Joan,
According to my radiation oncologist at Mass General, a ct scan can pick up anything bigger than 1 mm. in size. I don't know what the comparable size for manual palpitation by a doctor but I would be surprised if it is that small. PET scans can find activity at the cellular level but the problem remains the number of false positives and negatives.
The recent edition of CURE magazine had an article on a newer technology which combines pet/ct scans and fuses the images of both technologies. You can read the article online at
http://www.curetoday.com/currentissue/departments/housecall/index.html.
I don't know if you are being treated at a comprehensive cancer center, but most of the people on this board would recommend making sure that you are. Second, I would suggest getting a second opinion from another institution that has a team-based approach to treating oral cancer (the team should include at least an ENT surgeon, an oncologist and radiation oncologist). You should ask about the scheduling of your follow ups and need for scans. I see my ENT surgeon every six weeks and this will continue at least until the end of the first year post treatment. I see the radiation oncologist every 3 months and have a ct scan each time. Others on the board might want to provide information about their follow up schedules so that you will have more data with which to compare.
Best wishes and good luck fighting this nasty disease. - Sheldon