Having spent a little time reading up on the medical physics for radiation and AN, I can unequivocally state that I am no expert on range uncertainty or how best to control it. That said, the notion behind many smaller doses (e.g. @1.8g with proton treatment) with a long duration between each dose (24 hours) is to minimize impacts to the critical organs at risk. The key appears to be allowing enough time for any intentionally or unintentionally radiated healthy tissue to recover somewhat before the next treatment. Proportioning the time between doses with the dose strength, results in an effective, safe treatment even when the cumulative dose may be 54 Gy. Again, it is neither done all at once nor in five days, yet more like over 5-6 weeks, at least for fractionated SRT using protons. As far as evidence in support of multiple sessions, there are many papers that indicate fractionation is a viable treatment option, especially for larger AN:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4247447/
to show but one. Some of the references go back over 20 years ago.
In addition, in a presentation to the Boston AN Support Group in 2017, titled, The Evolution of the Management of Acoustic Neuromas (available on the ANA online video library), Dr. McKenna of MEEI emphasized how a fractionated protocol delivers less toxicity to the cranial nerves, though, to PaulW's point, he also noted that there is no demonstrated difference between SRS and fractionated SRT in the tumor control rate; however, it is noteworthy that the implication is that less damage to nearby organs at risk is more likely with a fractionated approach. In the presentation, a study at MGH showed that when SRS with a dose of 12 Gy was compared with fractionated proton SRT at a daily dose of 1.8 gy for 30 days, 16% of people had trigeminal nerve complications with SRS, while only 4% showed such complications with the fractionated approach. Tissues seem to tolerate well spaced smaller doses better than a single large one, even when the cumulative dose can be much higher. Every radiation approach has risks, and as is often stated, we need to do our homework then decide, at least we can all agree on that.
Hope for the best of outcomes, whatever choice is made.