model of late time-to-onset, cerebral radiation necrosis (RN) with serial evaluation by magnetic resonance imaging (MRI) and histology.Methods and MaterialsMice were irradiated with the Leksell Gamma Knife® (GK) PerfexionTM (Elekta AB; Stockholm, Sweden) with total single-hemispheric radiation doses (TRD) of 45- to 60-Gy, delivered in one to three fractions. RN was measured using T2-weighted MR images, while confirmation of tissue damage was assessed histologically by hematoxylin & eosin, trichrome, and PTAH staining.ResultsMRI measurements demonstrate that TRD is a more important determinant of both time-to-onset and progression of RN than fractionation. The development of RN is significantly slower in mice irradiated with 45-Gy than 50- or 60-Gy, where RN development is similar. Irradiated mouse brains demonstrate all of the pathologic features observed clinically in patients with confirmed RN. A semi-quantitative (0 to 3) histologic grading system, capturing both the extent and severity of injury, is described and illustrated. Tissue damage, as assessed by a histologic score, correlates well with total necrotic volume measured by MRI (correlation coefficient = 0.948, with p<0.0001), and with post-irradiation time (correlation coefficient = 0.508, with p<0.0001).ConclusionsFollowing GK irradiation, mice develop late time-to-onset cerebral RN histology mirroring clinical observations. MR imaging provides reliable quantification of the necrotic volume that correlates well with histologic score. This mouse model of RN will provide a platform for mechanism of action studies, the identification of imaging biomarkers of RN, and the development of clinical studies for improved mitigation and neuroprotection.

Purpose Accurate non-invasive diagnosis is vital for effective treatment planning. Presently, standard anatomical MRI is incapable of differentiating recurring tumor from delayed radiation injury, as both lesions are hyperintense in both post-contrast T1- and T2-weighted images. Further studies are therefore necessary to identify an MRI paradigm that can differentially diagnose these pathologies. Mouse glioma and radiation injury models provide a powerful platform for this purpose. Methods and Materials Two MRI contrasts that are widely employed in the clinic were chosen for application with a glioma/radiation-injury model: diffusion weighted imaging, from which the apparent diffusion coefficient (ADC) is obtained, and magnetization transfer contrast, from which the magnetization transfer ratio (MTR) is obtained. These metrics were evaluated longitudinally, first in each lesion type alone – glioma vs. irradiation - and then in a combined irradiated glioma model. Results MTR was found to be consistently decreased in all lesions compared to nonlesion brain tissue (contralateral hemisphere), with limited specificity between lesion types. In contrast, ADC, though less sensitive to the presence of pathology, was increased in radiation injury and decreased in tumors. In the irradiated glioma model, ADC also increased immediately after irradiation, but decreased as the tumor regrew. Conclusions ADC is a better metric than MTR for differentiating glioma from radiation injury. However, MTR was more sensitive to both tumor and radiation injury than ADC, suggesting a possible role in detecting lesions that do not enhance strongly on T1-weighted images.

Purpose/Objectives To quantify the effectiveness of SB415286, a specific inhibitor of GSK-3β, as a neuroprotectant of radiation-induced, CNS (brain) necrosis in a mouse model. Methods and Materials Cohorts of mice were treated with SB415286 or DMSO prior to irradiation with a single 45-Gy fraction targeted to the left hemisphere (brain) using the Leksell Perfexion Gamma Knife. The onset and progression of radiation necrosis were monitored longitudinally by non-invasive in vivo, small-animal MRI, beginning 13 weeks post-irradiation. MRI-derived necrotic volumes for SB415286- and DMSO-treated mice were compared. MRI results were supported by correlative histology. Results Mice treated with SB415286 showed significant protection from radiation-induced necrosis, as determined by in vivo MRI with histologic validation. MRI-derived necrotic volumes were significantly smaller at all post-irradiation time points in SB415286-treated animals. While the irradiated hemispheres of the DMSO-treated mice demonstrated many of the classic histologic features of RN, including fibrinoid vascular necrosis, vascular telangiectasia, hemorrhage, and tissue loss, the irradiated hemisphere of the SB415286-treated mice consistently showed only minimal tissue damage. These studies confirmed that treatment with a GSK-3β inhibitor dramatically reduced delayed time-to-onset necrosis in irradiated brain. Conclusions The unilateral cerebral hemispheric stereotactic radiosurgery mouse model, in concert with longitudinal MRI monitoring, provides a powerful platform for studying the onset and progression of radiation necrosis and for developing and testing new neuroprotectants. SB415286’s effectiveness as a neuroprotectant for necrosis motivates potential clinical trials of it or other GSK-3β inhibitors.