Hi, William - and welcome, although I'm sure you wish you didn't have the acoustic neuroma diagnosis that brought you here.
I won't venture to answer your question because I simply don't have the kind of medical expertise to offer a credible response. I do know that your AN is on the outer edge of the usual parameters for radiation treatment but if Dr. Linseed at UMPC is confident that you're a candidate for Gamma Knife treatment, I won't argue the point. I do suggest that you consult other doctors before making a treatment decision.
For what it's worth, In 2006, when I was just a lad of 63, I was diagnosed with a large (4.5 cm) acoustic neuroma that was impacting my brain stem and giving me real problems. My neurosurgeon, (with 30+ years of AN removal experience) presented me with a plan to debulk the AN, sever it's blood supply and then (90 days later) radiate the remaining tumor using FSR. His approach was intended to avoid facial immobility as a result of the surgery and to ensure (as much as possible) that the AN would not re-grow. Long story short: his plan was a great success. Tumor debulked, all pre-op AN symptoms disappeared, home from the hospital in 5 days, a relatively quick recovery. FSR 3 months later was uneventful and later MRI scans showed the beginning of necrosis and shrinkage. Today, almost 6 years later, I'm doing great!
AN surgery is no picnic, of course, but if your health is otherwise good (mine was) 'debulking' the tumor, then radiating it could be a solution. However, only a medical doctor can offer you a credible opinion on the feasibility of this approach in your case and, as always, the ultimate decision for treatment lies with you, alone. I trust this website and the discussion forums can be of help to you as you make that decision.
Jim