Most radiation-induced osteosarcomas of the skull are reported to arise in the 
facial bone or paranasal sinus after radiotherapy for retinoblastoma and/or 
pituitary adenoma. Here we report two cases of radiation-induced osteosarcoma in 
the paranasal sinus after treatment for frontal glioma. Case 1 was a 56-year-old 
woman who underwent surgical resection of a left frontal tumor in October 1990. 
The histological diagnosis was a low-grade glioma, and radiotherapy of 54 Gy was 
administered. Sixteen years later, in September 2006, the patient noted an 
enlarging subcutaneous mass in the right frontal region. CT showed an osteolytic 
mass in the right frontal sinus. An open biopsy established the 
histopathological diagnosis of osteosarcoma, and the patient subsequently died 
of rapid tumor regrowth. Case 2 was a 58-year-old man who underwent partial 
removal of a bifrontal tumor in May 1996. The histological diagnosis was 
anaplastic oligoastrocytoma, and radiotherapy of 56 Gy was administered. Twelve 
years later, in March 2008, the patient was readmitted to our hospital for 
reasons of marked deterioration in general physical condition. Tumor recurrence 
was suspected in the left frontal lobe, and CT demonstrated an osteolytic mass 
in the left frontal and ethmoid sinus. A secondary operation was performed, and 
the pathological specimens were diagnosed as osteosarcoma. Radiotherapy was 
readministered, but the subject died of rapid tumor regrowth. From these 
clinicopathological findings, both cases were diagnosed as radiation-induced 
osteosarcoma. Radiation-induced osteosarcomas appeared 16 and 12 years after 
radiotherapy in cases 1 and 2, respectively. As the prognosis of 
radiation-induced osteosarcoma is poorer than that of primary osteo-sarcoma, 
careful attention is required for consideration of the long-term survival of 
patients with glioma.