The imaging findings of extraosseous Ewing sarcomas are variable in appearance and location,
with a common theme.
The imaging appearance is typically aggressive and complex.
The lesions are large,
solid,
and heterogeneous,
with some having cystic changes.
Familiarity with the imaging appearance on various modalities is critical.
Generally,
these tumors are commonly seen in the deep soft tissues of the extremities,
with lower extremities having a higher propensity than upper extremities,
though they can be found anywhere.
Extraosseous Ewing sarcomas are often close to,
though not originating from,
bone.
Often the masses are well circumscribed though can be infiltrative,
and multilobulated.
Radiographs may demonstrate a soft tissue mass [1-7].
Computed tomography (CT) findings usually show a soft tissue mass with similar attenuation compared to skeletal muscle (Figure 1).
Adjacent bone involvement is uncommon,
as is bony metastatic disease in early stages (Figure 2).
Calcification is exceedingly rare (Figure 1).
Sonographic findings are variable and nonspecific,
and often demonstrate a hypoechoic,
heterogenous mass with internal vascularity (Figure 3).
Cystic change and necrosis may be seen (Figure 4).
Findings on magnetic resonance imaging (MRI) include a mass with signal similar to skeletal muscle on T1 weighted imaging,
which may be mildly increased (Figure 5) or decreased (Figure 7).
Areas of hemorrhage will demonstrate T1 hyperintensity.
T2 weighted images often demonstrate heterogeneous intermediate to hyperintense signal.
High signal areas representing foci of fluid or cysts are common (Figure 7).
On contrast enhanced images,
there is often heterogeneous enhancement (Figure 5 and 7).
High flow vascular channels or flow voids may also be seen (Figure 5).
Some of these masses may be quite complex and have undergone cystic degeneration/necrosis with internal septations (Figure 9).
CT evaluation of these complex masses demonstrate a heterogenous mass with mixed soft tissue and fluid density secondary to the cystic changes,
with calcification almost never seen (Figure 9).
MRI of these lesions will demonstrate multiple cysts,
hyperintense on T2 weighted images,
with enhancement of the soft tissue component,
cysts,
and septations (Figure 10).
Fluid-fluid levels may also be observed [8,9].
On gross pathological specimen extraosseous Ewing sarcoma often appears gray-yellow or gray-tan with lobulations and a soft texture.
Histopathology confirms extraosseous Ewing sarcoma via monotonous proliferation of small blue round cells solidly packed with intracellular glycogen which may indent nuclei (Figure 6,
8).
Cystic/necrotic regions demonstrate rich vascularity,
and areas of hemorrhage are often present.
Membrane staining is almost always positive for CD99 (Figure 6,
8).
Histologic staining for FLI1,
demonstrating t(11;22),
will provide definitive diagnosis (Figure 8).
Differential considerations of extraosseous Ewing sarcoma mainly include rhabdomyosarcoma and synovial sarcoma.
Neuroblastoma and lymphoma are less common considerations.
Benign entities such as a venous malformation,
especially the microcystic type,
soft tissue abscesses,
and hematomas can also mimic a soft tissue tumor.
Rare tumors such as extraosseous mesenchymal chondrosarcomas may also be included in the differential for an extraosseous soft tissue sarcoma.
Although metastatic disease to the soft tissues may mimic an extraosseous Ewing sarcoma,
this is rarely seen in children.
Rhabdomyosarcomas are the most common soft tissue malignancy and may be painless though rapidly growing.
CT will demonstrate a soft tissue density with heterogeneous enhancement +/- adjacent bony destruction.
MRI demonstrates T1 isointensity to muscle +/- areas of hemorrhage,
and T2 hyperintensity to muscle with avid enhancement.
There may be prominent flow voids (in alveolar subtype).
Embryonal,
alveolar,
and pleomorphic histologic subtypes all show skeletal muscle differentiation,
which is key to histopathological diagnosis (Figure 11) [10,11].
Synovial sarcomas are one of the most common childhood malignancies in the lower extremities.
These slow growing masses have predilection for juxtaarticular regions.
CT examination will demonstrate a heterogeneous soft tissue mass +/- calcifications.
MRI demonstrates a mass generally T1 isointense to muscle,
with T2 hyperintense signal with bands of fibrosis and variable enhancement.
Although the poorly differentiated subtype is histologically similar to extraosseous Ewing sarcoma, monophasic subtype reveal uniform atypical spindle cells while biphasic subtypes have an epithelial element (Figure 12).
“Synovial” is a misnomer,
as these tumors are not derived from synovium.
Cytogenetic studies show translocation of (x;18) [12].
Neuroblastomas are often paravertebral in location and in a younger demographic than extraosseous Ewing sarcoma.
Laboratory assessment will commonly show elevated urinary catecholamine levels.
Bony metastases are common and may be the presenting finding.
Although the masses may appear heterogeneous from necrosis or hemorrhage,
calcifications are common.
MRI demonstrates high signal on T2 and low signal on T1 weighted imaging with heterogeneous signal related to calcification,
hemorrhage and necrosis.
Sonographic images will often show increased,
heterogeneous,
echogenicity with shadowing from calcifications.
Microscopic features include immature,
undifferentiated sympathetic cells [13].
Lymphomas are differentiated from extraosseous Ewing sarcoma via identification of lymph node involvement; extraosseous Ewing sarcoma rarely involves lymph nodes.
Extraosseous mesenchymal chondrosarcomas are painless,
slowly growing masses with chondroid matrix (Figure 13).
On CT,
a soft tissue mass with similar attenuation to muscle is demonstrated with either central or eccentric mineralization.
Necrosis may be present,
and enhancement is heterogeneous.
MRI findings include a soft tissue mass containing variable low signal mineralization with isointensity (to muscle) on T1 weighted imaging and hyperintensity on T2 weighted imaging with intense heterogeneous enhancement (Figure 13).
Histopathologic features include a bimorphic appearance with well-differentiated cartilage surrounded by sheets of closely packed undifferentiated cells.
Stains are positive for S100,
neuron-specific enolase,
and Leu-7,
and negative for actin,
epithelial membrane antigen,
and cytokeratin [14-16].
Benign entities including venous malformations,
soft tissue abscesses,
and hematoma can be distinguished based on several key features.
Rapidly growing venous malformations can be distinguished by identifying a highly vascular lesion with thrombus,
or phlebolith,
on imaging.
Soft tissue abscesses are often thick rimmed,
with irregular peripheral enhancement,
and the patient will commonly have systemic symptoms of infection.
Hematomas often demonstrate fluid-fluid levels and lacks solid regions of enhancement [17-19].