I believe radiation necrosis is linked to how much brain receives 12Gy or more.
Its a real problem for people that have very large tumours, People that are treated with Whole Brain Radiotherapy and repeat radiosurgery..
I dont believe it is a significant problem at all for AN's
If you have Glioma they will zap your brain as well as the tumour as often there are cancer cells within the brain itself.
The risk of radiation necrosis then becomes much higher.
Radiation Necrosis can be picked up on the follow up MRI's
Hydrocephalus after radiation is a complication but is directly related to the volume of the AN treated with radiation.
I believe AN's smaller than 2cm this would be a very rare occurrence if at all..
As for Malignant Transformation... over 50% of malignant transformations have occurred in people with NF2
Less than 5% of people with AN's have NF2. There is also a theory that people that get malignant transformation may have a predisposition to cancer.
If you have this you could be at higher risk, if you dont the risk maybe lower again..
Interesting paper here linking radiation failure to existing cancer
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3379795/