If your heart is really set on "retreating with radiation", then I would certainly contact as many radiation specialist throughout the country as possible to find out if that's possible, and their opinion for this delicate and more complicated matter.
Here are some of the top 8 places for radiosurgery, and the best neurosurgeons in the USA to start you off. I would simply mail a copy of your most recent MRI and medical history for their free consultation over the phone.
Dr. Lunsford - Gamma Knife at UPMC Pittsburgh, PA
Dr. Chang - Cyber Knife at Stanford, CA
Dr. Kondziolka - New Jersey, Manhattan, New York
Dr. Sheehan - Univ. Virginia - Gamma Knife - UVA
Dr. Schwartz - House Ear Clinic - Los Angeles, CA
Dr. Friedman - Keck Univ- California
Stephen Tatter and Michael Chan - Wake Forest University North Carolina - Gamma Knife
Clinton A. Medbery, III, M.D. Southwest Radiation Oncology - Oklahoma
And perhaps your local Gamma Knife and Cyber Knife specialists