The problem with a PET scan is that it can give you easily wrong positives especially for oral scans. The more metabolically active cells are the more they grab glucose which what you see. Inflammation and scar tissue (healing) will also show up.
The other thing about the PET is the detection limit. You need a bunch of active cells to show up, if there are just a few you will not see them regardless how active they are.

Having said this the the scan shows areas of concerns to be watched and or biopsied which will tell you what is going on.

Hope all goes well.

M