Secondarily, infection associated with dental implant may become a possible cause of ORN. In radiotherapy including the oral cavity, gingivitis is frequently observed adjacent to fixed metal dental restorations because they cause significant dose enhancement around them [12]. It is easy to speculate that the same occurs around dental implant prostheses (peri-implant mucositis). The presence of mucositis (gingivitis) causes poor oral hygiene and leads to a vicious spiral of poor oral hygiene and increased bacterial infection. Radiation therapy may also interfere with normal wound healing mechanisms. Changes in vasculature, effects on fibroblasts, and varying levels of regulatory growth factors result in the potential for altered wound healing. Radiation also induces alterations of the immune response (immunosuppression). Ionizing radiation directly affects the immune system. These conditions reduce the peri-implant tissue resistance to oral bacteria, thus increasing the risk of peri-implantitis. Slama et al. [9] reported the existence of peri-implantitis prior to the development of ORN. In our case, the peri-implantitis progressed to ORN. It is well known that the defense function of peri-implant tissue is weaker than that surrounding natural teeth. It is likely that the peri-implant mucositis caused by radiotherapy can easily progress to infected peri-implantitis and subsequently more severe infection of the jaw bone.