Although I had FSR, the Gamma Knife procedure has been around for over 20 years and if it somehow instigated normal cells to turn cancerous, I think we've have heard about it by now.
Actually, GammaKnife has been around for over 40 years in the U.S. (the University of Pittsburgh Medical Center began using it on patients over 40 years ago). But that only reinforces Jim's point that we would've heard it by now if GammaKnife causes cancer.
Pat, the odds of getting cancer from radiation treatments (GammaKnife, CyberKnife or FSR) is virtually no greater than the risk to the general public of getting cancer ; the difference in risk is a fraction of one percent (essentially statistically insignificant).
Why is the risk so small? With GammaKnife (GK) and CyberKnife (CK) at least (the two types of radiation treatment I can speak most knowledgeably about), the radiation is delivered from many different angles so that the beams all converge on the tumor. Each beam is by itself relatively weak compared to the combined total, and it affects only a very tiny part of the brain. For example, with CK there are between 100 and 140 beams of radiation focused on the tumor. So each beam only presents at most 1% (1/100th to 1/140th) of the total dose to healthy tissue in between the skull and the tumor.
Also, the doses of radiation have been reduced a lot since it's early use. Doctors have learned that lower doses are effective in controlling an AN's growth and have reduced the total dosage accordingly to minimize side effects and long-term risks.
I have heard of only one case -- out of thousands -- of malignancy growing at the sight of an AN treated with radiation. When the incidence is that low (less than 1%), coincidence cannot be ruled out (in my mind, at least).
Each type of treatment (radiation and surgery) pose their own risks, but malignancy is not one you should concern yourself with. Statistically speaking, there is a far greater risk of facial-nerve damage (in a functional sense, if not anatomically), hearing loss, and chronic headaches with surgery, in general, than there is a risk of malignancy with radiation treatments.
The biggest question, however, is whether the risk of disabilities during the relative prime of your life (from surgery) should be more of a concern than some theoretical risk decades later (from radiation) that may never materialize. Perhaps I'm a fatalist, but I can't say I won't be killed in a car crash sometime in the next thirty years. I'd rather have those be as full (healthy) as possible. I'm more concerned with today. But everyone must weigh their tolerance for different risks for themselves, as no treatment plan is without considerable risk. My advice would be to concern yourself about those risks that are proven to be statistically significant. Malignancy is not one of those risks.