[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Bones, joints and soft tissue](https://durgical.com/en/general-pathology/areas/bone-soft/) › Synovial sarcoma and undifferentiated pleomorphic sarcoma

# 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.

Updated Nov 12, 2026

## Key points

- Synovial sarcoma: age 15–40, around the knee and thigh; t(X;18) SS18-SSX1/2/4.
- Biphasic (glands and spindle cells) or monophasic spindle; keratin and EMA positive in some cells, TLE1 nuclear, SS18-SSX fusion-specific antibody.
- Calcifications on X-ray are common; spreads to lungs and sometimes lymph nodes.
- Undifferentiated pleomorphic sarcoma: older adults, deep thigh; bizarre pleomorphic cells, storiform pattern; complex karyotype; formerly 'malignant fibrous histiocytoma'.
- Radiation-associated sarcomas are often undifferentiated pleomorphic.
- Solitary fibrous tumour: patternless spindle cells, staghorn vessels, nuclear STAT6; can cause hypoglycaemia by IGF2 secretion.

## 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.

## Memory aid

Synovial sarcoma is X-meets-18 near the knee of a young adult; it borrows epithelium, not synovium.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/general-pathology/synovial-sarcoma-and-undifferentiated-pleomorphic-sarcoma/)

[More Pathology questions →](https://durgical.com/en/practice/?f=pathology)

## Connections

## Related 6

- [Translocations, amplifications and epigenetic changes in cancer](https://durgical.com/en/general-pathology/translocations-amplifications-and-epigenetic-changes-in-cancer/)
- [Differentiation and anaplasia: how a tumour cell looks under the microscope](https://durgical.com/en/general-pathology/differentiation-and-anaplasia-how-a-tumour-cell-looks-under-the-microscope/)
- [Soft tissue tumours: principles](https://durgical.com/en/general-pathology/soft-tissue-tumours-principles/)
- [Joint cysts and tenosynovial giant cell tumour](https://durgical.com/en/general-pathology/joint-cysts-and-tenosynovial-giant-cell-tumour/)
- [Lipoma and liposarcoma](https://durgical.com/en/general-pathology/lipoma-and-liposarcoma/)
- [Pleural tumours: mesothelioma and solitary fibrous tumour](https://durgical.com/en/general-pathology/pleural-tumours-mesothelioma-and-solitary-fibrous-tumour/)

## Related conditions 1

- [Soft tissue sarcomas (liposarcoma, leiomyosarcoma, rhabdomyosarcoma)](https://durgical.com/en/conditions/soft-tissue-sarcomas/)

[← All of general pathology](https://durgical.com/en/general-pathology/)

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](https://www.cancer.gov/types/soft-tissue-sarcoma/hp/adult-soft-tissue-treatment-pdq) (National Cancer Institute, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/general-pathology/synovial-sarcoma-and-undifferentiated-pleomorphic-sarcoma/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/synovial-sarcoma-and-undifferentiated-pleomorphic-sarcoma/