Low-grade central osteosarcoma of the skull

A case of low-grade central osteosarcoma of the skull in a 36-year-old male is described. A 10 cm mass of the left temporo-occipital bone, which was incidentally found 6 years previously on the occasion of a head trauma, was osteolytic and osteoplastic, affecting soft tissues and the subdural space. The patient was treated with intralesional curettage. Histologically, the tumor was characterized by spindle cell proliferation with woven and lamellar bone formation, resembling fibrous dysplasia, parosteal osteosarcoma, low-grade fibrosarcoma, or desmoplastic fibroma. The spindle cells were fairly uniform with mild atypia and low-mitotic activity (3 per 50 high power fields), and cellularity varied from low to high. Clinicians and pathologists should be aware of this type of low-grade osteosarcoma of the skull and distinguish it from its mimics, including fibrous dysplasia, parosteal osteosarcoma, and low-grade fibrosarcoma. Careful microscopic correlation with radiographic findings is the clue to the correct diagnosis.