Ans. b. Ewing's sarcoma Ewing's sarcoma: A type of primitive neuroectodermal tumour (PNET) On histopathological examination shows small round cells, which show positive immunohistochemical staining or CD-99 and MIC-2 gene Although the classical periosteal reaction seen in Ewing's sarcoma is onion peel appearance but sometimes it can present as Codman's triangle. Sunray appearance and Codman triangles are usually associated with osteosarcoma, but they are just as common in Ewing's sarcoma. For the diagnosis of osteosarcoma, histopathology must show the presence of immature new bone cells, i.e. osteoid. Classical radiological features Found in Codman's triangle, Sun ray appearance OsteosarcomaQ Onion peel appearance Ewing's SarcomaQ Soap bubble appearance OsteoclastomaQ `0. ring sign EnchondromaQ Wormian Bones Osteogenesis imperfectaQ Honey comb appearance AdmantinomaQ Driven snow appearance Pindborg tumorQ Speckled/ Mottled/ Patchy calcification ChondrosarcomaQ