Ans. a (Ewing's sarcoma). (Ref. Robbin, Pathology, 6th ed., 1244)EWING'S SARCOMA# Ewing's sarcoma constitutes -10-15% of all bone sarcomas# Is common in adolescence (peak incidence in the second decade of life).# It typically involves the diaphyseal region of long bones, Also has an affinity for flat bones.# X-ray: characteristic 6ionion peel" periosteal reaction with a soft tissue mass (better seen by MRI).# The soft mass is composed of sheets of monotonous, small, round, blue cells and can be confused with lymphoma, embryonal rhabdomyosarcoma, and small-cell carcinoma.# It is composed of sheets of uniform small, round cells that are larger than lymphocytes, rich in glycogen with Homer- Wright rosettes (tumor cells with PAS positive Glycogen-filled cytoplasm characteristic).# The presence of p30/32, 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 Ewing's sarcoma (and other members of PNETs).# In approximately 85% of patients t (11;22) (q24;ql2) translocation is seen.# It is Undifferentiated--high grade tumor (Ewing's is always classed G4).# It is very aggressive, and it is therefore considered a systemic disease.# Common sites of metastases are lung, bones, and bone marrow.# Ewing's sarcoma can be curable, even in the presence of metastatic disease, especially in children <11 years.# Systemic chemotherapy is the mainstay of therapy, often being used before surgery.# Local treatment for this tumor includes surgical resection, usually with limb salvage or radiation therapy.# More PNETs are known to arise in soft tissues; which include -- Peripheral neuroepithelioma,- Askin's tumor (chest wall), and- Esthesioneuroblastoma.Other small round cell tumors includeQ TumorClassic cytogenetic abnormality1.Wilm's tumor11p13deletion/mutation2.Lymphoblastic lymphoma (ALL)t (9;12)3.Rhabdomyosarcomat (2;13), llpdeletion4.Neuroblastomalpdeletion,N-myc amplification,DNA hyperploidy5.Medulloblastoma17q6.Retinoblastoma13q14deletion/mutation7.Ependymoblastoma 8.Burkit's lymphomat (11;22)9.Small cell carcinoma Note: Osteosarcoma is not a round cell tumor.