Malignant synovial and perisynovial tumor in the stifle joint of a juvenile cat: histopathological and immunohistochemical findings.

Case summary: A 1-year-old domestic cat was presented with lameness and progressive swelling of the right stifle joint. The condition had been present for 1 month; during the consultation, it was evident that the cat was not bearing weight on the limb, which also exhibited muscle atrophy and tremors, as well as signs of pain upon manipulation. After sedation, crepitus was noted at the femorotibial-patellar joint. Radiographic examination showed osteolysis, and during clinical exploration, tumor cells destroying the joint were confirmed by cytology. Surgical excision was performed via limb amputation, and the mass was found to measure 3 × 5 × 3 cm. Histopathological analysis showed a biphasic neoplasm composed of spindle-shaped mesenchymal-like cells and large epithelial-like cells. Normal joint structures, including the capsule and articular cartilage, were completely effaced by neoplastic infiltration, including similar neoplastic changes in distal femur and proximal tibia epiphyses, metaphyses and diaphyses. Tumor cells were immunopositive for either vimentin or cytokeratin. Based on the 2020 World Health Organization (WHO) classification, the tumor was diagnosed as a malignant synovial and perisynovial neoplasm. The cat died 3 months after surgery; at the owner's request, no post-mortem examination was conducted. Relevance and novel information: To the authors' knowledge, this is the first reported case of malignant synovial and perisynovial neoplasia diagnosed in a juvenile cat in accordance with the 2020 WHO classification. This case provides new insights into the histopathological findings of this rare tumor type in feline patients.