Malignant fibrous histiocytoma of the deep peri-articular tissue of the stifle in a dog

A Belgian shepherd dog aged 4 years and 9 months was presented with acute o nset of non-weight bearing lameness and stifle effusion of the left hind li mb, caused by the deep form of a malignant fibrous histiocytoma originating in the deep musculature and fascia surrounding the stifle joint. The tumou r progressed rapidly in the tissues along the femoral diaphysis with marked periosteal new bone formation. Cytology of a stifle joint aspirate reveale d numerous large polygonal neoplastic cells with considerable anisocytosis and anisokaryosis. These cells were present in clusters, with cytoplasmic p rojections between the cells, but occasionally also occurred singly. Severa l cells contained multiple cytoplasmic vacuoles and occasional giant cells were also encountered. Adequate tumour-free margins were not possible with radical limb amputation and the dog was euthanased. Macroscopically the tum our appeared as an extensive pale tan, firm mass with scattered small haemo rrhages and foci of yellow discolouration. Histologically the tumour consis ted of dense neoplastic expanses or multiple nodules, composed of spindle-s haped fibroblastic cells, polygonal histiocytic cells or cell clusters and scattered giant cells with 2-3 nuclei. The polygonal neoplastic cells were frequently present around and invading lymphatics and blood vessels, causin g neoplastic emboli. This is the Ist report of the clinical behaviour, radi ography and cytology of the deep form of malignant fibrous histiocytoma in the dog.