From all the info I've gathered, retreatment with radiosurgery or resection should not be considered until 2 years after the original radiosurgery, unless there is significant growth beyond what can be caused by swelling due to central necrosis, or the swelling causes severe symptoms that constitute emergency surgery.
Here's some info from IRSA you may find comforting:
"Shrinkage actually is found in the vast majority of tumors when they are followed long enough. One year after the Gamma Knife treatment, shrinkage is confirmed in about one-third of the tumors. After four years, two-thirds of the tumors are smaller, and by 10 years, more than 90% have shrunk.
Signs of lack of response to radiosurgery, in general, appear within one to three years of treatment. At least in my experience, failure is extremely unlikely to occur when five years or more have elapsed. This statement may not apply for acoustic neuromas associated with neurofibromatosis 2 (NF2) in which case recurrence may occur later following Gamma Knife treatment as well as microsurgery.
I have found that Gamma Knife treatment can be repeated without increased risks if the acoustic neuroma did not respond as expected (unchanged size/shrinkage) to the first treatment. Microsurgery can also be selected, depending on the patient's preference.
Acoustic neuromas sometimes increase in size temporarily as a reaction to the Gamma Knife treatment. This is actually a favorable sign indicating a brisk response. Such swelling usually is most obvious between 6 and 18 months after the procedure. It should not be confused with increase due to lack of response in which case the tumor size will not return to the baseline but continue to increase. A definite assessment should be made two years after the treatment: was the swelling merely temporary or did the tumor fail to respond to the treatment? In any case, resection should not be considered during this two-year wait."
http://www.irsa.org/acoustic_neuroma.html
