Boxerma, that's exactly where Chester's cancer was - his teeth between the incisors had disappeared or were very, very tiny.
We had no idea it was a problem, he was ten years old and we thought it was normal 'wear and tear' on a dog's teeth, altho we were never told that there was a connection. Good luck - it's not the prettiest site - we used to have unfeeling kids say to us 'Oo what's wrong with his mouth' when we took him for walks, and it was almost as if he knew what they were saying.