Synovial Cell Sarcoma

Synovial cell sarcoma (SCS) is a rare, malignant mesenchymal neoplasm that arises from the synovial lining of joints, bursae, or tendon sheaths. In veterinary medicine, it is most commonly diagnosed in dogs, with occasional reports in cats. The tumor is characterized by a biphasic cellular pattern, consisting of epithelioid and spindle cell components, which mimics the normal synovium. SCS is locally invasive and has a moderate to high metastatic potential, primarily to the lungs and regional lymph nodes. It typically affects the large appendicular joints, such as the stifle, elbow, and hock, and presents as a painful, progressively enlarging periarticular mass. Surgical excision with wide margins (often limb amputation) is the treatment of choice, with adjunctive chemotherapy or radiation therapy considered for high-grade tumors or incomplete margins. The prognosis is guarded, with median survival times ranging from 1 to 3 years depending on grade and stage.

The exact etiology of synovial cell sarcoma in animals is unknown, but several factors have been hypothesized. Chronic inflammation or trauma to the synovial membrane may predispose to neoplastic transformation, although no direct causal link has been established. Genetic mutations, such as chromosomal translocations (e.g., t(X;18) in human SCS), have not been consistently identified in canine cases. In dogs, no specific breed or sex predilection is confirmed, but large-breed dogs may be overrepresented. The tumor arises from primitive mesenchymal cells with the capacity for synovial differentiation, rather than from mature synovial cells. Environmental factors, such as exposure to carcinogens, have not been documented in veterinary patients. The role of oncogenic viruses remains speculative. Overall, the etiology is likely multifactorial, involving spontaneous genetic alterations in synovial progenitor cells.

Synovial cell sarcoma is a rare tumor in dogs and cats, accounting for less than 1% of all canine neoplasms. It is the most common malignant tumor of the joint in dogs, but still uncommon compared to other sarcomas. The median age at diagnosis is approximately 8 years (range 2-13 years). Large-breed dogs, such as Labrador Retrievers, Golden Retrievers, Rottweilers, and German Shepherd Dogs, may be at increased risk, possibly due to their larger joint surfaces and higher body weight. No consistent sex predilection is reported, though some studies suggest a slight male predominance. The stifle (knee) joint is the most frequently affected site, followed by the elbow and hock (tarsus). In cats, SCS is extremely rare, with only isolated case reports. Working and athletic dogs may be at higher risk due to repetitive joint trauma, but this is not proven. The tumor is locally aggressive, with a metastatic rate of 25-50% at the time of diagnosis, primarily to the lungs and regional lymph nodes.

Synovial cell sarcoma originates from undifferentiated mesenchymal cells in the synovial membrane of joints, bursae, or tendon sheaths. The tumor grows as a solid, lobulated mass that invades the joint capsule, periarticular soft tissues, and adjacent bone. Histologically, it exhibits a biphasic pattern with epithelioid cells forming gland-like structures and spindle cells arranged in fascicles. The tumor is highly vascular and may contain areas of necrosis, hemorrhage, and cystic degeneration. As the mass enlarges, it causes mechanical disruption of the joint, leading to pain, lameness, and decreased range of motion. Local invasion into bone results in periosteal reaction, cortical lysis, and pathological fracture. The tumor can also extend along tendon sheaths and bursae, facilitating local recurrence after incomplete excision. Metastasis occurs hematogenously, with the lungs being the most common site, followed by regional lymph nodes. The aggressive biological behavior is associated with high histologic grade, mitotic index, and the presence of metastasis at diagnosis.

Predisposing factors for synovial cell sarcoma include age (middle-aged to older dogs), large breed size, and possibly chronic joint inflammation or trauma. Dogs with a history of previous joint surgery or intra-articular fractures may have an increased risk, though this is not well-documented. Genetic predisposition is suspected in certain breeds, but no specific genetic markers have been identified. Obesity and high body weight may contribute to increased mechanical stress on joints, potentially promoting neoplastic transformation. Environmental factors, such as exposure to radiation or chemical carcinogens, are theoretical but not confirmed. In cats, no specific predisposing factors are known. The presence of pre-existing joint disease, such as osteoarthritis, may mask early signs of SCS, leading to delayed diagnosis. Overall, the condition appears to arise spontaneously, with no strong modifiable risk factors identified.

