Dedifferentiated parosteal osteosarcoma.

Parosteal osteosarcoma is an uncommon malignant tumor of bone, and in a review of Mayo Clinic records we identified eleven cases of so-called dedifferentiated parosteal osteosarcoma. Ten of the eleven patients had had a long history of treatment for multiple recurrences of the tumor as a low-grade parosteal osteosarcoma and then for a definite recurrence as a high-grade undifferentiated osteosarcoma. The prognosis in this group of patients was similar to that in patients with conventional osteosarcoma.