Mostly depends upon the doctors. I had Petscans at 3 months and 6 months out (my RO & MO) and a MRI within a month (ENT).
In my case both the Petscans were worthless, falsely indicating cancer on the right side while the MRI was clear. But that is common for Base of Tongue cancer patients. Subsequent Petscans completely missed the tumor recurrence on the left side, but an MRI caught it. I wish my doctors had been as knowledgeable as David's appear to have been about BOT Petscan false positives and never ordered any on me.
The above is supposed to be reassuring to you in case the doctors do not order a Petscan or if the results appear bad.
I admit to a big bias in favor of MRIs which have been extremely accurate for me. Do a search on Petscan in the box on the upper right to catch up on prior posts on this if you want a quick overview of OCF experiences.
Good luck
Charm