Cytology examination of a synovial fluid sample from the right elbow was performed. This revealed low cellularity composed predominantly of large mononuclear cells (occasionally vacuolated), with lower numbers of small mononuclear cells and spindle cells. Large mononuclear cells (Figure 1A) had round nuclei with prominent single or multiple small nucleoli and moderate amounts of mid-basophilic cytoplasm. There was mild anisocytosis and anisokaryosis and occasional mitotic figures were present (Figure 1B). Occasional binucleated or multinucleated spindle cells were also noted (Figure 1C).
Sampling of structural lesions with biopsy for histopathology examination was advised to rule out neoplasia.
Biopsies were submitted from joint capsule and adjacent muscle of right forelimb. Throughout the connective tissue there were infiltrates of plump spindle cells forming short interweaving bundles. The cells had oval nuclei with irregular outlines and reticulate chromatin patterns. Cells contained small nucleoli. They had low to moderate amounts of fibrillar cytoplasm with indistinct borders. There was mild anisocytosis and within this population there were mitotic figures (Figure 2). Sections from the joint capsule consisted of dense fibrocollagenous connective tissue with scattered isolated blood vessels and the same plump spindle cells forming interweaving bundles (Figure 3A) or whorls (Figure 3B) and aggregates.
A diagnosis was made of a canine soft tissue mesenchymal tumour (sarcoma) which appears to be of low to intermediate grade. Differentials include fibrosarcoma, perivascular wall tumour or peripheral nerve sheath tumour.
Historically, synovial cell sarcomas have been reported commonly in dogs. However, more recently studies using immunohistochemistry have shown that many different types of mesenchymal tumours can develop in and around joints in dogs. The majority are not of synovial cell origin. This particular tumour was considered to be of connective tissue origin. Soft tissue sarcomas (mesenchymal tumours) tend to behave in the same way regardless of the exact cell of origin. Given the cytological features, mitotic activity and absence of necrosis this may be a low-grade tumour, although on examination of larger areas of the tumour this grade may increase. Such tumours are locally infiltrative and frequently recur following surgery due to difficulty in achieving adequate excision margins at awkward surgical sites and the ability of these tumours to dissect between fascial planes. Metastasis, however, is uncommon or develops late in the course of the disease, often by haematogenous rather than lymphatic spread.
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