a malignant neoplasm which occurs usually before the age of 20 years, about twice as frequently in males, and in about 75% of patients involves bones of the extremities, including the shoulder girdle, with a predilection for the metaphysis; histologically, there are conspicuous foci of necrosis in association with irregular masses of small, regular, rounded, or ovoid cells (2-3 times the diameter of erythrocytes), with very scanty cytoplasm. SYN: endothelial myeloma, Ewing sarcoma.