Do a search of the main OCF site for the word osteoradionecrosis (ORN). The bones of the mouth (the mandible and the maxilla) do not become brittle, but they do lose much of their micro vascularization from the radiation treatments. This means that they heal poorly. Extractions and other extreme procedures after these treatments can cause a chronic non healing wounds that lead to far worse things. So before beginning treatments the oral environment must be in the best possible shape. Teeth that are compromised by periodontal disease or are not restorable have to be extracted prior to treatments.