Parosteal osteosarcoma: a case report.

Parosteal osteosarcoma is a low-grade malignant tumor arising from the periosteum. It involves mainly adults of third to sixth decades and the favorite site is the posterior aspect of distal femoral metaphysis. Microscopically, it is composed of woven to lamellar bone trabeculae within spindle cell stroma. We present a case of parosteal osteosarcoma that has very well-formed lamellar bone trabeculae and fat marrow under microscopic examination. Without the help of radiologic study, it is impossible to make the diagnosis of parosteal osteosarcoma.