We present a rare case of radiation-induced osteosarcoma following Gamma Knife® 
surgery (GKS) for a vestibular schwannoma (VS). A 49-year-old female with 
sporadic VS underwent GKS. Serial follow-up imaging showed that the tumor size 
decreased. Six years after GKS, magnetic resonance imaging demonstrated regrowth 
of the tumor. The tumor was removed via the retrosigmoid approach. 
Interestingly, the final pathology report confirmed osteosarcoma arising in 
schwannoma with direct transition (osteosarcoma component: 90 %, schwannoma 
component: 10 %). The osteosarcoma was considered to be a radiation-induced 
malignancy. The possibility of this rare complication should be explained to the 
patient before GKS, and the patient should be screened periodically after GKS.