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Osteosarcoma of the talus in a dog.

A six-and-a-half-year-old, 41 kg, male, black Labrador retriever was referred to the Ontario Veterinary College Teaching Hospital for evaluation of a progressive left hind limb lameness of 10-weeks duration. There was no history of injury or previous lameness in the affected limb. Six weeks prior to presentation, the dog had been examined by another veterinarian who had noted a mild swelling of the hindpaw. No specific diagnosis was made at that time, and empirical treatment with prednisone was initiated. Although the swelling of the phalanges resolved, the tarsus remained swollen and painful. Despite continued prednisone therapy, the swelling was markedly worse four weeks later. On presentation, the dog was not bearing weight on its left hind limb, and there was firm swelling from the tarsus distad. Serosanguineous fluid oozed from the skin over the tarsus and metatarsus, and a fetid, purulent discharge exuded from the swollen phalanges. A fistulous tract was also present on the medial aspect of the tarsus. Pronounced pain and swelling of the tarsus precluded assessment of the range of motion and degree of stability of the joint. There were no significant abnormalities present in the results from a complete blood count and a routine biochemistry profile, aside from a mild elevation in alkaline phosphatase (290 pIL; reference range 0-200 pIL). Radiographically, severe osteolysis of the talus was apparent, with possible involvement of the distal end of the tibia and part of the calcaneus (Figure 1). An outwardly streaking "sunburst" effect could be seen radiating from the talus, particularly over the plantar aspect of the calcaneus on the lateral view. Diffuse soft tissue swelling from the tarsus distad was also evident. Differential diagnoses based on these findings included septic arthritis/osteomyelitis and several types of neoplasm. Given the apparent intra-articular nature of the lesion, synovial sarcoma was the most likely diagnosis, if the lesion was indeed neoplastic. Osteosarcoma, chondrosarcoma, fibrosarcoma, and hemangiosarcoma were other diagnostic considerations. No evidence of metastatic lesions was detected on right and left lateral and ventrodorsal radiographic views of the thorax. Fine needle aspirates of the tumor were examined; a cell population consisting of spindle and epithelioid cells was seen. These cells had a high nuclear to cytoplasmic ratio, multiple nucleoli, marked anisocytosis and anisokaryosis, and numerous mitotic figures. No inflammatory cells were observed. A cytopathological diagnosis of stromal cell tumor was made. Based on this presumptive diagnosis and the degree of destruction within the tarsus, midfemoral amputation was elected.