Diagnostic Radiology/Musculoskeletal Imaging/Tumors Basic/Ewing sarcoma

Ewing's sarcoma is a malignant tumor that belongs to a broader family of tumors that includes PNET (primitive neuroectodermal tumors). In simple terms, these tumors are all pathologically characterized by the so-called "small round blue cells." Ewing's sarcoma almost always occurs in bone, although other members of the PNET family can occur anywhere. Ewing's is pathologically differentiated from other members of the family on the basis of immunohistochemical staining.

Ewing's sarcoma was described by James Ewing in 1920. Similar tumors were reported earlier than this, but it was the work of Ewing which established that the disease was separate from lymphoma or neuroblastoma.

Clinical Findings
Usually presents in childhood/early adulthood with a peak between 10 and 20 years of age, although it can occur in younger children and older adults. Can occur anywhere, but most commonly in the pelvis and proximal long tubular bones. The metaphysis and diaphysis of the femur are the most common sites, followed by the tibia and the humerus. Thirty percent are overtly metastatic at presentation.

The most common clinical findings are pain and swelling.

Imaging Findings
 Plain Radiographs:  The most common osseous presentation is a permeative lytic lesion with periosteal reaction. The classic description of lamellated or "onion skin" type periosteal reaction is often associated with this lesion.



AP and lateral views of proximal tibia in child with Ewing's sarcoma. Note the permeative lesion with adjacent lamellated periosteal reaction.

 CT:  19 year old male with Ewing's sarcoma of proximal femur, with pathologic fracture. On CT, note the large soft tissue mass anterior to the femoral neck. Evaluation of the extraosseous structures is an advantage of CT over plain films.



 MR:  21 year old male with pain and swelling. Typical MRI findings here (in order T1, T2, and FSE T2 images) show low signal intensity in T1 images and high signal intensity in T2.





 Radionucleotide/PET:  Same patient as above, bone scintigraphy shows the typical increased uptake of radionuclide in the femur at the site of the lesion (there are rare reports of cold" lesions).



Imaging Work-up
Plain films add valuable information in the initial evaluation or screening. The wide zone of transition (e.g. permeative) is the most useful plain film characteristic in differention of benign versus aggressive or malignant lytic lesions. Zone of transition is not useful in MRI.

MRI should be routinely used in the work-up of malignant tumors. MRI will show the full bony and soft tissue extent and relate the tumor to other nearby anatomic structures (e.g. vessels). Gadolinium contrast is not necessary as it does not give additional information over noncontrast studies.

CT can also be used to define the extraosseous extent of the tumor, especially in the skull, spine, ribs and pelvis. Both CT and MRI can be used to follow response to radiation and/or chemotherapy.

Bone scintigraphy can also be used to follow tumor response to therapy.

Differential Diagnosis
Other entities that may have a similar radiologic presentation include osteomyelitis, osteosarcoma (especially telangiectatic osteosarcoma) and eosinophilic granuloma. Soft tissue neoplasms such as malignant fibrous histiocytoma that erode into adjacent bone may also have a similar appearance.

Treatment
Because almost all patients with apparently localized disease at diagnosis have occult metastatic disease, multidrug chemotherapy as well as local disease control with surgery and/or radiation is indicated in the treatment of all patients (2). Treatment often consists of adjuvant chemotherapy generally followed by wide or radical excision, and may also include radiation. Complete excision at the time of biopsy may be performed if malignancy is confirmed at that time.

Prognosis
Staging generally separates localized versus metastatic disease. Most commonly, metastasis occur in the chest, bone and/or bone marrow. Less common sites include CNS and lymph nodes.

Survival for localized disease is 65-70% when treated with chemotherapy. Survival for metastatic disease is 25-30% (3).