Just as a side note: radiation doses are lifetime accumulative, so everything that you get; treatments, all your follow up scans, dental xrays, etc. all add up. Assuming that you get follow up scans the rest of your life after treatment it all adds up. At some point in time radiation begins to cause its own malignancies. For instance by year ten of survival, approximately 3% of all patients with the maximum treatment levels of radiation develop osteosarcomas of the mandible. This study and statistic was done before the mass implementation of IMRT, so hopefully these numbers will go down. Many dentists now have digital xray machines that expose you to 1/10th the amount of radiation of normal dental xrays. If your dentist doesn't have one of these machines, another dentist in town does have it. As to lifetime doses and more, I am going to leave those answers to Gary who has forgotten more about radiation than I know.