Clinical signs of synovial cell sarcoma typically include progressive lameness, joint swelling, and pain. The lameness is often chronic and unresponsive to conservative management with rest and nonsteroidal anti-inflammatory drugs (NSAIDs). The affected joint may be visibly enlarged, warm to the touch, and painful on palpation and manipulation. Range of motion is often decreased, and crepitus may be present. As the tumor invades adjacent bone, severe bone pain and pathological fracture can occur. In advanced cases, systemic signs such as lethargy, anorexia, and weight loss may be observed, especially if metastasis is present. The tumor may be palpable as a firm, fixed mass around the joint. In some cases, regional lymphadenopathy is detected due to lymphatic metastasis. The clinical signs are often indistinguishable from other joint diseases, such as septic arthritis, rheumatoid arthritis, or other periarticular tumors, necessitating advanced imaging and biopsy for definitive diagnosis.

Differential diagnoses for synovial cell sarcoma include: 1) Septic arthritis: typically presents with acute onset of severe lameness, joint effusion, fever, and systemic signs; synovial fluid analysis reveals septic neutrophilic inflammation and positive culture. 2) Immune-mediated polyarthritis: often affects multiple joints, with shifting lameness and positive response to immunosuppressive therapy; synovial fluid shows non-septic neutrophilic inflammation. 3) Osteoarthritis: chronic, progressive lameness with joint effusion and osteophytes on radiographs; synovial fluid is typically non-inflammatory. 4) Other periarticular sarcomas: such as fibrosarcoma, histiocytic sarcoma, or osteosarcoma arising from periarticular soft tissues or bone; histopathology is required for differentiation. 5) Synovial cyst or ganglion cyst: benign, fluctuant swelling with minimal pain; imaging shows well-defined cystic structure. 6) Rheumatoid arthritis: rare in dogs, but can cause erosive joint disease; serology and synovial biopsy may be needed. 7) Joint luxation or ligament injury: acute onset, with specific orthopedic findings on palpation and stress radiography. 8) Bone tumors (e.g., osteosarcoma) involving the periarticular bone: radiographs show characteristic bone lysis and proliferation. Definitive diagnosis requires histopathology of a biopsy sample.

The diagnostic algorithm for synovial cell sarcoma begins with a thorough history and physical examination, focusing on the affected joint. Orthopedic examination should include palpation, range of motion assessment, and joint manipulation to localize pain and swelling. Radiographs of the affected joint are the first imaging step; they may reveal soft tissue swelling, joint effusion, periarticular bone lysis, and periosteal reaction. If radiographs are inconclusive, advanced imaging such as computed tomography (CT) or magnetic resonance imaging (MRI) is recommended to assess the extent of the mass, bone invasion, and regional lymph node involvement. Thoracic radiographs (three views) are essential to screen for pulmonary metastasis. Synovial fluid analysis is performed to rule out inflammatory or septic arthritis; however, cytology of synovial fluid is often non-diagnostic for SCS. Definitive diagnosis requires a biopsy of the mass, either via needle core biopsy or incisional biopsy, submitted for histopathology. In some cases, arthroscopy or exploratory arthrotomy may be performed to obtain tissue samples. Staging may include abdominal ultrasound and lymph node aspiration if metastasis is suspected. The diagnostic workup should be completed before surgical planning to determine the appropriate treatment and prognosis.

Laboratory findings in synovial cell sarcoma are often non-specific. Complete blood count (CBC) may reveal mild anemia of chronic disease or leukocytosis due to inflammation. Serum biochemistry profile may show elevated alkaline phosphatase (ALP) or other markers of bone turnover, but these are not specific. Synovial fluid analysis is crucial: typically, the fluid is non-inflammatory or mildly inflammatory, with low nucleated cell counts (<5,000 cells/µL) and increased protein concentration. Cytology may show reactive synoviocytes, but neoplastic cells are rarely exfoliated. If septic arthritis is suspected, synovial fluid should be submitted for aerobic and anaerobic culture. Coagulation panel (PT/aPTT) is recommended if surgery is planned, especially if the tumor is large and vascular. Inflammatory biomarkers such as C-reactive protein (CRP) may be elevated but are not diagnostic. Urinalysis is part of the routine workup, especially in older animals. Overall, laboratory findings are supportive but not diagnostic; histopathology is required for definitive diagnosis.

