Daniel,
Brian and others know a lot more than I about this subject but I can shed a little bit of light for you. CT scans seem to be the standard follow up for oral cancer. PET scans reflect glucose or sugar buildups in various parts of the body which may show a tumor's presence because they consume inordinate amounts of sucrose.
PET scans are expensive, and therefore not preferred by insurance companies and , to my understanding, can often result in false positives that might cause you anxiety or to undergo invasive procedures to make sure that the tumors are not present.
I know that there are some archived threads here on the subject because I once asked the same question myself.

Danny G.