Approach to Pediatric Soft Tissue Masses

Is the Mass Cystic?

Ganglion cysts, meniscal cysts, and synovial cysts can be difficult to differentiate because of their location and similar imaging appearance on MRI. As will be a common theme, correlating clinical history with imaging is important in narrowing your differential diagnosis. On imaging, a communication with an adjacent joint or association with an effusion of the joint or tendon sheath can help determine the diagnosis. This can many times be done with a linear array transducer and close attention as to the flow direction of any internal echoes during compression, which may give the location of communication and origin.

Ganglion cysts are lesions with a dense fibrous connective tissue capsule lined with flat spindle-shaped cells. Although imaging of these lesions usually shows a simple cyst, septations may be present. Characteristics on imaging that may suggest a ganglion cyst include internal septa with peripheral fluid-filled pseudopodia that shows up as a “bunches of grapes” appearance. Typically, T1-weighted imaging of a ganglion shows intermediate to high signal intensity because of the content being either mucinous or hemorrhagic. T2-weighted sequences will show homogeneous high signal ( Fig. 16.1 ). US will show a similar cystic appearance, and color Doppler imaging may show vascularity of the septa on low-flow settings. Although controversial, ganglion cysts are thought to arise from mucoid cystic degeneration in a collagenous structure under repetitive stress, and hence most commonly occur in the hand, wrist, and foot. The joint capsule and tendon incur the majority of the stress from repetitive activities and results in the periarticular soft tissue being prone to developing ganglia. Ganglion cysts may also occur in intraarticular, intraosseous, or periosteal locations.

Ganglion cysts are less common in children than in adults. When present, these lesions are usually asymptomatic. Management is conservative due to the tendency for ganglion cysts to spontaneously resolve, especially in the pediatric population. Repeated trauma, however, can result in progression of these masses and can lead to them becoming symptomatic or eroding adjacent bone.

Synovial cysts are commonly confused with ganglion cysts. The lesions primarily differ in that synovial cysts result from herniation of the synovial membrane through the joint capsule or fluid distention of a periarticular bursa and are thus lined by synovial cells. Although synovial cysts may occur in many locations, one of the most well-characterized areas is the popliteal region, where it is known as a Baker cyst. Baker cysts arise from the gastrocnemius-semimembranosus bursa and are specifically located in the medial aspect of the popliteal fossa between the medial head of the gastrocnemius muscle and the semimembranosus tendon. Complications of Baker or popliteal cysts include compression of adjacent structures, rupture, and, rarely, hemorrhage ( Fig. 16.2 ), which have associated changes in the MR signal characteristics but classic anatomic location.

In the pediatric population, imaging of these lesions is unlikely to be associated with a joint effusion or internal derangement. Juvenile idiopathic arthritis is the primary exception to this, where popliteal cysts are shown in the majority of cases and commonly have an associated joint effusion. In children, management is normally conservative with the typical natural history being that of spontaneous involution.

Clinical history of trauma with the presentation of a mass in a joint space may suggest a meniscal cyst. These cysts occur as synovial fluid accumulates within the degenerated tissue from the injury. These cysts develop in continuity with complex meniscal tears, especially if there is horizontal cleavage. These cysts may be contained in the torn menisci as an intrameniscal cyst or may be displaced through the tear. The latter is more common and results in an expansion of the meniscocapsular margin and displacement of the capsule outward into the adjacent tissues becoming a “parameniscal cyst.”

MRI shows meniscal cysts, as well as circumscribed lesions that are often described as septated fluid collections. The continuity with the horizontal cleavage or complex meniscal tear that caused their formation can often be appreciated on imaging as well. Unique features that may be seen with these lesions include low signal intensity contents on T2-weighted imaging. This finding is likely the result of water resorption by parameniscal tissue or hemorrhage. T1-weighted imaging may show these lesions as being isointense to muscle, likely because of hemorrhage or highly proteinaceous content within the cyst.

Synovial venous malformations, most frequently seen in the knee, may appear as an intraarticular cystic mass on an unenhanced MRI examination. Clues to the diagnosis include recurrent hemarthrosis and hemosiderin deposition within the joint. If a synovial venous malformation is suspected, repeat contrast-enhanced MRI can be performed for diagnostic confirmation.

Does the Mass Enhance With a Gadolinium Contrast Agent?

