A child 10 yrs of age presents with a mass on his left thigh. The mass seems to be arising from diaphysis of femur, and involving the soft tissue of thigh. The child is having fever also. Give your most probable diagnosis:

Ewing sarcoma usually involves the diaphysis of long bones. Pain at the site of the primary tumor is the most common presenting sign, with or without swelling and erythema. No specific laboratory findings are characteristic of Ewing sarcoma, but an elevated LDH may be present and is of prognostic significance. Associated symptoms include fevers and weight loss. Radiographic findings: Lytic tumor with permeative margins and extension into the surrounding soft tissue. Codman triangle may be present. Classic radiographic lesion of Ewing sarcoma is the "onion skin" periosteal reaction. Ref: Graham D.K., Quinones R.R., Keating A.K., Maloney K., Foreman N.K., Giller R.H., Greffe B.S. (2012). Chapter 31. Neoplastic Disease. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.