Data suggests that regions of the normal brain exposed to radiation doses that has otherwise been regarded as safe and not limited by current radiation treatment planning may contribute to the risk of late neurocognitive injury. Radiation dose-dependent subclinical vascular effects have been reported in irradiated normal brain tissue and have been hypothesized to be a potential mechanism of action. Direct neuronal injury is another potential mechanism of injury. 1)Estimate the degree of cognitive loss following RT. 2) Demonstrate evidence of radiation induced subclinical vascular and neuronal injury in adjacent brain regions receiving exit doses of radiation. Subjects will include patients with malignancies of the skull of the skull base or patients with low grade glioma who require radiotherapy. Baseline MRI imaging of the brain utilizing established techniques will be used to identify and characterize the regions of interest anatomically adjacent to the regions of intended high dose irradiation. The MRI data for the ROIs will be registered with the radiotherapy treatment planning CT in order to create a single volume of data where each voxel corresponds to a vector containing the multi-parametric information. Subsequent repeat MRI imaging will be at 1.5, 4.5, 12 and 24 months following completion of the radiotherapy for patients with low grade glioma and 1.5 and 12 months post radiotherapy for patients with malignancies involving the skull base. Both cohorts will repeat standard neurocognitive evaluation at 1.5, 4.5,12 and 24 months following completion of radiotherapy. Analysis: Neurocognitive domains will be evaluated at the designated time points.