Synovial sarcoma and undifferentiated pleomorphic sarcoma
- Lesion
Synovial sarcoma has nothing to do with synovium; it is a fusion-driven sarcoma of young adults. Other sarcomas show no line of differentiation at all.
What it is
Some soft tissue tumours have no normal tissue counterpart. Synovial sarcoma, named for its frequent location near joints, is a sarcoma of adolescents and young adults defined by the t(X;18) SS18-SSX fusion; it is usually in the deep soft tissue around the knee and thigh and can be biphasic (epithelial glands plus spindle cells) or monophasic. Undifferentiated pleomorphic sarcoma is a high-grade sarcoma of older adults with no recognizable differentiation, a diagnosis made after other lines are excluded. Solitary fibrous tumour, with its NAB2-STAT6 fusion, occurs in the pleura and many soft tissue sites.
Structure
- Biphasic synovial sarcoma: gland-like spaces lined by epithelial cells within uniform, densely packed spindle cells.
- Monophasic: only spindle cells in fascicles, with staghorn vessels and calcifications.
- Undifferentiated pleomorphic sarcoma: sheets of large atypical spindle and pleomorphic cells, many bizarre mitoses and necrosis.
- Solitary fibrous tumour: hypo- and hypercellular areas with ropy collagen and branching thin-walled vessels.
Step by step
- In synovial sarcoma, the SS18-SSX fusion protein enters the BAF chromatin remodelling complex and evicts its SMARCB1 subunit.
- The altered complex activates genes that drive proliferation and partial epithelial differentiation.
- Cells form spindle fascicles and sometimes glands.
- In undifferentiated pleomorphic sarcoma, genomic chaos (TP53, RB1 loss) produces bizarre cells without a program.
- Both spread mainly by the blood to the lungs.
Markers
- TLE1 nuclear and SS18-SSX antibody (synovial sarcoma)
- Keratin and EMA focal (synovial sarcoma)
- STAT6 nuclear (solitary fibrous tumour)
Clinical notes
- A slowly growing deep mass near the knee of a young adult can be synovial sarcoma, even if present for years.
- Undifferentiated pleomorphic sarcoma presents as a large, painless deep thigh mass in an older adult.
Test yourself
Think of the answer, then tap the question to see it.
Which translocation defines synovial sarcoma?
t(X;18), fusing SS18 with an SSX gene.
Does synovial sarcoma come from synovium?
No. Despite its name, it has no synovial origin; it often lies near but outside joints.
What was undifferentiated pleomorphic sarcoma formerly called?
Malignant fibrous histiocytoma.
Which nuclear marker identifies solitary fibrous tumour?
STAT6, from the NAB2-STAT6 fusion.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Soft Tissue Sarcoma Treatment (PDQ®)–Health Professional Version (National Cancer Institute, 2025).
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Representative model, for educational purposes only.