Clinical history An older mixed breed dog had been treated for lameness in the pelvic limbs for several years. The pain was increasing and now the dog was being considered for a total hip replacement. Radiographs were made of the pelvis prior to surgery. The radiographic changes are bilateral and those of arthrosis in the hip joints assumed to be secondary to hip dysplasia. It was noted that soft tissue atrophy was prominent on the left. Further palpation of the left pelvic limb suggested additional stifle joint pain not noted on earlier physical examination. Radiographs of the stifle joint were made. The highly destructive lesion in the proximal tibia included cortical destruction (arrows). The zone of transition between lesion and normal bone was indistinct. The lesion appeared to extend to the subchondral bone proximally, but did not enter the stifle joint. Periosteal new bone was not a part of the radiographic features. Soft tissue swelling was not evident. What is your diagnosis considering these radiographic changes? Diagnosis The changes are typical for a primary bone tumor. The location in the tibia is a less common site but the location in the metaphysis/epiphysis if typical for a primary bone tumor. The extent of cortical destruction and the finding of a solitary lesion makes a metastatic tumor unlikely. The absence of periosteal response and indistinct zone of transition makes osteomyelitis unlikely. Pulmonary metastasis were evident on thoraxic radiographs and the dog was euthanized.
