<prompt> An 8-year old boy presents with swelling in mid-thigh region. On examination, femoral thickening was found with Codmans triangle. On aspiration, greyish white liquid was aspirated that showed atypical round cells with MIC-2 positivity. The most likely diagnosis isOsteosarcomaEwings sarcomaTubercular osteomyelitisPyogenic osteomyelitis  <response>: Ans. b. Ewings sarcoma Ewings 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 Ewings sarcoma is onion peel appearance but sometimes it can present as Codmans triangle. Sunray appearance and Codman triangles are usually associated with osteosarcoma, but they are just as common in Ewings sarcoma. For the diagnosis of osteosarcoma, histopathology must show the presence of immature new bone cells, i.e. osteoid. Classical radiological features Found in Codmans triangle, Sun ray appearance OsteosarcomaQ Onion peel appearance Ewings 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 