100%? I'm not sure that ever happens.
If you read a radiologist's report on your MRI, it will probably contain a sentence like "most compatible with a vestibular schwannoma", which is radiologist-speak for "you have an AN". The radiologists will never say they are 100% certain. Of course, they would probably say "most compatible with a nose on my face", where the rest of us would just say "that's my nose."
I did hear of one case this year, where a woman was diagnosed with an AN, and went in for middle fossa surgery. When she woke up, they said they were sorry, but they couldn't find a tumor! I haven't heard more about it, other than that she is fine, so I can't tell you what the final result was. It is a scary thought, but it can happen - fortunately not very often, though.
Hopefully they will be able to make a confident diagnosis, and get you on the right path, whatever it is.
Steve