This will depend on how accurate the machine is. I assume that not only are there restraints to keep the head in the right place, but that the machine targets t

This will depend on how accurate the machine is. I assume that not only are there restraints to keep the head in the right place, but that the machine targets the beams based on where the head actually is rather than where it expects it to be. I also imagine that ultrasound is also being used to precisely locate the brain or even the targeted section. For the Alzheimers treatments, it seems the damage (if it can be called that?) is subtle enough that it only lasts 24-48 hours, and shouldn't have side effects if that is true (the drugs themselves will of course have side effects). For other treatments, it is essentially pinpoint surgery, so has all the same risks, but less so because pinpoint. I can certainly imagine that destroying a few neurons to treat essential tremor could have drastic side effects.