Neoplastic Arthritis in Dogs and Cats

Neoplastic arthritis is joint inflammation and structural damage caused by primary or metastatic neoplasia affecting the joint or periarticular structures. Although rare in dogs and cats, neoplastic arthritis must be considered in cases of chronic joint swelling, pain, or nonresponsivelameness, particularly in older animals.

Although synovial tumors are relatively uncommon in dogs, there are several distinct types, including synovial cell sarcoma, histiocytic sarcoma, synovial myxoma, and other undifferentiated sarcomas. It is crucial to note that modern veterinary literature advises against using the broad term "synovial cell sarcoma," favoring the precise classification of these lesions as synovial histiocytic sarcomas or synovial myxosarcomas. These tumors typically affect older large-breed dogs.

Clinical signs of neoplastic arthritis include lameness, joint swelling, or a periarticular mass, often mimicking immune-mediated polyarthritisor degenerative joint disease (DJD).

Because of the similar clinical and histological appearance of these various tumors, immunohistochemical (IHC) testing—particularly looking for markers like cytokeratin for synovial cell sarcoma and CD18 for histiocytic sarcoma—is crucial for accurate diagnosis. Without IHC testing, tumor types are often misclassified.

Testing for cytokeratin in veterinary medicine is now considered unnecessary for the diagnosis and prognostic evaluation of tumors believed to be of synovial origin, because it lacks diagnostic specificity and has limited clinical relevance. The emphasis has shifted to accurate classification using CD18 and morphology.

In cats, neoplastic arthritis is even rarer than in dogs. Possible neoplastic causes of joint disease in cats include synovial sarcoma, histiocytic sarcoma, and lymphoma.

Synovial cell sarcoma, once a broad diagnosis, has been more precisely defined. The tumor arises from primitive mesenchymal cells outside the synovial membrane. Synovial cell sarcomas occur in older dogs, more commonly in males, and some breeds (eg, Flat-Coated Retriever and Golden Retriever) might be overrepresented.

Clinical signs of synovial cell sarcoma in dogs and cats include lameness and joint swelling.

Radiography reveals soft tissue swelling, bone lysis, and periosteal reaction (see synovial cell sarcoma image). Synovial fluid analysis might reveal hemorrhage, high cellularity, or neoplastic cells.

Synovial cell sarcoma, radiograph, dog

Image

Courtesy of Dr. Ronald Green.

This type of tumor has an 8–32% metastatic rate at diagnosis, and regional lymph nodes and lungs are the most common metastatic sites (1).

The prognosis in cases of synovial cell sarcoma in dogs varies. One study reported a mean survival time of approximately 32 months, particularly when treated with amputation and, in some cases, chemotherapy. Mean survival time is approximately 93 days with no treatment, and 455 days to 17 months overall (2, 3).

Synovial cell sarcoma is uncommon in cats.

In contrast, histiocytic sarcoma, which has been found to be more common than was previously thought, especially in Rottweilers and in dogs with chronic stifle joint inflammation, carries a much poorer prognosis. Affected dogs have a mean survival time of 5.3 months, with a high rate of metastasis to the lungs, lymph nodes, and liver (3).

Synovial myxosarcoma accounted for approximately 17% of cases in one retrospective study, and Doberman Pinschers were overrepresented. These tumors had a more favorable outcome, with mean survival times > 30 months (3). Some dogs were successfully treated with local excision; however, amputation remains the standard approach for most synovial tumors when metastasis is not present.

Definitive diagnosisof neoplastic arthritis in dogs and cats requires evidence, via biopsy and histological examination, of a soft tissue tumor. IHC testing is essential for accurate diagnosis of the tumor type. Staging (via thoracic radiography/CT, abdominal ultrasonography, and lymph node assessment) and advanced imaging for surgical planning are recommended to determine the most adequate treatment and prognosis.

Although amputation is the mainstay of treatment for most synovial tumors, survival and metastatic risk vary substantially by tumor type, underlining the importance of distinguishing between types. Chemotherapy may also be recommended, especially in tumors with high metastatic potential.

New information is coming out regarding incorrect use of the term "synovial cell sarcoma" (SCS) and the use of cytokeratin immunohistochemistry to determine its location. The original use of the term "synovial cell sarcoma" in dogs was based on a human tumor, the human synovial sarcoma. However, the human tumor is known not to be of true synoviocyte origin, but rather a sarcoma with epithelial-like features (hence the cytokeratin positivity).

In dogs, the tumors previously classified as SCSs on the basis of cytokeratin expression are now recognized as a group of tumors that simply have epithelial differentiation (or cytokeratin positivity) but do not necessarily arise from the synovial lining. The presence of cytokeratin positivity does not greatly affect the patient's prognosis or survival time. Therefore, testing for it is not clinically helpful for predicting patient outcome.

The current focus is on accurately identifying tumors that are of true synovial origin—namely, synovial histiocytic sarcomas (SHSs) and synovial myxosarcomas (SMs)—which have distinct biological behaviors and require different diagnostic markers: SHSs are confirmed with histiocytic markers like CD18. Diagnosis of SMs is based on their characteristic myxoid histological pattern (3).