Ewing's sarcoma.

Ewing's sarcoma is a highly malignant tumor of uncertain origin. A strong relationship is suggested between Ewing's sarcoma and tumors of neural origin. The radiologic manifestation of Ewing's sarcoma are protean and lesions may be lytic, mixed lytic-sclerotic, or rarely, predominantly sclerotic. The lower extremity long bones are predominantly affected and most lesions are diaphyseal or metadiaphyseal. CT and particularly MR imaging are invaluable in further delineating the extent of disease not readily manifested on plain radiographs. Gallium scintigraphy and gadolinium-enhanced MR images are best for following the response to therapy. Ewing's sarcoma must be distinguished from other round cell tumors including lymphoma, neuroblastoma, and primitive neuroectodermal tumor of bone as well as from osteosarcoma.