Case Name: Synovial Cell Sarcoma Signalment: 6-year old castrated male Rottweiler Presenting Complaint and Patient History: Severe lameness of the left rear leg, with a recent onset of swelling around the left stifle.  No history of trauma. Physical Examination: Toe-touching lame on the left rear limb, with moderate edema extending distally from the stifle.  Physical exam was otherwise within normal limits. Diagnostic Imaging Studies: VD view     Radiographs of the left stifle show lytic lesions of the distal femur, proximal tibia and patella. Metallic implants from a prior surgery are visible in the proximal tibia.   Thoracic radiographs (not shown) are unremarkable. Cytology Images: Cytology Description Impression smears were made from a biopsy of the mass located near the stifle.  The smears are densely cellular and most of the nucleated cells are a population of neoplastic mesenchymal cells that are pleomorphic in shape, varying from round to angular or spindled.  They typically have a single nucleus with a moderate amount of medium blue cytoplasm, and often show moderate cytoplasmic vacuolation.  The nuclei of these cells are round or slightly indented in shape.  They have a finely stippled chromatin pattern, and most contain 1-2 prominent nucleoli.  There is moderate anisocytosis and anisokaryosis.  Rare binucleated cells are present.  A few heavily vacuolated macrophages, nondegenerate neutrophils and small lymphocytes are admixed with the neoplastic cells. Opinion:  Sarcoma.  Based on the location and the vacuolated appearance of the cells, likely differentials include synovial sarcoma or histiocytic sarcoma. Biopsy with histopathology is recommended to make a definitive determination of tumor type. Histopathology Images: H&E; 10X alpha-1-anti-trypsin; 20X Vimentin; 20x Histopathology Description H&E:  Sheets of pleomorphic, spindle-shaped to polygonal cells invade the stroma of the joint capsule.  Note the variation in nuclear size (anisokaryosis), a feature of neoplasia. Vimentin immunostained section:  Neoplastic cells bind antibody to vimentin, confirming the mesenchymal origin of the neoplasm. Alpha-1-anti-trypsin immunostained section:  Neoplastic cells do not bind antibody to alpha-1-anti-trypsin, a macrophage marker.  This demonstrates that the neoplasm is not a histiocytic sarcoma. Diagnosis:  Synovial cell sarcoma, left stifle