A PET scan cannot see tiny cancer cells floating around, only after they settle in and start growing into a tumor. The rads and chemo are necessary to catch those free cells (if there are any) and kill them before they move on anywhere else. Did your dad have a PET scan as part of his diagnosis to see if there was anything of concern elsewhere? Likely it will be recommended in the future for a time. I know you're worried about how he will fare with rads and chemo, none of it is easy or pain free, but it is necessary to be sure they got it all. I just had a small lesion on my tongue with it close to a nerve but not in it, the doctor did not check my lymph nodes. He took my case to the tumor board and the consensus was to do radiation but no chemo. I had a clear PET scan before surgery and will get another next month (fingers crossed). Two doctors have told me I should have no more problems with this but my ENT I think is cautious. There really is no positive way for them to know for sure that there are no cancer cells left after surgery. My opinion about oral cancer after research is that it is tricky and can be aggressive, if the doctors think your dad needs the rads and chemo, do it. But insist that they help with the side effects, that is what the nurses are for, if you need to talk to them every day about something, do so. If you look at many of the bios of people on this site, there are recurrences, so it's best to treat aggressively. Hope you and your dad can hang in there!