When considering your differential for an articular mass, enhancement with gadolinium contrast can be an important differentiator from a cystic lesion. Benign tumors that may be considered include pigmented villonodular synovitis (PVNS) and giant cell tumor of the tendon sheath (GCTTS). PVNS and GCTTS are classified as benign fibrohistiocytic tumors (although there is no relationship with true histiocytes). Synovial sarcoma is an important entity to keep in mind with gadolinium-enhancing lesions due to its malignant potential.

PVNS is an idiopathic tumor of synovial origin that can occur as a diffuse or intraarticular form. This lesion can be seen in the pediatric population (usually in those older than 10 years), although it is much more prevalent in those around 20 to 50 years old. Patients have swelling and pain that develops over time; an acute presentation is rare. It most often affects the knee joint. In the knee, PVNS most commonly occurs in the Hoffa fat pad, the suprapatellar bursa, and the posteromedian synovial joint recess. Additional areas that are affected by PVNS include the ankle, hip, shoulder, and elbow. Multiple joint involvement has been reported in the pediatric population.

The difficulty in assessing some of these areas with arthroscopy (especially posteromedian synovial joint recess) highlights the importance of assessment with MRI. Findings of PVNS on MRI includes an irregularly thickened or frondlike T1- and T2-hypointense synovium (caused by hemosiderin deposition) ( Fig. 16.3 ). The resulting low signal intensity is exaggerated by signal loss or blooming with gradient-echo or susceptibility-weighted sequences. Despite hypointensity on T2-weighted imaging, PVNS may appear hyperintense on Short Tau Inversion Recovery (STIR) imaging. Avid enhancement after gadolinium is the norm. PVNS may be associated with joint effusions or multiseptated popliteal cyst. Bone involvement (e.g., edema, erosion) is rare in children. Although radiographs may detect any bone involvement and US may detect a large soft tissue mass and reactive effusion, the finer details, anatomy, and characterization of the articular lesions are best delineated with MRI.

Histologically, PVNS is characterized by synovial cell hyperplasia; accumulation of giant cells, macrophages, and fibroblasts; and intracellular and extracellular hemosiderin deposition. Treatment for PVNS is synovectomy. Recurrence for PVNS is estimated to be around 20% to 40%, highlighting the use of MRI in postoperative evaluation as well.

Another entity known as GCTTS is thought to be a continuation of PVNS because of their histological similarities. GCTTS has subclassifications that include a localized extraarticular, localized intraarticular (nodular synovitis), and diffuse extraarticular focus. Localized extraarticular GCTTS is the most common variant. The signal characteristics of GCTTS on MRI are similar to that of PVNS.

Synovial sarcoma, the second common pediatric soft tissue sarcoma after rhabdomyosarcoma, arises from primitive mesenchymal cells (versus synovial, despite the name). It is mostly seen in the first five decades of life. Thirty percent of cases occur in patients younger than 20 years, and it has been reported to occur in children as young as 2 years. Synovial sarcoma usually occurs in the extremities and clinically manifests as a painless, slow-growing mass that has been present for weeks to several years. Other less common clinical presentations include pain and tenderness before the mass becomes palpable, acute arthritis or bursitis, or chronic contracture.

The majority of cases of synovial sarcoma occur in a juxtaarticular location, close to the joint capsule, tendons, and bursa. Only a minority of cases are intraarticular. Plain films findings may include a soft tissue density and spiculated calcifications, seen in up to 30% of cases. With US, synovial sarcoma can appear as a well-defined hypoechoic vascular mass near, but not within, the joint ( Fig. 16.4 ).

A classic MRI description of synovial sarcoma based on the most common findings is a juxtaarticular mass with a heterogeneous or triple-signal pattern (high-, intermediate-, and low-intensity areas) on T2-weighted imaging with fluid–fluid levels secondary to intralesional hemorrhage. Synovial sarcoma tends to be a well-defined mass isointense to muscle on T1-weighted images. Smaller lesions are homogenous, sometimes simulating cysts on unenhanced images. These small lesions, because of their homogenous hyperintense appearance on T2-weighted imaging and their propensity to displace, rather than infiltrate, adjacent structures, have been misdiagnosed as benign entities. Gadolinium is important in the evaluation of these masses. Unlike a cyst, these lesions will enhance.

Synovial sarcoma does often have an intimate relation to adjacent bone. About half of cases will be contiguous with bone, and about 20% will result in adjacent cortical thinning or medullary invasion, serving as a clue to the diagnosis.