Twindy ~
The frequency of MRI scans following tumor irradiation is up to your doctor. Generally - and in my experience (I underwent FSR on a debulked AN) - MRI scans are ordered at six month intervals for the first two years or so. Again, this is not a rule but simply what many doctors do for AN radiation patient follow-up. However, some doctors follow a different MRI schedule.
My doctor ordered an MRI at six months post surgery/radiation. It showed some swelling so he ordered another, three months later, just as a precaution. The swelling had subsided by then but I underwent another MRI at my one-year FSR 'anniversary'. My doctor ordered another MRI at my two-year surgery/radiation anniversary. That one indicated tumor necrosis and slight shrinkage. I had not experienced AN symptoms in the two years following my surgery & radiation so my doctor felt confident that I could go five years before my next MRI, assuming no symptoms arose. They haven't. I'll probably have an MRI scan in 2013.
Post-radiation swelling is typical at six months or so. I experienced it but no steroids were prescribed and it went away on it's own. No guarantee that post-radiation swelling or the lack of it will affect hearing retention but that is a question your doctor can better explain.
Jim