Joint Tumors – Synovial Cell Tumors

Key Points

Commonly soft tissue tumors of joints may require additional testing to make a specific diagnosis

Some joint tumors carry a good prognosis with amputation of the limb

Histiocytic tumors of the joint, commonly seen in Burnese Mountain dogs and Flat-coated retreivers carry a poor prognosis



Synovial cell tumors originate from the synovium or the lining of joints.  Normally there are three types of cells that form the synovium: type 1, phagocytic macrophages (they eat up debris…a cleaning cell); type 2, antigen-presenting dendritic cells (helps the body to recognize foreign material and bacteria in the joint); and type 3, fibroblast-like mesenchymal cells that produce glyosaminoglycans (joint lubricant).  Other kinds of tumors that affect the joint include fibrosarcoma, rhabdomyosarcoma, osteosarcoma, malignant fibrous histiocytoma (MFH), liposarcoma, hemangiosarcoma, myxoma, malignant giant cell tumor of soft tissue, and undifferentiated sarcoma.  All of these tumors have roots that extend well beyond the primary tumor. The spread rate of these tumors depends on they type and grade of the tumor.


Common features

Synovial cell tumors have a common set of features.  They can develop in any joint in the body, have poorly defined microscopic margins, have roots that infiltrate through tissue planes around the joint, commonly regrow after conservative surgery, spread through the blood stream, and respond poorly to radiation and chemotherapy when used without the addition of surgery.



A swelling surrounding a joint is the most common finding of a synovial cell sarcoma. Most sarcomas have a firm consistency, however, some have a softer texture.  Once these tumors get larger they may become ulcerated and develop a secondary infection.  As the tumor grows, it commonly will cause the pet to become lame on the affected limb.  If the tumor has spread to the lungs, breathing difficulty may be seen.  Spread of the tumor to other organs may cause loss of appetite, weight loss and malaise.



Diagnosis of synovial cell sarcoma or soft tissue sarcoma of the joint is based upon a biopsy performed by aspirating the tumor with a small needle or surgically collecting a piece of tissue from the mass.    These tumors may not exfoliate cells very well as the cells may be tightly fixed together, thus necessitating a surgical biopsy.  Special imunohistochemical stains are needed to differentiate other tumors involving the synovium. Immunohistochemical staining for cytokeratin, CD18, and smooth muscle actin are recommended to make the diagnosis and predict the behavior of synovial tumors in dogs. An x-ray of the tumor commonly shows soft tissue swelling around the joint and in some cases erosion of the bones (see photo right).  CT scan is a much more sensitive test to look for bony invasion. A complete blood count, chemistry profile and urine testing are done to evaluate the health status of your companion’s internal organs prior to anesthesia and surgery.  Chest x-rays or CT scan are used to help identify spread of cancer to the lungs and lymph nodes in the chest. Abdominal ultrasound is also performed to identify evidence of spread of tumor to the internal abdominal organs.  It is important to note that microscopic spread of cancer to lungs and other internal organs cannot be identified with these imaging tests.


The day of surgery

Our anesthesia and surgical team will prescribe a pain management program, both during and after surgery that will keep your companion comfortable. This will include a combination of general anesthesia, injectable analgesics, local anesthetics, oral analgesics and anti-inflammatory medication.



For the best outcome, amputation of the limb is the treatment of choice for synovial cell tumors.

Chemotherapy may be indicated to help stunt the growth of metastatic cancer from a high-grade sarcoma.  If chemotherapy is recommended, it is administered every three weeks via intravenous injection for a total of four to five treatments.  The treatments are typically done on an outpatient basis and may take a total of 90 minutes to complete each visit.  Unlike humans, most dogs do not lose their hair and usually have only mild side effects from the medication such as transient loss of appetite and vomiting.



After surgery, a prescribed pain reliever should be given to minimize discomfort. It’s also extremely important to limit your companion’s activity and exercise level for three weeks after surgery.  The incision should be checked daily for signs of infection.  If indicated, adjunctive chemotherapy can be started 2 weeks after surgery.



The grade of the tumor, based on the biopsy report, will have a dramatic impact on the long-term survival times.  Grade 1 and 2 synovial cell sarcomas are commonly cured with survival times commonly exceeding 3 years.  Grade 3 synovial cell sarcomas generally have a high rate of tumor metastasis with a reported median survival time of 7 months.  Mean survival times is 5.3 months for synovial histiocytic sarcoma, and 3.5 months for other sarcomas. Synovial myxomas rarely metastasize, thus amputation is usually curative. Special stains of the synovial tumor can be done to help differentiate the specific types of joint tumors and predict potential patient survival.

Short-term complications following surgery are uncommon and may include temporary dehiscence (opening) of the incision, bleeding and infection.   Local recurrence is uncommon with amputation of the limb, unless the tumor is involving a joint that cannot be removed with large margins.  Metastatic disease is always a potential complication, therefore chest x-rays are recommended every 3 months for the first year, then once every 6 months thereafter.



  1. Setling KA, et al. Outcome of dogs with high-grade soft tissue sarcomas treated with and without adjuvant doxorubicin chemotherapy:  39 cases (1996–2004).  J Am Vet Med Assoc 2005;227:1442-1448.
  2. Ehrhardt N. Soft-tissue sarcomas in dogs: a review.  J Am Anim Hosp Assoc 2005;41:241-246.
  3. Fox DB, Cook JL, Kreeger JM, et al. Canine Synovial Sarcoma: A Retrospective Assessment of Described Prognostic Criteria in 16 Cases (1994-1999). J Am Anim Hosp Assoc 2002;38:347–355.
  4. Craig LE, Julian ME, Ferracone JD.  The diagnosis and prognosis of synovial tumors in dogs:  35 cases.  Vet Pathol 2002;39:66-73.

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