<prompt> Which of the following bone tumors occurs in young age group, arises in medullary cavity, and characteristically shows sheets of small uniform sized round cells and Homer Wright rossets?OsteosarcomaEwings tumorChondrosarcomaOsteoid osteoma  <response>: Ans. b Ewings tumor Ref. Robbins pathology, 7thpg. 1301; 558.The four main types of rosettes in pathology :1. Homer Wright rosette: typically seen in neuroblastomas, medulloblastomas, and primitive neuroectodermal tumors PNETs. It consists of a halo of tumor cells surrounding a central region containing neuropil.2. Flexner-Wintersteiner Rosette: characteristic of retinoblastomas. It consists of tumor cells surrounding a central lumen that contains cytoplasmic extensions from the tumor cells.3. True Ependymal Rosette: consists of tumor cells surrounding an empty lumen. Are characteristic of ependymoma, but not seen in all cases.4. Perivascular Pseudorosette: consists of tumor cells collected around a blood vessel. Its called a pseudorosette because the central structure isnt part of the tumor. These rosettes are common in ependymomas, but you also see them in medulloblastoma, PNET, central neurocytomas, and glioblastomas.EWINGS SARCOMA Incidence: Constitutes 10 to 15 of all bone sarcomas. Age: Peak incidence in the second decade of life. Origin: Arises in the medullary cavity of the bones. Location:involves the diaphyseal region of long bones especially femur flat bones especially pelvis. X-ray: permeative destructive lytic lesion with onion peel periosteal reaction with soft tissue mass. Histopathology:- It is composed of sheets of monotonous, small, round, blue cells and can be confused with lymphoma, embryonal rhabdomyosarcoma, and small-cell carcinoma.- The cells are rich in glycogen PAS positive cells.6- The presence of Homer-Wright rosettes is indicative of neural differentiation.- Most PNETs arise in soft tissues; they include: Peripheral neuroepithelioma, Askins tumor chest wall, and Esthesioneuroblastoma.- The presence of p3032, the product of the mic-2 gene which maps to the pseudoautosomal region of the X and Y chromosomes is a cell-surface marker for Ewings sarcoma PNETs.- The classic cytogenetic abnormality associated with this disease and other PNETs is t11;22.- CD99 . It is very aggressive tumor and considered a systemic disease and mimics acute osteomyelitis. Common sites of metastases are lung, bones bone to bone metastases, and bone marrow. Rx: Systemic chemotherapy is the mainstay of therapy, often being used before surgery. Local treatment for the primary tumor includes surgical resection, usually with limb salvage or radiation. Ewings is a curable tumor, even in the presence of obvious metastasis, especially in children 11 years old.Also Know:TranslocationTtimor 22q11Chronic myelogenous leukemia 12;22q 13 ;q 12Malignant melanoma of soft parts MMSP 11;14q13;q32Mantle cell lymphoma 14; 18q32;q21Follicular lymphoma 1;22q24;q12Ewings sarcoma 1;7p34;q35T cell acute lymphocytic leukemia ALL 8;14q24;q32Burkitts lymphoma, B cell ALL 11;22pl3;q12Desmoplastic small round cell tumor DSRCT 2;13q35;ql4Alveolar rhabdomyosarcoma 1; 13p36;q 14Alveolar rhabdomyosarcoma 10; 17q 11.2;q23Papillary thyroid carcinomas x, 18Synovial cell sarcoma 