Ewing's sarcoma is a malignant neoplasm that is probably of neuroectodermal origin. Among primary malignant neoplasms of bone, it follows plasmocytoma, osteosarcoma, and chondrosarcoma in frequency.
There is a male predominance, with a male-to-female ratio of 2:l. Its peak incidence is in the second decade, with 90% of cases occurring between 5 and 25 years of age.
Ewing's sarcoma may involve any bone but has a predilection for the long bones and pelvis. In the long bones, it is localized to the diaphysis or metaphysis, and until the growth plate is open, it does not extend to the epiphysis.
MRI facilitates staging of Ewing's sarcoma and is better than CT for demonstrating bone marrow and soft tissue involvement. MRI signal intensity characteristics of Ewing's sarcoma are not specific and are similar to those of other malignant neoplasms. The tumor's signal intensity is lower than or equal to that of muscle on TI-weighted images and is higher on T2-weighted images.
MRI and CT are helpful in evaluating response to treatment. If the tumor is sensitive to treatment, it decreases in size, the periosteal reaction matures, and the bone becomes sclerotic. High signal intensity on T2-weighted images after treatment does not always indicate a poor response, since it may represent necrosis, reactive granulation tissue, or hemorrhage. Enhancement after Gd-DTPA does not aid in distinguishing reactive changes from residual tumor, but the lack of enhancement indicates tumor necrosis.