The patient was a 79-year-old woman. In August 201X–15, she noted a mass in the floor of the mouth in the left mandibular molar region, and consulted the dental department of a local hospital. The resected mass was diagnosed as neurofibroma. Thereafter, in June 200X–6, she consulted our hospital primarily due to a mass and pain in the same site. In December of the same year, she underwent resection of the floor of the mouth, and the histopathological diagnosis of the resected specimen was primary adenoid cystic carcinoma of the sublingual gland. From January 200X–5, she received postoperative radiotherapy (total dose: 60Gy) and was followed-up. After 5 years, in April 201X, she noted swelling and pain of the floor of the mouth in the left mandibular region. Panoramic radiography showed a fracture line in the left mandible, and radiation-induced osteomyelitis of the mandible was suspected. Although she was treated with antimicrobial medication, around July of the same year, she was admitted to our department due to aggravation of the swelling of the left floor of the mouth and submental region. The mass was diagnosed histopathologically as soft tissue sarcoma, which was suspected to be myogenic, and was considered as radiation-induced sarcoma.
