Soft tissue
Uncertain differentiation
Synovial sarcoma

Editor-in-Chief: Debra L. Zynger, M.D.
Farres Obeidin, M.D.
Borislav A. Alexiev, M.D.

Topic Completed: 1 July 2018

Minor changes: 2 July 2021

Copyright: 2003-2021,, Inc.

PubMed Search: Synovial sarcoma [title] soft tissue pathology AND free full text[sb] AND Humans[Mesh]

Farres Obeidin, M.D.
Borislav A. Alexiev, M.D.
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Cite this page: Obeidin F, Alexiev B. Synovial sarcoma. website. Accessed July 28th, 2021.
Definition / general
  • Malignant soft tissue tumor of uncertain histogenesis with variable epithelial differentiation
Essential features
  • Can occur in or around any tissue in the body
  • Multiple morphologies, including monophasic spindle cell, biphasic, poorly differentiated, myxoid, ossifying and monophasic epithelial
  • Monotonous spindle cells with vesicular, plump and overlapping nuclei with hemangiopericytic vessels
  • TLE1+, Cytokeratin+, EMA+, BCL2+, CD99+
  • Has a characteristic chromosomal translocation t(X;18)(p11;q11) involving genes SS18 and either SSX1, SSX2 or SSX4
  • Obsolete terms: malignant synovioma
ICD coding
  • C49.9: Malignant neoplasm of connective and soft tissue, unspecified
  • Accounts for 5 - 10% of all soft tissue sarcomas
  • Median age 35 years old, age range 0 to 85+
  • Age range: 0 to 85+
  • Slight male predominance (1.2:1)
  • 17.6% of cases occur in children and young adults 0 - 19 years old (Pediatr Blood Cancer 2011;57:943)
  • Can occur anywhere in the body
  • Primary site distribution (Pediatr Blood Cancer 2011;57:943):
    • Extremities: 68.7%
    • Trunk: 15.7%
    • Head and neck: 6.3%
    • Intra-thoracic: 5.3%
    • Intra-abdominal: 1.8%
    • Other: 2.2%
  • Driven by the chromosomal translocation t(X;18)(p11;q11) involving genes SS18 and either SSX1, SSX2 or SSX4
  • Translocation has multiple effects on oncogenetic pathways, including the SWI/SNF chromatin remodeling complex, polycomb repressor complex and canonical Wnt pathway (Cancer Discov 2015;5:124)
  • Translocation partner affects epithelial differentiation (Int J Clin Exp Pathol 2013;6:2272):
    • SS18-SSX1: 60 - 70% monophasic, 30 - 40% biphasic
    • SS18-SSX2: 97% monophasic, 3% biphasic
    • SS18-SSX1 inhibits Snail gene while SS18-SSX2 inhibits Slug gene
    • Interfering with Snail leads to stronger derepression of E-cadherin than does interfering with Slug
    • Stronger E-cadherin and increased extracellular matrix protein MMP2 expression are needed for biphasic morphology to develop
  • Despite the name, the cells of origin are not synovial cells
  • Histogenesis is still debated
  • One study has shown stem cell marker expression suggesting that origin may be a multipotent mesenchymal stem cell (Stem Cells 2010;28:1119)
  • One study using mice identified satellite cells (immature myoblasts) as a potential source (Cancer Cell 2007;11:375)
Clinical features
  • Very rarely associated with prior radiation therapy (Mod Pathol 2002;15:998)
  • Not classically known to be associated with any syndromes
Radiology description
  • Most present as round to oval, lobulated masses
  • Bone involvement is rare
  • Ossifying synovial sarcomas have characteristic spotty radio-opacities caused by focal calcifications (Pathol Res Pract 2009;205:195)
Radiology images

Contributed by Farres Obeidin, M.D.

Elbow mass

Shoulder mass

Arm mass

Contributed by Mark R. Wick, M.D.



Prognostic factors
Case reports
  • Primarily surgical
  • Radiotherapy and adjuvant chemotherapy have been evaluated in high risk situations (tumors > 5 cm or difficult to resect)
  • Radiation therapy: adjuvant use has shown improvement in oncologic outcome and overall survival (J Surg Oncol 2015;111:158)
  • Adjuvant chemotherapy:
    • Data is conflicting on its use but can be considered in high risk patients
    • Ifosfamide was shown to increase disease specific survival (Ann Surg 2007;246:105)
    • Two large EORTC database clinical trials showed no benefit of doxorubicin on overall survival (Ann Oncol 2014;25:2425)
  • Novel therapies:
    • Initial PDL1 and CTLA4 studies have shown no benefit (Sarcoma 2013;2013:168145)
    • Receptor tyrosine kinase inhibitor pazopanib has a promising trend toward better overall survival
    • Tazemetostat, an EZH2 inhibitor, has also been studied in INI1 deficient tumors including synovial sarcoma (Curr Treat Options Oncol 2018;19:13)
    • Early studies on T cell receptor based immunotherapy directed towards NY-ESO-1 in HLA-A*0201+ patients have demonstrated radiological and clinical benefit (J Clin Oncol 2011;29:917)
Gross description
  • Varies depending on growth rate
  • Slow growing tumors are well circumscribed, round or multilobular and have a smooth pseudocapsule
  • Poorly differentiated and rapidly growing tumors are poorly circumscribed with a variegated appearance, often with necrosis or hemorrhage
Gross images

Contributed by Farres Obeidin, M.D.

