Synovial cell sarcoma

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Synovial sarcoma in a 6-year-old Golden Retriever dog, showing obliteration of the shadow of the infrapatellar fat pad due to joint effusion and proliferative synovial tissue[1]

Synovial cell sarcoma (malignant synovioma) are a malignant mesenchymal spindle cell tumors similar to mesenchymomas, arising from connective tissue elements and characterized by soft-tissue masses associated with skeletal joints[2].

These tumors arise from primitive mesenchymal cells outside of the synovial membranes, tendons heaths and bursa of joints. These precursor cells differentiate into epithelioid (synovioblastic) and fibroblastic (spindle) cells.

The elbow, stifle and scapulohumeral joints are more commonly affected in middle-aged to older large-breed dogs[3], but other joints including the tarsus[4], carpus and jaw[5] may also be involved. The disease is characterized by a chronic, slowly progressive lameness[6].

The affected joint is usually swollen and tender to touch, but no evidence of other joints is a common finding, ruling out polyarthropathies such as immune-mediated arthritis. Reduced joint movement and pathological fractures may be evident.

On radiograph, a periarticular mass is usually evident, associated with soft tissue calcification, cortical osteolysis with poorly defined margins and a markedly irregular periosteal reaction. Pulmonary metastases are common in advanced cases.

On arthroscopic examination, these tumors appear to adhere to the outer surface of the joint capsule or may invade it. These tumors may extend along fascial planes and tendon sheaths. Involvement of the whole joint is common.

Diagnosis can be difficult histologically and these may be interpreted as malignant histiocytomas or malignant fibrosarcomas based on their microscopic appearance.

Immunohistochemistry may be required to clearly differentiate the tumor type, with these tumors staining positively for vimentin and epithelial membrane antigen[7].

A differential diagnosis would include osteosarcoma, fibrosarcoma, chondrosarcoma, histiocytoma, osteochondrosarcoma and joint diseases such as cruciate ligament injury, elbow dysplasia and osteochondritis dissecans.

The recommended treatment of canine synovial sarcoma is amputation, as local resection often results in recurrence[8]. Adjuvant chemotherapy may improve survival rates, with [[cyclophosphamide] and doxorubicin recommended.

Radiation therapy should be considered as an adjuvant or palliative therapy in at-risk patients[9].

Treatment aggressiveness may be more appropriate than clinical staging or tumor grading in predicting survival[10], with an average survival time of 2 - 3 years[11].


  1. Harasen G (2002) Joint tumors. Can Vet J 43(12):975-976
  2. Fisher C et al (2001) Synovial sarcoma. In: Fletcher CDM, Sundaram M, Rydholm A, Coindre JM, Singer S, editors. WHO Classification of Soft Tissue Tumours. Lyon, France: IARC Press. pp:127–130
  3. Karayannopoulou M et al (1992) Synovial sarcoma in a dog. Zentralbl Veterinarmed A 39(1):76-80
  4. Peremans K et al (2007) Use of [123I]-2-iodo-L-phenylalanine as a tumor imaging agent in two dogs with synovial cell sarcoma. Vet Radiol Ultrasound 48(5):471-474
  5. Griffith JW et al (1987) Synovial sarcoma of the jaw in a dog. J Comp Pathol 97(3):361-364
  6. Bellah JR & Patton CS (1986) Nonweightbearing lameness secondary to synovial sarcoma in a young dog. J Am Vet Med Assoc 188:730–732
  7. Loukopoulos P et al (2004) Canine biphasic synovial sarcoma: case report and immunohistochemical characterization. J Vet Sci 5(2):173-180
  8. Vail DM et al (1994) Evaluation of prognostic factors for dogs with synovial sarcoma: 36 cases (1986-1991). J Am Vet Med Assoc 205(9):1300-1307
  9. Gibbons DS et al (2011) Palliative radiation therapy in the treatment of canine appendicular synovial sarcoma. J Am Anim Hosp Assoc 47(5):359-364
  10. Fox DB et al (2002) Canine synovial sarcoma: a retrospective assessment of described prognostic criteria in 16 cases (1994-1999). J Am Anim Hosp Assoc 38(4):347-355
  11. Craig LE et al (2002) The diagnosis and prognosis of synovial tumors in dogs: 35 cases. Vet Pathol 39(1):66-73