Radiography: In early cases, radiographs may show only soft tissue swelling and joint effusion. As the tumor progresses, periarticular bone lysis, periosteal reaction, and soft tissue mineralization may be seen. The joint space may be widened or narrowed. In advanced cases, pathological fracture may be evident. Stress radiography may be used to assess joint stability, but is not specific for SCS. Ultrasonography: Can be used to evaluate the soft tissue mass, its echogenicity, and vascularity via Doppler. It is useful for guiding fine-needle aspiration or biopsy. Computed Tomography (CT): Provides detailed cross-sectional imaging of the joint and surrounding tissues, allowing assessment of bone invasion, cortical destruction, and the extent of the mass. CT is also useful for surgical planning and for detecting pulmonary metastasis (CT of the thorax is more sensitive than radiographs). Magnetic Resonance Imaging (MRI): Offers superior soft tissue contrast, allowing detailed evaluation of the tumor's relationship to the joint capsule, tendons, and neurovascular structures. MRI is particularly helpful for surgical planning, especially if limb-sparing surgery is considered. Arthroscopy: Allows direct visualization of the joint and biopsy of the synovium, but is invasive and may be limited by the size of the mass. Angiography/Fluoroscopy: Rarely used, but may be helpful for embolization or intra-arterial chemotherapy. Overall, advanced imaging is essential for staging and surgical planning.

Cytology: Fine-needle aspiration of the joint mass or effusion may yield clusters of spindle cells and epithelioid cells, but cytology is often non-diagnostic due to the firm, fibrous nature of the tumor. Synovial fluid cytology typically shows non-inflammatory or mildly inflammatory changes, with reactive synoviocytes. Histopathology: Definitive diagnosis requires biopsy. The tumor is characterized by a biphasic pattern with epithelioid cells forming acinar or glandular structures and spindle cells in interlacing fascicles. The cells exhibit moderate to marked pleomorphism, with a variable mitotic index. Immunohistochemistry (IHC) is often used to support the diagnosis: SCS is typically positive for vimentin, cytokeratin (in epithelioid cells), and sometimes for epithelial membrane antigen (EMA). It is negative for S-100, desmin, and CD34, which helps differentiate it from other sarcomas. Histologic grading (based on mitotic count, necrosis, and differentiation) is important for prognosis. Surgical margins should be evaluated for completeness of excision. Special stains, such as Alcian blue, may highlight mucin production. In some cases, electron microscopy may be used to confirm the diagnosis.

The treatment of choice for synovial cell sarcoma is wide surgical excision. For tumors involving the appendicular joints, limb amputation is often recommended to achieve complete margins, especially if the tumor is large or has invaded bone. In cases where limb-sparing surgery is feasible (e.g., small, well-defined tumors), local excision with wide margins may be attempted, but the risk of local recurrence is high. Amputation is well-tolerated in most dogs, especially those with good body condition and no concurrent orthopedic disease. Preoperative staging, including thoracic radiographs or CT, is essential to rule out metastasis. If metastasis is present, surgery may still be performed for palliative pain relief, but adjunctive chemotherapy is recommended. Chemotherapy protocols commonly used include doxorubicin (30 mg/m² IV every 3 weeks) alone or in combination with cyclophosphamide (50-100 mg/m² PO every 3 weeks) or ifosfamide. Radiation therapy may be used as an adjunct for incomplete margins or for palliation of pain. Postoperative pain management includes opioids (e.g., morphine 0.5-1 mg/kg IM/SC q4-6h, or fentanyl CRI 2-5 µg/kg/h) and NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h, or meloxicam 0.1 mg/kg PO q24h). Physical rehabilitation is important for recovery after amputation. The prognosis is guarded, with median survival times of 1-3 years, depending on grade and stage.

The prognosis for synovial cell sarcoma is guarded to poor. Factors associated with a worse prognosis include high histologic grade, presence of metastasis at diagnosis, incomplete surgical margins, and large tumor size. The median survival time for dogs with SCS treated with amputation alone is approximately 1-2 years. With adjunctive chemotherapy, survival may be extended to 2-3 years, especially in dogs without metastasis. Local recurrence is common if excision is incomplete. The 1-year survival rate is approximately 60-70%, and the 2-year survival rate is 30-50%. Negative prognostic indicators include mitotic index >10 per high-power field, tumor necrosis, and invasion of bone. Dogs with pulmonary metastasis at diagnosis have a median survival of less than 6 months. Early detection and aggressive surgical excision offer the best chance for long-term control. However, even with optimal treatment, the metastatic rate is high, and regular monitoring for metastasis is essential.

