Lynn, it's impossible to say why your doctor wants to talk with you so suddenly. But I can tell you that it is almost certainly not because your tumor is malignant. A malignant tumor looks very different from a benign AN, and the first MRI would've alerted him already to the possibility of malignancy if that were the case.
ANs and other benign tumors have relatively well-defined and smoothly contoured borders and look self-contained. Malignant tumors, on the other hand, look kinda like spaghetti, with thin projections radiating outwards into surrounding tissue. So, I don't think you need to worry about malignancy. That's probably already been ruled out.
It's certainly nerve-wracking to get this kind of call and having to wait to find out why they want to talk with you again so soon. Please let us know what you find out. We're hoping the best for you.
Best wishes,
TW