Leigh,
the radiologists are pretty good at determining growth changes but I have seen a few posts where the growth was slightly different to the pictures.
As for MRI's, it's very advisable to use the same machine. They are all different in strengths, so for a consistant trend, use the same machine.
As for radiotherapy causing the tumor to go malignant, the odds are very, very low. there are stats on various studies in this site, just do a serach on the key words.
I had middle fossa done 3.5 years ago and it has returned so I am really p...d off. I am currently on watch and wait and am having MRI's 6 monthly. When it gets to a point of growth spurt I will be having Gamma Knife treatment on it. I will not go under the knife again if I can help it. There are other people with different stories to me that have been very happy with middle fossa, trans lab or retri... You need to research the options, make an informed decision and stick with it.
Laz