I just found your question. I just last week had my large (5cm+) AVM treated with the X-Knife in Dunedin, New Zealand.
It had to be prepared first with a partial embolisation as it was too large to treat in one go, and I had more side-effects from the embolisation than the Radiotherapy, but that's not to say Matt will experience the same - or different.
Everyone's AVM is in a different spot and configuration, and I think of my self as one of the incredibly lucky amongst us that it seems not to have had any side effects - yet. Perhaps in a year I shall experience headaches and so on, but at the moment all I'm taking is a steadily reducing course of Cortisone that makes me hungry, wake up far too early and feel a bit "mad".
And honestly, I had some mad phases before anyway - definitely associated with the AVM, as they were strongest after the embolisation; I would stand in the supermarket trying to make a decision whether to buy an extra loaf of bread or something, and end up pulling it out and putting it into the trolley, then changing my mind and putting it back. Over about ten times, while standing there!
More recently I did the same with making a sandwich, opening and shutting it repeatly as I decided whether to add tomato or not. Didn't in the end, but the bread got pretty knocked about. Probably should have got an extra loaf :)
But that stuff is nothing, and I am pleased that as yet there's nothing undue showing up.
I found the whole procedure relatively un-scary for some reason, probably that I was doing something positive with the AVM that had been such a epilepsy-causing hassle to me for nearly 30 years.
No-one can, of course, predict what may happen as an after-effect. The rays passed through some functional tissue at the same time, so perhaps there is some further complications to come for me?
Anyway, I know you must go ahead with what could prove to a life-saving treatment for Matt, try not to worry too much about what may or may not come to pass -
I wish you all the best