LissieP:
The bottom line is you should not rely on one doctor's opinion in deciding your course of action (or inaction). I agree with Steve that you should seek consultations with House Ear Clinic and with at least one radiosurgery doctor (I highly recommend Dr. Chang, who is included in the list of CyberKnife doctors that Steve provided a link for). Also, I would recommend you contact Staten Island University Hospital, as they will reportedly treat with radiosurgery tumors over 3 cm in size that most other facilities will not. (Most facilities rule out Cyberknife or GammaKnife treatment for tumors over 3 cm in size. SIUH uses FSR radiosurgery to treat much larger tumors.) Radiosurgery is widely regarded as easier and safer than surgery for elderly or infirm patients.
At the risk of speaking too bluntly, I'll say this: a 34x24x29mm tumor is somewhat large and the rate of growth over the past 4 years that you have reported is above average. If your mother-in-law's overall health is so poor that doctors expect she hasn't got many more years ahead of her before she passes, then "watching and waiting" may indeed be the best course of action. However, if she is expected to live quite a bit longer than that, doing nothing may not be the best option because the tumor may cause new symptoms (one-sided facial numbness or paralysis) and more severe balance symptoms (disabling vertigo or oscillopsia) that could be far worse than what side effects treatment might bring. An untreated and growing AN (not all ANs continue to grow) does have the potential to eventually kill, although western medicine has all but eliminated mortality as a risk -- provided the patient receives some form of treatment. The only way to assess the risk-benefit ratio for various courses of action is to consult several top professionals who are experts in treating ANs.
There is also a wealth of information available in posts on this board. Welcome. We're here to help with any other questions you may have and with emotional support.
Best wishes,
Tumbleweed