CONDITION
Synovial Cell Sarcoma
Synovial cell sarcoma is a rare tumour that arises from the soft tissues around joints, tendons, and bursae — the structures that support movement and cushion bone surfaces. It tends to grow locally and can, in some cases, spread to other parts of the body. The condition is uncommon in dogs and exceptionally rare in cats. Owners often first notice a firm swelling or lump near a limb joint, sometimes accompanied by lameness or stiffness that develops gradually. The mass may have been present for weeks or months before it becomes large enough to prompt concern. Because the signs can resemble other soft-tissue growths or joint problems, the nature of the lump is typically clarified through imaging and sampling. This page explores the patterns that may bring synovial cell sarcoma to attention, what is understood about how these tumours behave, the investigations used to characterise them, and the range of approaches that may be considered once a diagnosis is established.
Why this matters now
Synovial cell sarcoma tends to appear in middle-aged to older dogs, typically between six and twelve years of age, though younger animals can occasionally be affected. Large and giant breed dogs appear to be represented more frequently, though the rarity of the tumour makes breed-specific risk difficult to establish with certainty. There is no clear association with sex, and no known environmental, dietary, or injury-related trigger has been identified.
The tumour often grows slowly over a period of weeks to months, expanding within the soft tissues that surround a joint or tendon sheath. In many cases, the mass remains confined to the area where it started, but in a subset of dogs the cells can spread to the lungs or nearby lymph nodes. The rate of growth and likelihood of spread vary between individuals, and some tumours remain locally contained for extended periods whilst others show more aggressive behaviour early in their course.
Signals & patterns
Early signals
Firm swelling near a joint
An owner may notice a lump close to the elbow, stifle, or hock that feels solid rather than soft or fluid-filled. The swelling may be subtle at first and can appear similar to other benign masses or areas of thickening.
Intermittent lameness
The affected limb may show occasional stiffness or reluctance to bear full weight, particularly after rest or first thing in the morning. The lameness can come and go, which may delay recognition that something is changing.
Reduced range of movement
The dog may extend or flex the nearby joint less fully than usual, especially if the mass is pressing against tendons or the joint capsule. This can be most apparent during activities that require full limb extension, such as climbing stairs or jumping.
Lump growing over weeks
A mass that was initially small and easy to overlook may gradually enlarge, becoming more obvious to touch or visible beneath the skin. The pace of growth can vary, and some owners notice the change only when comparing the limb to the opposite side.
Later signals
Persistent lameness
The limb may now be favoured consistently, with the dog shifting weight away from the affected leg during standing and walking. Pain or discomfort may become more apparent, particularly when the area is touched or the joint is moved through its range.
Visible or ulcerated mass
As the tumour continues to expand, it may become prominent beneath the skin or, in some cases, the overlying tissue may break down, leading to an open wound that does not heal in the expected way.
Respiratory changes
If the tumour has spread to the lungs, the dog may develop a cough, increased breathing effort, or reduced tolerance for exercise. These signs can appear weeks to months after the initial limb swelling is noticed.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a physical examination and history, establishing the location, size, and texture of the mass, alongside any changes in mobility or comfort. Imaging follows, allowing the clinician to assess the extent of local involvement and whether other sites in the body show evidence of spread. Sampling of the mass itself — either with a needle or by removing tissue surgically — provides the cellular detail required to classify the tumour and distinguish it from other soft-tissue growths.
Physical examination
Radiography
Computed tomography
Fine-needle aspiration cytology
Biopsy and histopathology
Options & trade-offs
Management is shaped by the size and location of the tumour, whether imaging suggests spread to other sites, and what the owner finds acceptable in terms of cost, recovery, and quality of life. Many approaches are used in combination rather than in isolation, and the sequence or selection of options often evolves as new information becomes available. What works well for one animal and household may be less suitable for another, and there is rarely a single path that fits all cases.
Surgical resection
The mass is removed along with a margin of surrounding normal tissue, aiming to excise all tumour cells. This often involves removing portions of muscle, fascia, or tendon sheath, and in some cases amputation of the affected limb is considered if the tumour is large or involves structures that cannot be reconstructed. Surgery is typically performed under general anaesthesia, and healing requires several weeks of restricted activity.
Trade-offs: Complete removal can be difficult if the tumour has grown around nerves, major blood vessels, or into joint spaces, and incomplete excision increases the chance of local regrowth. Amputation removes the tumour entirely but alters mobility, and not all owners or animals adapt comfortably to limb loss.
Radiation therapy
High-energy beams are directed at the tumour site, damaging the DNA of rapidly dividing cells and slowing or halting growth. Radiation may be used after surgery to address residual tumour cells, or as a standalone treatment when surgery is not possible. The protocol typically involves multiple sessions over several weeks, each requiring brief general anaesthesia.
