A. Enhanced T1-weighted magnetic resonance imaging. Enhanced mass occupies sphenoid sinus entirely. The tumor has invaded the bilateral cavernous sinuses surrounding internal carotid arteries, but pituitary gland and stalk seem to be totally intact (sagittal view). B. Histology of the tumor. Hematoxylin and eosin staining shows high cellularity in the tumor tissue (a), and osseous components mixed in with the tumor (b) suggesting osteosarcoma. Molecular immunology Borstel-1 (MIB-1) index was high, 15% (c). C. Postoperative magnetic resonance imaging. Magnetic resonance imaging showed a reduction of the tumor volume in the sphenoid and ethmoid sinuses, but residual tumor definitely exists in the sinuses with strong enhancement.