Deb, from both the (considerable) research I've read and what I've been told by Dr. Steven Chang (esteemed neurosurgeon and CyberKnife practitioner at Stanford U. Medical Center), the risk that radiation treatments for an AN would cause a malignancy is no greater than the risk to the general population of getting cancer. CK and GK are very similar in their delivery.
The reason for the low risk is that the radiation is split up into roughly 80 to 140 beams that arrive at the tumor from all directions. Thus, the tumor gets a big dose where the radiation beams all converge on it, but other tissue that the radiation passes through on the way to the tumor only gets 1/80 (at most) to 1/140 (at the least) of the radiation dose that the tumor itself receives. Not to minimize the risk of intentionally and directly irradiating the brain, but it's good to remember that anyone who flies in a commercial airliner also gets a good dose of cosmic radiation each time they fly. That is to say, almost nobody escapes receiving some radiation these days, even if they avoid getting X-rays.
All that said, there are often side effects from receiving radiosurgery, although these are usually short-lived. I felt pretty wiped out after getting CK (for only a couple months, though, and I reacted stronger than most people to the treatment). But the risk of getting cancer from CK, GK or any of the other "flavors" of FSR is so little that it should not be a factor in your decision of which treatment to have.
Best wishes,
Tumbleweed
P.S. I'm now 9 months post-CK treatment, and I feel better now than I did before getting treatment. It might be a rocky road at first, but it does get better pretty quickly. I am a firm believer in radiosurgery and, in particular, radiotherapy ( the latter being the more accurate term for multiple, fractionated radiation treatments).