Trade-offs: Radiation requires access to a specialist centre and carries costs for both the treatment itself and repeated anaesthesia. Side effects can include skin changes, delayed wound healing, and, in the long term, tissue fibrosis or stiffness. It may slow progression but does not always prevent eventual regrowth or spread.
Chemotherapy
Drugs are given intravenously or by mouth to target cancer cells throughout the body, with the aim of slowing metastasis or reducing tumour burden. Protocols vary depending on the drug chosen and the individual response, and treatment may continue for weeks to months. Blood tests are monitored periodically to assess bone marrow and organ function.
Trade-offs: The evidence for chemotherapy in synovial cell sarcoma is limited, and response rates are less predictable than for some other tumour types. Side effects such as nausea, diarrhoea, and low white blood cell counts can occur, and not all animals tolerate the drugs comfortably.
Pain management and supportive care
When curative or disease-modifying treatment is not pursued, or when the tumour progresses despite intervention, the focus shifts to maintaining comfort and quality of life. This may include non-steroidal anti-inflammatory drugs, opioid analgesics, or adjunctive therapies such as physiotherapy to support limb function and reduce stiffness.
Trade-offs: This approach does not slow tumour growth or prevent metastasis, and the time during which the animal remains comfortable varies. It suits owners who prefer to avoid anaesthesia, surgery, or the uncertainty of more intensive treatment, and can offer weeks to months of acceptable quality of life in many cases.
Common misconceptions
"A lump that has been present for months without growing much is unlikely to be serious."
Synovial cell sarcomas can remain stable in size for extended periods before entering a phase of more rapid expansion or spread. The pace of growth at any given moment does not reliably predict the tumour's eventual behaviour, and even slow-growing masses can metastasise.
"Removing the lump surgically will always prevent it from coming back or spreading."
Surgery aims to remove all visible tumour tissue, but cells can extend beyond the palpable edge of the mass, and incomplete excision allows local regrowth. Metastasis to distant sites such as the lungs may already have occurred before surgery, even if imaging does not detect it at the time.
"Radiation or chemotherapy will cure the tumour if surgery is not possible."
Radiation and chemotherapy can slow progression, reduce tumour size, or delay spread, but they do not reliably eliminate all cancer cells in synovial cell sarcoma. These treatments are often palliative rather than curative, and outcomes vary considerably between individuals.
Related conditions
Osteosarcoma
Osteosarcoma and synovial cell sarcoma are both malignant tumours that tend to arise in or near the limbs of dogs, and both can present with lameness and a palpable mass. The distinction often rests on imaging patterns and biopsy, as osteosarcoma originates in bone itself whilst synovial cell sarcoma arises from the surrounding soft tissues.
Lipomas
Lipomas and synovial cell sarcomas can both present as soft-tissue masses in the limbs, and owners may initially notice a lump without other signs. The key difference is that lipomas are benign, freely moveable, and slow-growing, whilst synovial cell sarcoma tends to be firmer, more fixed to surrounding structures, and may cause lameness as it enlarges.
Septic Arthritis
Septic arthritis can produce localised swelling, pain, and lameness near a joint, which may prompt consideration of other causes of periarticular masses such as synovial cell sarcoma. In septic arthritis the swelling reflects fluid and inflammation within the joint space itself, often with systemic signs of infection, rather than a discrete soft-tissue tumour.
Osteoarthritis in Dogs
Osteoarthritis and synovial cell sarcoma can both cause progressive lameness and stiffness in a limb, and both may be suspected when an owner notices gradual difficulty with movement. Imaging and tissue sampling help distinguish chronic joint degeneration from a soft-tissue tumour arising near the joint.
Hypertrophic Osteopathy
Hypertrophic osteopathy produces new bone growth along the limbs and can cause swelling and lameness, which may overlap clinically with the signs of a soft-tissue mass near a joint. In hypertrophic osteopathy, the changes are typically symmetrical and involve all four limbs, and there is often an underlying thoracic condition driving the bone response.
Understanding the behaviour of synovial cell sarcoma often unfolds over weeks rather than days, as imaging results, biopsy findings, and the animal's response to initial management come into focus. Owners may find it helpful to explore broader patterns in how soft-tissue tumours are investigated and managed, as well as the physiological changes that accompany ageing in larger dogs. Conversations about prognosis, quality of life, and what signs might signal a shift in the tumour's behaviour can be revisited as the picture evolves.
Last reviewed: 31 July 2026 · Dr Alastair Greenway MRCVS