Postoperative follow-up for synovial cell sarcoma is critical. After amputation, the surgical site should be monitored for complications such as seroma, infection, or dehiscence. Sutures or staples are typically removed 10-14 days postoperatively. Thoracic radiographs (three views) should be repeated every 3 months for the first year, then every 6 months thereafter, to monitor for pulmonary metastasis. If chemotherapy is administered, CBC and biochemistry should be checked before each dose to monitor for myelosuppression and organ toxicity. Physical rehabilitation is important to help the dog adapt to amputation; this may include controlled leash walks, swimming, and range-of-motion exercises. The dog's activity should be restricted for 4-6 weeks postoperatively to allow healing. Long-term, the dog should be monitored for signs of lameness in the remaining limbs, as they may develop osteoarthritis or other orthopedic issues due to increased weight-bearing. Regular veterinary check-ups every 6-12 months are recommended.

Pearls: 1) Always obtain a biopsy before amputation to confirm the diagnosis, as other conditions (e.g., septic arthritis) may mimic SCS. 2) Use advanced imaging (CT/MRI) to assess the extent of the tumor and plan surgical margins. 3) Consider limb amputation as the primary treatment, as it provides the best chance for complete excision. 4) Administer perioperative antibiotics (e.g., cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery) to prevent surgical site infection. 5) Provide aggressive postoperative pain management, including multimodal analgesia (opioids, NSAIDs, local blocks). 6) Monitor for metastasis with regular thoracic radiographs. Pitfalls: 1) Attempting limb-sparing surgery without adequate margins, leading to local recurrence. 2) Delaying surgery due to misdiagnosis as arthritis, allowing tumor progression. 3) Failing to stage the patient for metastasis before surgery, leading to unnecessary surgery in cases of widespread disease. 4) Inadequate pain control, leading to poor recovery and patient suffering. 5) Not considering adjunctive chemotherapy for high-grade tumors, missing the opportunity to delay metastasis.

Perioperative antimicrobial prophylaxis: Cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. Postoperative antibiotics are not routinely needed unless infection is present. Analgesia: Preoperative: Opioids such as morphine (0.5-1 mg/kg IM/SC) or hydromorphone (0.05-0.1 mg/kg IV/IM) may be given. Intraoperative: Fentanyl CRI (5-10 µg/kg/h) or ketamine CRI (0.5 mg/kg/h) for balanced anesthesia. Postoperative: Opioids (e.g., buprenorphine 0.01-0.02 mg/kg IV/IM q8-12h) and NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h, or meloxicam 0.1 mg/kg PO q24h) for 3-5 days. Local anesthesia: Epidural with morphine (0.1 mg/kg) or bupivacaine (1-2 mg/kg) may provide prolonged analgesia. Chemotherapy: Doxorubicin (30 mg/m² IV every 3 weeks) is the most commonly used agent. Ifosfamide (375 mg/m² IV every 3 weeks) may be used in combination. Cyclophosphamide (50-100 mg/m² PO every 3 weeks) is an alternative. Metronomic chemotherapy with cyclophosphamide (10 mg/m² PO daily) and piroxicam (0.3 mg/kg PO every 24-48h) may be considered for palliative or adjunctive therapy. Supportive care: Antiemetics (e.g., maropitant 1 mg/kg IV/PO q24h) and gastroprotectants (e.g., omeprazole 1 mg/kg PO q24h) may be needed during chemotherapy. All dosages should be adjusted based on renal and hepatic function.

The veterinary literature on synovial cell sarcoma is limited to retrospective case series and case reports. A landmark study by Vail et al. (1994) evaluated 26 dogs with SCS and found that amputation alone resulted in a median survival of 1.5 years, with a 25% metastatic rate. Another study by Craig et al. (2002) reported that histologic grade was a significant prognostic factor, with high-grade tumors having a median survival of 6 months compared to 2 years for low-grade tumors. A more recent study by Ehrhart et al. (2013) evaluated the use of doxorubicin-based chemotherapy as an adjunct to amputation and found a median survival of 2.5 years, suggesting a benefit in delaying metastasis. However, no prospective randomized trials have been performed due to the rarity of the disease. The ACVS and ECVS consensus guidelines recommend amputation as the standard of care for SCS, with chemotherapy considered for high-grade tumors or incomplete margins. Radiation therapy is reserved for palliative treatment or for cases where surgery is not feasible. Overall, the evidence supports aggressive surgical excision and careful staging, with adjunctive chemotherapy potentially improving outcomes.