<prompt> A 12 year old girl complains of pain persisting in his leg for several weeks with a low grade fever. A radiograph reveals a mass in the diaphyseal region of the left femur with overlying cortical erosion and soft tissue extension. A biopsy of the lesion shows numerous small round cells, rich in PAS positive diastase sensitive granules. The most likely histological diagnosis is :Osteogenic sarcoma.Osteoblastoma.Ewings sarcoma.Chondroblastoma  <response>: Ans. is c i.e. Ewings Sarcoma Small round cells rich in PAS positive diastase sensitive granules confirms the diagnosis of Ewings sarcoma.The histopathological features of Ewings sarcoma are:Small round cells having scant cytoplasmThese cells are rich in glycogen this glycogen filled cytoplasm is detected by staining with periodic acid - shiff PAS veThese cells are diastase sensitive. Diastase is an enzyme which splits glycogen.Homer wright rosettes the tumor cells are arranged in circles around a central fibrillary spaceRemember: The other round cell tumours of boneOsteosarcomaReticulum cell sarcomaMetastatic neuroblastomaAlso RememberGross pathological features of Ewings sarcoma:The tumour arises in the medullary cavity from the endothelial cells.New bone formation may extend along the shaft and sometimes it appears as fusiform layers of bone around the lesion, onion peel effectThe tumour erodes the cortex early often extending into the soft tissues with radiating streaks of ossification and reactive periosteal bone at the proximal and distal margins, these features the sunray appearance and codmans triangle are usually associated with osteosarcoma but they are just as common in Ewings sarcomaClinical features of Ewings sarcomaIt occurs most commonly between the age of 10-20 years especially in the tibia, fibula or clavicle.The patient presents with pain and swelling often arising in the diaphysis Sometimes fever is also associated with it confusing it with osteomyelitis 