None of the scans are 100 percent accurate, and none can prove cancer 100 percent. Only a direct biopsy can, followed by a FNAB, which is pretty close. If a lesion was small, visible then maybe a punch, brush or scalpel biopsy in the office can be performed, but it can effect future scans if any are redone, and if it is cancerous, although maybe not likely as the dr said, they don't want to start poking all around in your mouth looking for something that can't b seen, and possibility of spreading cancer too, and the things you mentioned are considered. There are many reasons for abnormal scans, not always cancerous, but they may want to do a direct biopsy in the hospital, if anything suspicious is seen, while doing a scope of the mouth, and throat under general anesthesia. As far as a recurrence goes, majority of recurrences occur the first two years, and you are over four years out. Try not to worry, which is easier said than done. I know how it is since I had 8 PET/CT scans, 3 CT, 2 MRI, and 5 FNAB in three years, and waiting is the worst. Try to keep busy! Good luck.