Hi Samsmom and welcome,
I have re-growth after 3 years from my op. If I read your post correctly, in 3 months the growth has grown 4.5 cm - THATS IMPOSSIBLE for an AN. they do not grow that fast. I am also suspicious re the Cat scan. to the best of my knowledge, the cat scans can't pick up AN's.
I suspect that what the cat scan has seen is the fat that was placed in the gap left behind by the tumor.
You need to have another MRI with the fat suppressant box ticked on the form (if its the same as in Oz). The radiologist reading the cat scan wouldn't be able to distinguish tumor from fat on the film. In fact, unless the MRI is set for fat suppression, the fat looks very much like the tumor. - How do I know, coz my #$%#@ ENT surgeon didn't put fat suppression on a follow up MRI that I had and it scared the living daylights out of me. I then had to have another one which at that time was clear.
However, these things can grow back but certainly not at the rate you indicated so its important to have annual follow up MRI's with fat suppression.
Good Luck
Laz