Here's the description from the UPMC website regarding the long term risk of malignancy from GK:
Malignant schwannomas are rare, but have been reported de novo, after prior resection, and after irradiation. We answer that this is always a risk after irradiation, but that the risk should be very low. We have not seen this yet in any of our 5,400 patients during our first 15 years experience with radiosurgery, but quote the patients a risk between 1:1000 and 1: 20,000. We reported one patient with a malignant mesenchymal tumor of the cerebellopontine angle that resembled an acoustic tumor. One report from Japan found a malignant tumor four years after resection, and six months following radiosurgery. The time interval after irradiation was too short to be causative. A second report noted the development of a temporal lobe glioblastoma 7.5 years after radiosurgery for a nearby acoustic neuroma. The temporal lobe had received a low radiation dose. In contrast, we have a patient who had initial management of a frontal lobe astrocytoma, and years later developed an acoustic neuroma. Was the development of these tumors related in some oncogenetic way, or were they radiation related? We believe the risk of developing a tumor years after radiosurgery is much less than the risk of mortality immediately after a resection, and likely less than the risk of the patient developing another tumor on their own in another body location.