Ewing sarcoma

small, round-cell sarcoma of mesenchyme of medullary bone

age 5 - 14 yrs
most lethal of all 1' bone tumors
any bone
- < 20 y/o: long bones
- > 20 y/o: flat bones (where there's still red marrow)
purely lytic (62%), purely sclerotic (15%)
periosteal reaction (onion-skin or perpendicular)
similar lesion at different age
- < 5 y/o -- neuroblastoma
- > 30 y/o -- mets, reticulum cell sarcoma