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.