Your ENT is very misinformed about radiosurgery apparently, which continues to amaze me as to how may "doctors" are woefully unprepared to accurately guide patients in decisions with good information.
The general and well accepted size rule on ANs being treated by radiosurgery is around 3 cm, subject to review of the scans of course. Your husbands AN is slightly smaller than mine was at the time of treatment and more so than some others on the board.
There are many posts in the archives of this forum regarding the absurdity of the "malignant transformation issues". In short, there have been somewhere between 6-8 occurences over the years, although several followed whole brain radiation treatment as opposed to true radiosurgery and there were a couple that may have been malignant tumors before treatment. At any rate, the statistical probability is approximately the same as open surgery, which is to say, virtually nil.