Marg,
Yes, depth is considered, but anything can happen when you are talking this type of cancer. Notice the depth of my lesion was nothing--1 mm, supposedly anyway. I've read that if it is a sore they should measure what the mucosa would be if it were covering it, so that would make it thicker perhaps. Mine was fairly flat. After I had a metastatic lymph node, that changed everything. My case was unusual, nevertheless one cannot completely disregard caution. As far as I know there is no accepted minimum depth. The size of your lesion also figures into risk. If there are no apparent involved lymph nodes from whatever screening tools are being used (CT & PET most usual), then most will watch it, but not all doctors as sometimes these tests are wrong.

Hope you are having medical doctors (ENT) looking at it, pathology report made (how aggressive the lesion is matters also), and in general getting the best care you can, for oral tongue cancer can be tricky.

Best,
Anne