Soft tissue tumours: principles
- Topic
Soft tissue tumours are named for the tissue they resemble. Benign ones are a hundred times more common; sarcomas are rare, graded and spread by blood.
What it is
Soft tissue tumours arise from the non-epithelial tissues outside bone, organs and the CNS: fat, fibrous tissue, muscle, vessels and nerve sheaths. They are classified by the tissue their cells resemble, not by a presumed cell of origin, and many of uncertain type exist. Benign tumours vastly outnumber sarcomas. Sarcomas fall into two genetic groups: those driven by a specific translocation or mutation with simple karyotypes (often in younger patients), and those with complex, unstable karyotypes (often older adults). Most appear in the thigh and other deep extremity sites and the retroperitoneum and spread by the blood, mainly to the lungs.
Structure
- Benign tumours are usually small, superficial and well circumscribed with uniform cells.
- Sarcomas are often large, deep, fleshy ('fish flesh') with haemorrhage and necrosis; they seem encapsulated but invade along a pseudocapsule.
- Histologic patterns: spindle cells, small round cells, epithelioid cells or pleomorphic cells.
- Immunohistochemistry and molecular tests (FISH, sequencing for fusions) refine the type.
Step by step
- A mesenchymal progenitor gains one driver (a fusion making a chimeric transcription factor or kinase, or an MDM2 amplification) or broad genomic instability.
- The driver imposes a differentiation program that resembles fat, muscle, fibrous tissue or nerve sheath.
- The tumour grows inside its compartment, compressing tissues into a pseudocapsule with tumour fingers beyond it.
- High-grade tumours invade veins and metastasize to the lungs.
- Local recurrence follows if the tumour is shelled out rather than removed with a margin.
Soft tissue tumours by line of differentiation
| Lineage | Benign | Intermediate | Malignant |
|---|---|---|---|
| Fat | Lipoma, hibernoma | Atypical lipomatous tumour | Liposarcoma (well-differentiated, myxoid, pleomorphic, dedifferentiated) |
| Fibrous | Nodular fasciitis | Superficial and deep fibromatosis (desmoid) | Adult fibrosarcoma |
| Skeletal muscle | Rhabdomyoma | — | Rhabdomyosarcoma |
| Smooth muscle | Leiomyoma | — | Leiomyosarcoma |
| Vessels | Haemangioma | Haemangioendothelioma, Kaposi sarcoma | Angiosarcoma |
| Nerve sheath | Schwannoma, neurofibroma | — | Malignant peripheral nerve sheath tumour |
Markers
- Desmin and myogenin (skeletal muscle)
- SMA and desmin (smooth muscle)
- S100 and SOX10 (nerve sheath)
- CD31 and ERG (vessels)
- MDM2 (well-differentiated liposarcoma)
- Beta-catenin nuclear (desmoid)
Clinical notes
- A painless, growing deep mass in the thigh of an adult is the classic presentation of a sarcoma.
- Retroperitoneal sarcomas grow large before symptoms.
- Treatment: surgery with clear margins, with radiation for high-grade tumours; chemotherapy for chemosensitive types.
Test yourself
Think of the answer, then tap the question to see it.
What is the most common soft tissue tumour?
Lipoma.
Which features of a soft tissue mass suggest malignancy?
Large size, deep location, growth over time and, under the microscope, high grade with necrosis.
Which translocation defines synovial sarcoma?
t(X;18), fusing SS18 with an SSX gene.
How do sarcomas usually metastasize?
Through the blood, mainly to the lungs.
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).
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.