Transformation of Cemento-Ossifying Fibroma into Osteosarcoma after Radiotherapy and Nine Endoscopic Surgeries

Cemento-ossifying fibromas (COFs) are benign fibro-osseous tumors usually found in the mandible and maxilla that can show malignancy in rare situations. COFs usually grow slowly and asymptomatically until they produce local bulges, pain, headaches, and visual impairment, and have a Ki-67 index within 3%, even in aggressive and recurrent cases. Surgical resection can usually provide a satisfactory prognosis. However, no cases of COF transforming into osteosarcoma have been reported. We present a case of COF involving a 58-year-old woman whose initial symptom was hearing loss in the right ear accompanied by tinnitus. Enhanced magnetic resonance imaging revealed a 3.1 cm x 3.2 cm mass centered on the right pterygoid process of the sphenoid bone. Postoperative pathology revealed a COF with a high Ki-67 index (8%–10%). Over the next two years, the tumor relapsed repeatedly. The patient underwent nine endoscopic surgeries and radiotherapy. The sixth postoperative biopsy showed that the COF had transformed into an osteosarcoma. The patient subsequently experienced respiratory and cardiac arrests and was receiving treatment in the intensive care unit with a poor prognosis at the time of writing this manuscript. Thus, a high Ki-67 index may be a risk factor for malignant transformation for COFs.