<prompt> Onion peel appearance is seen inEwings sarcomaOsteosarcomaChondrosarcomaAneurysmal bone cyst  <response>: Ans. a Ewings sarcoma Ref. RRM Radiology, 6th ed., p.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 