*Corresponding author: Jihye Choi Department of Veterinary Medical Imaging, College of Veterinary Medicine, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea This case report was based on a client’s patient with a high standard of veterinary care and involved informed client consent. The Seoul National University Veterinary Medical Teaching Hospital approved this study.
Synovial cell sarcoma is a rare articular tumor in dogs, accounting for only 14% of joint tumors, compared to histiocytic sarcoma, which accounts for 51% of cases [1,2]. The disease arises near the joints or tendon sheaths. Despite its name, it rarely originates from synovial cells or within an intra-articular location, and the specific cell origin remains unknown [3,4]. In dogs, it predominantly affects large joints, such as the stifle and elbow, but other joints or non-joint tissues can be involved [5]. In human medicine, the magnetic resonance imaging (MRI) features of synovial sarcoma are well described because precise imaging is crucial for differentiating synovial sarcoma from other soft tissue tumors and guiding treatment planning [6-8]. The ‘triple sign’ is one of the MRI features and refers to a heterogeneous mixture of hyperintense, isointense, and hypointense areas within the tumor on the T2-weighted (T2W) images. Generally, 85% of tumors are larger than 5 cm, and they usually show iso-hyperintensity compared to muscle on the T1-weighted (T1W) images and predominantly hyperintensity on the T2W images with heterogeneous and multilobulated with irregular and ill-defined features [8]. Despite this, reports in veterinary medicine are limited [3,9,10]. This study described the computed tomography (CT) and MRI features of a histologically diagnosed synovial cell sarcoma of the stifle, with suspected multiple metastases in a dog.
An 8-year-old, 29 kg, castrated male Golden Retriever presented with acute weight-bearing lameness of the left hindlimb for 2i weeks, accompanied by swelling in the left stifle and tarsal joints, and decreased appetite for one month. Previous medical examinations were unremarkable except for suspected left hip dysplasia. The dog had a fever (40.2°C), an enlarged left popliteal lymph node, and pain on the joint extension of the left stifle and hip joint. Blood tests revealed thrombocytopenia (90 k/µL; reference interval (RI), 155-498 k/µL), mild neutrophilia (16.29 k/µL; RI, 2.4-10.4 k/µL), and elevated C-reactive protein (168.7 mg/L; RI, 0-20 mg/L).
Radiographs of the hindlimbs revealed extensive soft tissue swelling around the left stifle and tarsal joints, with significant moth-eaten osteolysis of the proximal tibia and talus. The transition between normal and affected bone was ill-defined. Enlarged left popliteal lymph nodes (2.3 × 1.7 cm) were observed. Thoracic radiography revealed pulmonary nodules up to 1.2 cm in diameter, along with diffuse bronchointerstitial and patchy alveolar infiltrates. Abdominal ultrasonography revealed asymmetrical prostate enlargement with mineralization and cystic changes.
CT scans of the hindlimbs, thorax, and abdomen performed on a 160-multi-slice CT scanner (Aquilion Lightning 160; Canon Medical Systems, Japan) provided detailed images of the extent of the lesions. A 3.5 × 2.3 × 1.6 cm mass with a well-defined margin was observed on the caudolateral aspect of the left stifle joint, with hypo- to iso-attenuation compared to the surrounding muscle and focal stippled calcification (Fig. 1A-D). The mass exhibited mild rim enhancement, with central non-enhanced areas suggesting necrosis. The mass extended to the proximal epiphysis and metaphysis of the tibia, causing moth-eaten osteolysis and a mild amorphous periosteal reaction. Diffuse contrast enhancement of thickened joint capsule and the surrounding soft tissues from the caudal intercondylar area of the left stifle to the tibial tuberosity and along the lateral and medial femoral condyles was observed. The left tarsal joint showed marked periarticular edema, with extensive permeative and moth-eaten type osteolysis in the talus and amorphous periosteal bone formation (Fig. 1E and F). A rim-enhancing nodule, approximately 1 cm in size, was identified within the gastrocnemius muscle (Fig. 1G). CT images of the thorax revealed multiple pulmonary nodules up to 1.5 cm in diameter, consolidation in the right lung lobes, and multifocal hypoattenuating lesions in the myocardium of the left ventricle (Fig. 1H-K). Enlargement of the thoracic and abdominal lymph nodes was also detected.
MRI was conducted using a 1.5-T scanner (Sigma Creator 1.5 T; GE, USA) with a receive-only single-channel small flex coil (Fig. 2). MRI revealed a mass extending from the tibial medullary cavity to the lateral and posterior aspects of the tibia. The mass showed the triple sign with hypointense for the posterior compartment, isointense, and hyperintense for the anterior compartment on the proton density-weighted (PDW) and the T2W images. The fluid-fluid levels were also observed with a hypointense anterior portion and a hyperintense posterior portion within the mass. The post-contrast T1W images revealed marked rim enhancement, with various internal signals. Muscle invasion was visible in the right gastrocnemius muscle, with hyperintensity on the PDW and T2W images, and rim enhancement on the post-contrast T1W images. Peri-lesional edema was observed by T2W hyperintense signals extending into the surrounding tissues. Nodular lesions with T2W hyperintense signals and no contrast enhancement were also observed in the medial condyle of the left femur and ilium (Fig. 3).
