Inclusion Criteria:

• Age ≥ 4 years to ≤ 18 years old.
• Receiving intracranial radiation for a primary CNS malignancy.
• Histological or radiologic confirmation of intracranial disease.
• Able to use the computer for CogState assessment battery.
• Serum creatinine and total bilirubin obtained ≤ 35 days prior to study entry, with adequate kidney and liver function defined as follows:
  • Normal serum creatinine per institutional normal limits;
  • Total bilirubin ≤ 1.5 x upper limit of normal (ULN) OR Direct bilirubin ≤ ULN for patients with total bilirubin levels > 1.5 x ULN, Aspartate transaminase (AST) AND alanine transaminase (ALT) ≤ 2.5 x ULN.
• Provide informed consent if over age 18, or provide assent with consent from parent or legal guardian if age 7-17.
• Negative pregnancy test done ≤ 7 days prior to registration, for women of childbearing potential only. Patients capable of childbearing must use adequate contraception.

Exclusion Criteria:

• Patients with WHO Grade IV astrocytoma or glioblastoma tumors.
  • Note: A patient with Grade IV tumors of other histology can participate in the study if they meet all other criteria.
• Any prior intracranial radiation.
• Any contraindication or allergy to memantine.
• Use of short-acting benzodiazepines (may excite lethargy/dizziness with memantine).
  • Note: occasional use as a sleep aid or as needed  for anxiety or nausea is allowed.
• Intractable seizures while on adequate anticonvulsant therapy, defined as more than one seizure per month for the past 2 months.
• Co-morbid systemic illnesses or other severe concurrent disease which, in the judgment of the investigator, would make the patient inappropriate for entry into this study or interfere significantly with the proper assessment of safety and toxicity of the prescribed regimens.
• Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.