Arm mass

Shoulder mass

Lung mass

Elbow mass

Images hosted on other servers:
Missing Image

Tumor within the vastus intermedius muscle

Microscopic (histologic) description
  • General (Ann Diagn Pathol 2014;18:369):
    • Two main subtypes: biphasic and monophasic spindle cell
    • Rarer subtypes: poorly differentiated (round cell), monophasic epithelial, calcifying / ossifying and myxoid
  • Biphasic:
    • Architecture:
      • Two components, spindle cells and gland-like epithelial structures
      • Glandular lumina contain mucin
    • Cytologic features:
      • Epithelial component has moderate, distinct amphophilic cytoplasm with round to ovoid nuclei
      • Rarely squamous metaplasia can occur
  • Monophasic:
    • Architecture:
      • Infiltrative borders
      • Hypercellular fascicular architecture with little intervening stroma
      • Can rarely show hyalinization or myxoid change
      • Ill defined nuclear palisading may be seen
    • Cytologic features:
      • Monotonous cells with scant amphophilic cytoplasm, ovoid to spindled vesicular nuclei with evenly dispersed chromatin and inconspicuous nucleoli
      • Nuclei often close enough to overlap with adjacent nuclei
  • Poorly differentiated: highly cellular round cells with hyperchromatic nuclei and frequent mitotic activity and necrosis
  • Additional features:
    • May have staghorn, branching ("hemangiopericytic") vessels
    • Mast cells are common
    • Focal calcification can be seen in 1/3 of cases
Microscopic (histologic) images

Contributed by Farres Obeidin, M.D.

Monophasic synovial sarcoma

Hemangiopericytic vessel, 10x

Mitotic activity

Focal palisading

Post treatment

Biphasic synovial sarcoma

Patchy AE1/AE3+

Weak, patchy EMA+





Contributed by Debra Zynger, M.D.

Monophasic synovial sarcoma H&E

Monophasic synovial sarcoma TLE1+

Contributed by Mark R. Wick, M.D.

Monophasic synovial sarcoma H&E

Monophasic synovial sarcoma TLE1

Biphasic synovial sarcoma H&E

Biphasic synovial sarcoma TLE1

Positive stains
Negative stains
Electron microscopy description
  • Glandular formation of epithelioid tumor cells with sparse luminal microvilli
Electron microscopy images

Contributed by Mark R. Wick, M.D.


Biphasic spindle cell

Molecular / cytogenetics description
Molecular / cytogenetics images

Contributed by Mark R. Wick, M.D.

X18, in situ hybridization

Translocation in synovial sarcoma

Differential diagnosis
Board review style question #1

    Which of the following is true about the lesion pictured above?

  1. Commonly has PAX3 rearrangements
  2. It is associated with HHV8 infection
  3. It is negative for cytokeratin
  4. It occurs primarily in elderly patients
  5. SS18-SSX fusion is the characteristic chromosomal abnormality
Board review style answer #1
E. Synovial sarcoma has fusion of SS18 with SSX1, SSX2 or SSX4. It typically occurs in younger patients, with the peak incidence in the third - fourth decade and most cases show at least focal cytokeratin expression. It is not associated with PAX3 rearrangements, a typical finding in alveolar rhabdomyosarcoma and biphenotypic sinonasal sarcoma. HHV8 is seen in Kaposi sarcoma.

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Reference: Synovial sarcoma
Board review style question #2
    Which morphologic feature is commonly seen in synovial sarcoma?

  1. Chicken wire calcifications
  2. Extravasated red blood cells
  3. Hemangiopericytic vessels
  4. Nuclear pleomorphism
  5. Prominent storiform pattern
Board review style answer #2
C. Hemangiopericytic vessels and monotonous nuclei are commonly seen in synovial sarcoma. Chicken wire calcifications are typically seen with chondroblastoma. Extravasated red blood cells are common in nodular fasciitis, fibromatosis and Kaposi sarcoma. A storiform pattern is characteristic of dermatofibrosarcoma protuberans.

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Reference: Synovial sarcoma
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