Fine needle aspiration and punch biopsy of the stifle joint revealed atypical large cells with marked anisocytosis and anisokaryosis, consistent with sarcoma. Immunohistochemistry confirmed synovial cell sarcoma, with positivity for vimentin (1:200, Dako; Denmark) and negativity for CD18 (1:400, Dako) and desmin (1:200, Dako). CD18-positive cells were speculated to be infiltrated macrophages. Considering that synovial fibroblasts modulate the tissue microenvironment by secreting a variety of inflammatory mediators, including chemokines, it is not unusual to observe the infiltration of large numbers of macrophages in synovial cell sarcomas, as in this case [11]. The cytology of the left popliteal lymph node revealed mature small lymphocytes intermixed with highly dysplastic round to polygonal cells similar to those in the stifle mass, indicative of metastatic involvement. Euthanasia was performed because of the rapid clinical deterioration of the dog’s condition. Although a necropsy was not conducted, the CT and MRI findings strongly suggested metastatic spread to the lungs, lymph nodes, myocardium, the left tarsal joint, left femur and ilium, and right gastrocnemius muscle. Based on the imaging and biopsy findings, the dog was diagnosed with synovial sarcoma with multiple metastases.
Differentiating synovial cell sarcoma from histiocytic sarcoma is crucial because of their similar histological features but distinct biological behaviors and treatments. Amputation followed by adjuvant chemotherapy is commonly recommended for high-grade synovial cell sarcoma [12]. In contrast, histiocytic sarcoma is highly invasive and metastasizes extensively, with poor treatment responses [13-15]. Immunohistochemistry using markers, such as CD18 and vimentin, helps distinguish between the 2 [1,2].
Imaging modalities, such as CT and MRI, are crucial for evaluating joint tumors, providing detailed information on the tumor size, bone involvement, local invasion, and metastasis. CT is beneficial for evaluating bone destruction, while MRI provides superior soft tissue contrast for detecting soft tissue invasion and identifying the signal characteristics within the tumor.
In the present case, synovial cell sarcoma was diagnosed in the left stifle joint with extensive metastases suspected in the tarsal joint, lungs, lymph nodes, muscles, and myocardium. The MRI features characteristic of synovial sarcoma in humans [6,16,17] are reported. In human medicine, MRI helps differentiate synovial sarcoma from other soft tissue tumors, particularly large tumors (> 5 cm), which typically exhibit triple signs and fluid-fluid levels and are associated with a poorer prognosis [6,16,17]. The triple sign indicates the presence of necrotic, hemorrhagic, cystic, and cellular components within the tumor, which is reported in 35% to 57% of synovial sarcomas. The fluid-fluid levels are caused by tumor necrosis or sedimentation of blood products and are detected in 10% to 25% of cases. These features can also be observed in other soft tissue tumors and are not pathognomonic, but they are described more frequently in synovial sarcoma. In addition, calcification occurs in up to 30% of synovial sarcoma cases more frequently than in other soft tissue sarcomas, and extensive calcification has been associated with a more favorable prognosis [6,18]. The CT imaging findings of the present case, including minimal calcification, are similar to those of large-sized synovial sarcomas in human medicine. Rapid clinical deterioration is limited to an evaluation of the prognostic significance of calcification, but the prognosis would be poor given the presence of multiple metastases [9,16,17,19].
The regional and distant metastasis rate of synovial cell sarcoma was reported to be 22% to 31%, with common sites including regional lymph nodes and lungs [9]. In the present case, the extensive metastases were detected in unusual locations, such as the talus, skeletal muscles, and myocardium, which represented the aggressive nature of this sarcoma. Although histologic confirmation of all suspected metastatic lesions was not performed, the CT and MRI findings strongly suggested extensive tumor metastasis. Metastases of synovial sarcoma in dogs have been reported in the liver, kidney, spleen, pleura, heart, skin, and brain, but their imaging features have not been described [9,20]. This highlights the importance of comprehensive imaging to assess the extent of metastasis in synovial cell sarcoma.
This paper presented the clinical significance of CT and MRI for assessing bone lysis and calcifications, tumor extent and metastasis, and in identifying the ‘triple sign’ and fluid-fluid levels in a synovial cell sarcoma in a dog. Recognition of these imaging features may raise suspicion for synovial sarcoma, even in periarticular tumors with multiple metastases. Further studies will be needed to confirm their diagnostic value.
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