Proton therapy is better suited to larger tumours.
Challenges for proton therapy....
Range uncertainty... This is a physics thing.... Like heisenbergs uncertainty principle..
This limits accuracy to 3mm at best. Which means they must zap more to ensure they get all of the tumour.
Treatment is typically to the 120% isodose line critical structures right next to the tumour will receive significantly more radiation than photons... But 5mm away will get less.... Which is good for some tumours but not others.. Acoustic neuromas are often close to the trigeminal nerve and brainstem.
Search pubmed for research papers.. The results are not as good as gamma knife cyberknife or other linac.. Proton therapy has its place and has shown advantages for certain tumours. Small tumours like yours are not proton therapies strengths..
Facial and trigeminal nerve preservation rates are around 90% with proton therapy versus 99% with gammaknife http://www.ro-journal.com/content/7/1/210