Joint cysts and tenosynovial giant cell tumour
- Lesion
Most lumps around joints are cysts filled with jelly or fluid. A true tumour of the synovium, tenosynovial giant cell tumour, can be small or diffuse.
What it is
Reactive lesions are far more common than true tumours in joints. A ganglion is a cyst near a joint capsule or tendon sheath, usually on the dorsal wrist, filled with gelatinous fluid and lacking a true lining, formed by myxoid degeneration of connective tissue. A synovial cyst is a herniation of synovium through the capsule, such as the popliteal (Baker) cyst in knee disease. Tenosynovial giant cell tumour is a clonal neoplasm driven by CSF1 rearrangements: the localized type is a nodule on a finger tendon sheath, and the diffuse type (formerly pigmented villonodular synovitis) fills a large joint, usually the knee.
Structure
- Ganglion: cyst cavity with mucoid material and a wall of condensed fibrous tissue; no epithelium or synovium.
- Synovial cyst: wall lined by synovium, communicating with the joint.
- Tenosynovial giant cell tumour: sheets of round mononuclear cells, foamy macrophages, haemosiderin-laden macrophages and osteoclast-type giant cells.
- Diffuse type gross: tangled, matted red-brown to yellow villi (like a shaggy beard) from haemosiderin and lipid.
Step by step
- A CSF1 rearrangement arises in a small population of synovial cells.
- These cells secrete large amounts of CSF1.
- CSF1 recruits and activates macrophages through CSF1R; most of the tumour mass is these reactive cells.
- Macrophages fuse into giant cells and engulf blood pigments and lipid.
- The diffuse form grows along the synovium and erodes bone; it recurs if removal is incomplete.
Clinical notes
- Ganglia are painless or ache, may disappear on their own and recur after aspiration.
- Diffuse tenosynovial giant cell tumour causes a swollen, stiff knee with brown bloody fluid on aspiration.
- MRI shows haemosiderin as dark 'blooming' signal.
Test yourself
Think of the answer, then tap the question to see it.
What lines a ganglion cyst?
No true lining; the wall is condensed fibrous connective tissue.
Which gene is rearranged in tenosynovial giant cell tumour?
CSF1, which recruits CSF1R-bearing macrophages.
What was the diffuse type of tenosynovial giant cell tumour formerly called?
Pigmented villonodular synovitis.
Which condition can a ruptured Baker cyst mimic?
Deep vein thrombosis of the calf.
Elsewhere in DurgiPedia
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: Ganglion Cyst (StatPearls (NCBI Bookshelf), 2025); Soft Tissue Sarcoma Treatment (PDQ®)–Health Professional Version (National Cancer Institute, 2025).
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Representative model, for educational purposes only.