Dural fistula sigmoid sinus and gamma knife

dural fistula sigmoid sinus. Class 3.    very high risk for venous side coil embolization, extremely difficult location for surgery, only minor arterial embolization was possible to not much effect.  Recommendation now is gamma knife procedure.  no immediate risk, but risk is the 2 or more years you have to wait with the condition to before you even know if it worked or not. .  High risk to operate, high risk to wait.
My question please:  how favorable is cure with GK , fistula is 0.35 in wide,  0.70 inches long, quite high flow as well.
Also, in general, why does a blood vessel shrink when exposed to the GK radiation.  I understand that the dna in the vessel cells is damaged, but why does the vessel not stay the same size as it repairs itself, and why does it take 2 years or more for the effect of the radiation to show?  thanks kindly this matter concerns me alot I just need to know more. much appreciated