[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/) › Joint cysts and tenosynovial giant cell tumour

# 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.

Updated Nov 12, 2026

## Key points

- Ganglion: most common soft-tissue lump of the hand; dorsal wrist; cyst wall of fibrous tissue without lining cells; transilluminates.
- Baker cyst: synovial cyst in the popliteal fossa with OA, RA or meniscal tears; rupture mimics deep vein thrombosis.
- Tenosynovial giant cell tumour: CSF1 translocation in a minority of cells that recruit many macrophages expressing CSF1R.
- Localized type: small, well-circumscribed nodule on the fingers.
- Diffuse type: red-brown villous and nodular synovium in the knee or hip, recurrent haemorrhagic effusions, bone erosion; recurs after surgery.
- CSF1R inhibitors are used for diffuse disease.

## 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.

## Memory aid

A few cells shout CSF1 and a crowd of macrophages comes: that crowd is the giant cell tumour of tendon sheath.

## 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.

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## Elsewhere in DurgiPedia

- [Synovial joints: capsule, synovial membrane and articular cartilage](https://durgical.com/en/histology/synovial-joints-capsule-synovial-membrane-and-articular-cartilage/)

## Connections

## Related 3

- [Giant cell tumour of bone](https://durgical.com/en/general-pathology/giant-cell-tumour-of-bone/)
- [Synovial sarcoma and undifferentiated pleomorphic sarcoma](https://durgical.com/en/general-pathology/synovial-sarcoma-and-undifferentiated-pleomorphic-sarcoma/)
- [Rheumatoid arthritis: synovitis and pannus](https://durgical.com/en/general-pathology/rheumatoid-arthritis-synovitis-and-pannus/)

## Related conditions 2

- [Ganglion cyst](https://durgical.com/en/conditions/ganglion-cyst/)
- [Baker's (Popliteal) Cyst](https://durgical.com/en/conditions/baker-s-cyst/)

[← 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: [Ganglion Cyst](https://www.ncbi.nlm.nih.gov/books/NBK470168/) (StatPearls (NCBI Bookshelf), 2025); [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).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/joint-cysts-and-tenosynovial-giant-cell-tumour/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/joint-cysts-and-tenosynovial-giant-cell-tumour/