Pathology of Synovial Sarcoma
sarcoma is a well defined entity which commonly occurs in adolescents and
young adults. (between 15 and 40 years of age).
This tumour is commonly located in the paraarticular regions, close to tendon sheaths, bursae and joint capsules.
The tumour rarely involves synovial membrane.
The hallmark tumor marker is the t(X;18) translocation, which results in fusion of the SYT gene of chromosome 18 to the SSX gene of the X chromosome, creating most frequently either an SYT-SSX1 or SYT-SSX2 transfusion transcript.
These are usually located around knee and ankle joint, as well as around hip,shoulder and elbow.
variants occur in the oral cavity, anterior abdominal wall,
larynx, pleura, lung, pericardium, heart and mediastinum.
The gross appearance depends on the rate of growth and site of the tumour.
Slow growing tumours are well circumscribed , firm , round or multinodular lesion partly or fully encapsulated by pseudocapsule.
Cystic changes may be prominent.
Rapidly growing tumours are poorly circumscribed with a variegated , friable appearance.
There may be areas of haemorrhage
and necrosis with cyst
The two main variants are the monophasic and the biphasic types.
In both variants there are spindle shaped cells set in a collagenous backround.
I. Biphasic pattern of synovial sarcoma consists of an admixture of glandular structures lined by cuboidal or columnar epithelium set in a sarcomatous stroma.
In some occult cases clusters of plump cells with pale cytoplasm are present in a spindle cell stroma.
These nests of cells are accentuated by reticulin stain.
II. Monophasic pattern of synovial sarcoma is characterized by monomorphic population of spindle shaped cells arranged in fascicles.
The cells contain uniform tapering nuclei and pale cytoplasm.
Mitotic figures are present in variable numbers.
Other striking features include:
- Presence of mast cells
- Calcification with or without ossification. Calcification is usually preceded by hyalinization.
This is an important microscopic finding.
Prognosis is better for synovial sarcoma associated with osseous metaplasia or extensive calcification.
Hemangiopericytoma - like vascular pattern.
3 main groups :
- Round cell morphology associated with necrosis and high mitotic
- Polygonal larger cells which sometimes show rhabdoid morphology.
Synovial sarcomas are also described in the pleural cavity where they may be confused with both mixed and sarcomatoid variants of mesothelioma.
A diagnosis of synovial sarcoma should be considered particularly if an abdominal spindle cell neoplasm shows a haemangiopericytomatous pattern and diffuse CD99 and CD56 immunopositivity.
A confident distinction between abdominal synovial sarcoma and GIST requires KIT/PDGFRA mutation analyses and specific molecular testing for synovial sarcoma.
Synovial sarcoma demonstrates nuclear staining with TLE1 (transducin-like enhancer of split-1)
Distinction between poorly differentiated variant of Synovial sarcoma and Ewing's sarcoma/PNET:
FLI-1 has been described to be a useful marker for Ewing sarcoma
synovial sarcoma PNET/Ewing's sarcoma
FLI-1 Negative Positive
TLE1 95% cases positive Negative
and high power images of poorly differentiated variant of synovial
Poorly differentiated variant of synovial sarcoma is a round cell sarcoma which may be histologically indistinguishable from Ewing's Sarcoma / PNET.
The diagnosis of synovial sarcoma was established by :
Cytogenetic analysis: Detection chromosomal rearrangement of SS18
Poorly differentiated synovial sarcoma: Immunohistochemical distinction from primitive neuroectodermal tumors and high-grade malignant peripheral nerve sheath tumors.
cytokeratin subsets for distinguishing poorly differentiated
synovial sarcoma from peripheral neuroectodermal tumour.
Histochemistry- Secretions within the epithelial cells and pseudoglandular spaces are PAS positive and diastase resistant, alcian blue and mucicarmine positive.
The stromal mucin secreted by the spindle cells are alcian blue positive but PAS negative.
Reticulin stain demonstrate the biphasic pattern of the tumor. Nests of plump rounded cells are highlighted by the reticulin stain.
Synovial sarcoma demonstrates nuclear staining with TLE1 (transducin-like enhancer of split-1) in 95% of tumors and appears to be a very sensitive marker. TLE1 encodes for a transcriptional corepressor that is involved in epithelial and neuronal differentiation.
In Synovial sarcoma there is usually coexpression of mesenchymal (vimentin) and epithelial markers (cytokeratin and EMA).
The following immunomarkers are useful in the diagnosis:
- Cytokeratin (+)
Only synovial sarcoma is positive with cytokeratins 7 and 19 , other soft tissue sarcomas including synovial sarcoma is positive with cytokeratin 8 and 18 ;
In monophasic synovial sarcoma immunoreactivity of cytokeratin is reduced by almost 60% ;
- Cytokeratin positivity is noted in only 50% cases of PD
- EMA (+) (more sensitive than cytokeratin) ,
- S100 protein (+) in some cases,
- CD99 (+) in 2/3rd of cases .
- bcl-2 (+) in most cases.
Application of a panel of immunohistochemical markers is suggested to avoid diagnostic pitfalls.
Diagnostic clue: In
the differential diagnosis of 'round cell tumours expressing
epithelial markers' , poorly differentiated synovial sarcoma should be
As one of the first sarcomas to be defined by the presence of a specific chromosomal translocation leading to the production of the SS18-SSX fusion oncogene, it is perhaps a typical "translocation-associated sarcoma," and its translocation remains unique to this tumor type.
Differential diagnosis: Malignant peripheral nerve sheath tumor; Solitary fibrous tumor ; Abdominal spindle cell neoplasm (GIST) depending on the site of the lesion
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