This is to let you know that Memantine (Nameda, an anti-Alzheimer drug) can be used to prevent radiation-induced neurocognitive decline to a certain extent. My sister's radiation oncologist recommends it, it is apparently routinely prescribed for children undergoing brain radiation.
Patients usually take it a few days before radiation, during treatment, and for six months post-treatments. My sister's side effects so far have been headache, body ache, tiredness, and nausea.
But it might be worthwhile to ask for it.
Wishing you all the best,