A study to assess the effect of Gocovri (extended-release amantidine) to improve disability as assessed by the disability rating scale (DRS) and cognition as assessed by the Montreal Cognitive Assessment (MoCA) test in patients with radiation encephalopathy.

This is a prospective study to assess the effect of Gocovri (extended-release amantidine) on disability and cognition in patients with radiation encephalopathy. Patients with radiation encephalopathy will be seen in the office, and both clinical and short structured neuropsychological assessments will be collected longitudinally as medication adjustments are performed over time. The investigators will also measure functional glutamate activity in these subjects through use of F-FPEB PET/CT, which identifies mGluR5 expression in the brain. This imaging tracer serves as a surrogate for neurodegeneration in other conditions, such as Parkinson's Disease.

Inclusion Criteria:

• Individuals ≥ 18 years of age.
• Individuals with caregivers who are able to complete survey assessments for this study.
• Prior brain radiation treatment.
• Evidence of moderate-severe confluent white matter hyperintensity on MRI brain scan as judged by neuroradiology impression 0-5 years prior to enrollment.
• DRS > 6
• The effects of Gocovri on the developing human fetus are unknown. For this reason, women of child-bearing potential and men must agree to use adequate contraception (e.g. hormonal or barrier method of birth control) prior to study entry and for the duration of study participation; female subjects will be required to demonstrate a negative urine pregnancy test prior to study entry and subjects who are lactating should be excluded.

Exclusion Criteria:

• Pregnant or breastfeeding.
• Patients with growing brain lesions or those requiring escalating doses of tumor-directed treatment..
• Patients requiring doses of Dexamethasone higher than 4 mg or escalating doses of Dexamethasone
• Patients taking Amantadine or with prior usage of Amantadine.
• Patients taking other dopaminergic, GABAergic, glutamatergic, or noradrenergic drugs.
• Patients with a history of suicidality and depression.
• Patients with end stage renal disease (creatinine clearance less than 15 mL/min/1.73 m2).