History

A 5-year-old spayed female Golden Retriever–Chow Chow crossbred dog was evaluated because of a progressively enlarging subcutaneous mass overlying the caudolateral surface of the right femur. The mass had been approximately 10 cm in diameter when it was first noticed 5 months earlier. On evaluation, the dog had a non–weight-bearing lameness of the right hind limb. Direct palpation of the mass did not elicit signs of pain. Texture varied regionally from very hard to soft. The dog had no clinical signs of systemic illness. Findings on CBC, serum biochemical analysis, and thoracic radiography were unremarkable. There was no history of previous infection or surgery of the limb. The dog resided on a farm, and unobserved trauma was possible. Radiographs of the pelvis and right femur were obtained (Figure 1).

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Lateral (A) and ventrodorsal (with legs abducted at 90°, frog-legged; B) radiographic views of the pelvis and right femur of a 5-year-old spayed female Golden Retriever–Chow Chow crossbred dog evaluated because of a 5-month history of lameness and a subcutaneous mass.

Citation: Journal of the American Veterinary Medical Association 237, 1; 10.2460/javma.237.1.33

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Determine whether additional diagnostic imaging is required, or make your diagnosis from Figure 1—then turn the page →

Radiographic Findings and Interpretation

Overall bone opacity of the right hind limb is reduced, especially in the femoral head, consistent with disuse osteopenia (Figure 2). A 21 × 22 × 14-cm mass surrounds the right femur. The caudal portion of the mass contains a marked amount of well-organized mineralization; a small amount of poorly organized mineralization is seen in the cranial aspect. Several areas of cortical thinning are seen along the caudal aspect of the femur.

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Same radiographic views as in Figure 1. Notice the large amount of well-organized mineralization in the caudal aspect of the mass (black arrow), compared with the poorly organized and smaller amount of mineralization in the cranial aspect (white arrow). Several areas of cortical thinning are seen (arrowheads). Assessment of medullary involvement is difficult because the circumferential nature of the tumor results in superimposition.

Citation: Journal of the American Veterinary Medical Association 237, 1; 10.2460/javma.237.1.33

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Initial differential diagnoses for the mass included calcinosis circumscripta, mineralized hematoma, and either periosteal or parosteal osteosarcoma. The cortical defects were thought to represent local erosion from the mineralized mass or neoplastic invasion.

Comments

Cytologic examination of fine-needle aspirates obtained from multiple sites within the mass yielded small numbers of moderately pleomorphic mesenchymal cells with pink cytoplasmic granules. The extracellular matrix varied regionally from myxomatous to either chondroid or osteoid. These findings were inconsistent with calcinosis circumscripta and suspicious for sarcoma; however, poor cellularity precluded a definitive diagnosis.¹ Given the proximal and medial extent of the mass, simple amputation was unlikely to be curative.

Computed tomography or magnetic resonance imaging and an incisional biopsy were recommended to determine resectability and prognosis. Further diagnostic tests were declined, and the dog was euthanatized 6 months later following development of limb edema. On gross postmortem examination, the mass abutted the femoral cortex. The caudal portion consisted of multifocal 3- to 5-mm-diameter osseous foci embedded within a gray matrix admixed with abundant necrosis and hemorrhage. The cranial portion was translucent yellow and devoid of overt osseous foci or hemorrhage. Microscopically, the cortical and medullary trabecular bone of the proximal portion of the right femur were multifocally effaced by markedly disorganized pleomorphic chondrocytes embedded within an Alcian blue positive matrix. The soft tissue component was divided into lobules that contained regions of cavitation and mineralization but no osteoid. Mitotic figures were not seen. The histologic diagnosis was grade II chondroid chondrosarcoma. There was no evidence of metastatic disease or histologic involvement of the pelvis.

Chondrosarcoma (CSA) is the second most common primary bone tumor in dogs and occurs more frequently in flat than in long bones.² Chondrosarcomas can arise within the bone (medullary or central CSA) or from the periosteum (peripheral or periosteal CSA).² The origin of this mass is difficult to determine. Radiographically, there was little cortical bone destruction relative to tumor size. This suggests periosteal origin because peripheral CSAs often have substantial soft tissue expansion with delayed cortical and medullary bone invasion.³ A large soft tissue component and minimal medullary involvement can also occur when central CSAs breach cortical bone early in the course of disease.² As a result, radiographic appearance does not reliably distinguish between the 2 types of CSAs. Site of origin has prognostic importance for humans because peripheral CSAs are more frequently low grade.^3,4 Differences in biological behavior between central and peripheral CSAs have not been investigated in dogs, and this distinction is rarely made.²

Treatment for CSAs consists of wide surgical excision.⁴ No effective adjuvant chemotherapy has been defined.⁴ This dog survived > 11 months without treatment and was euthanatized for local disease progression with no evidence of metastasis.