In addition to what Jim mentioned about surrounding structures, the other major concern is that larger tumors might swell after radiation, and cause even greater compression. The main structures that can be threatened in such event in addition to the nerves, are the brainstem and the 4th ventricle - CSF fluid flows through the latter, and it if is blocked this can cause hydrocephalus. To avoid swelling it is common practice to use less amounts of radiation in GK, or more fractionated treatments. I think this is why your doctor recommends FSR, and it sounds like a reasonable choice. From what I have seen in the Cyberknife forum, the doctors there say that there is no absolute size limit for CK. It all depends on the location, surrounding structures etc, and of course only qualified doctors can make this judgement.
Yes, indeed whatever compression you have will continue even with a dead AN. The assumption is that if you don't have major symptoms, your brain has already adjusted, and as long as you don't get additional growth, things will stabilize (and sometimes shrinkage is an added bonus).
Good luck with your decision it's a tough one, and there are many factors to consider. I am one of the few that did chose radiation for a similar sized AN. It was an agonizing decision, so I totally understand how you feel. I have explained my reasons in my previous posts and would be glad to discuss if you send me a PM.
Marianna
Marianna