Hi Eric,
There is a posting in the news section of the OCF site about various imaging techniques and their relative advantages and disadvantages.
See
http://www.oralcancerfoundation.org/news/story.asp?newsId=1072
for details.

It seems that every ENT and radiation oncologist have their own strategy for follow up. I'm about 18 months past the end of my radiation treatment and my ENT orders a CT scan every 6 months at this stage. The first year I had a CT scan every 3 months. As the article I referred to notes, it depends what the docs are looking for - nodal spread, recurrence at the original tumor site or distant mets. Each imaging technology has advantages and disadvantages.

I would suggest getting advice from a comprehensive cancer center as to what the best follow up regime should be. It must vary from patient to patient and doctor to doctor.
Best wishes, Sheldon