While I don't know all the details, it is best if teeth which are periodontally or otherwise compromised be extracted prior to radiation. Removing these after treatment could cause serious implications that may even lead to loss of the mandible itself if it got out of hand. Given these dire consequences, doctors err on the side of caution. My teeth were all healthy when I went into treatment, and I had no extractions. However I came from a strong dental background, and had above average dental hygiene and teeth in great condition. Given that this patient has only five teeth left in his lower arch (out of 16) I wonder if those other teeth were lost due to periodontal disease. If they were, it would not bode well for any dentist evaluating the patient to assume that his ability to maintain perfect hygiene and stay on top of compromised teeth, when they will be further complication factors produced by the lack of saliva in the future. Blood supply isn't going to be the issue. It will be health of the teeth in either arch today, and the patients ability, and/or desire to work extra hard to maintain his oral health after treatments, and what teeth are going to fall into the field of radiation. For five remaining teeth I think the better long term solution would be a full denture which surely would allow him to function better when this was all over.