Primary intracerebellar osteosarcoma arising within an epidermoid cyst

○ The authors report a case of a primary extraskeletal osteosarcoma arising within an epidermoid cyst in the parenchyma of the cerebellum in a 64-year-old woman. On initial presentation, the tumor involved the midline cerebellum without attachment to the surrounding dura mater or calvarium. Complete medical and radiologic evaluation failed to reveal a primary skeletal or other extraskeletal osteosarcoma. To our knowledge, this is the first reported case of a primary extraskeletal osteosarcoma within the cerebellum. Osteosarcoma as a primary brain tumor is exceedingly rare, and only three cases (all occurring within the cerebral hemispheres) have been reported previously. The histogenesis of primary sarcomas of the brain is not evident. The associated finding of an epidermoid cyst suggests the tumor originated from a teratoma.