Effects of glioma presence during radiotherapy on tissue damage

Animal models have been used to quantify the genomic and functional changes post-radiotherapy in brain tissue. However, in these models the effects of tumor presence during radiotherapy is not taken into account. We have recently measured significant genomic changes in the rat brain tissue that was occupied by the tumor post-radiotherapy compared to normal irradiated tissue. In our current work we are characterizing the long term functional changes caused by the presence of the tumor during radiotherapy. Sprague Dawley male rats (7wks) were divided between three experimental groups: Sham implant, RT+sham implant, and RT+Tumor (C6-GFP tumor implant glioma). Hypofractionated irradiation (8-6Gy/day for 5 days) was localized to a 1cm strip of the cranium starting 5 days post implant, which resulted in a complete regression of the tumor and prolonged survival. The former tumor area was imaged 65 days post implant using a 9.4T Biospec MRI scanner (Bruker) with a 20cm bore using a quadrature rat brain array. 1H magnetic resonance spectroscopy (1H-MRS) was performed in the former tumor/implant region using a STEAM sequence with the following parameters: TR = 2s/TE = 2.22ms/ # of averages = 512/ voxel size 56mm3. Diffusion tensor imaging (DTI) was performed using a spin echo EPI sequence with a TR = 500ms, TE = 33.2ms, 2 repetitions, slice thickness = 12.8mm, b value = 800 s/mm2, 30 of directions, matrix size 128×256×16 giving a spatial resolution of 150×150×800mm. 1H MRS data was analyzed using LC Model and DTI was analyzed using Medinria. Intravital microscopy was also used to quantify blood brain-barrier (BBB) permeability and leukocyte activity. 1H-MRS revealed that both RT+sham implant and RT+C6-GFP tumor groups had a significant reduction in taurine levels (p <0.04) in the former tumor/implant area. However, the RT+C6-GFP tumor group had a significant increase in GABA levels (p = 0.02), which may indicate an effect on synaptic neurotransmission. Interestingly, myo-inositol levels ,which is associated with RT-induced brain damage, did not decrease significantly in either RT+sham implant or RT+tumor groups (p = 0.056 and p = 0.061, respectively). Fractional anisotropy (FA), which measures neuronal fiber density showed no significant changes when comparing RT+sham implant and RT+Tumor to sham. Using intravital imaging we also measured a significant increase in BBB permeability (p<0.05) in the RT+tumor implant compared to sham and an elevated level of leukocyte-endothelial adhesion. In conclusion, using our tumor implant model to study the effect of radiation alone versus radiation combined with the presence of tumor, we have measured several functional changes post-radiotherapy. Our results indicate that radiation might be the most significant influence on neuronal fiber disruption and damage while tumor presence might play an important role in disrupting synaptic transmission as well as exasperating the inflammatory side effects of radiation.