My response is not based on extensive research on whether GammaKnife (or any radio"surgery") can cause cancer, but I think that it's ironic when one does a Google search for "Gamma Knife causing cancer", all the responses are about Gamma Knife being used to TREAT cancer!
Gamma Knife has been around for decades. I haven't seen any reports popping up linking Gamma Knife treatment to CAUSING cancer. Perhaps you could put this question to, for example, someone at the House Clinic in LA (via email) or see if it's discussed on the CyberKnife patient site (Steve, what's that URL?). I do really agree with Steve that there are many things, including microsurgery for an AN, or totally unrelated things, that are more likely to be difficult or fatal than radio"surgery".
If I were you, I'd research the question more, try to find those who you sense have a balanced perspective, before rejecting Gamma or Cyber Knife. I can tell you from personal experience that Gamma Knife (which is what I had) is ALOTTTTTTT easier to recover from than brain microsurgery, which several members of my family have had in the past. Microsurgery can be a good choice, but I don't think that fear of cancer from radiosurgery is a good reason to choose microsurgery.
At age 40 I would have had no qualms about choosing radiosurgery, but that of course is a personal choice.
Keep us posted about your situation, and tell us if you find any balanced discussion or research about this question of radiosurgery for the younger.