Ewing sarcoma, primary site

**Definition:** A small round cell tumour that lacks morphologic, immunohistochemical, and electron microscopic evidence of neuroectodermal differentiation. It represents one of the two ends of the spectrum called Ewing's sarcoma/peripheral neuroectodermal tumour. It affects mostly males under age 20, and it can occur in soft tissue or bone. Pain and the presence of a mass are the most common clinical symptoms.