Osteosarcoma

What it is

Osteosarcoma is a malignant tumour of mesenchymal origin whose cells produce osteoid. It is by far the most common primary bone tumour in the dog. It destroys the bone in which it arises while producing disorganised tumour bone, so the lesion combines lysis and proliferation. Its biological behaviour is very aggressive: at the time of diagnosis most dogs already have pulmonary micrometastases, even if they are not visible on radiographs.

Who it affects

Large and giant breeds: Rottweiler, Great Dane, Irish wolfhound, greyhound, Saint Bernard, golden retriever, German shepherd, Dobermann. Body weight and height are more important risk factors than breed itself. Most cases occur in middle-aged to older dogs, with a small secondary peak in young animals.

Anatomy involved

Most cases arise in the appendicular skeleton, in the metaphysis of the long bones, and the thoracic limb is affected more often than the pelvic limb. The mnemonic "away from the elbow, toward the knee" sums up the preferred sites: distal radius and proximal humerus in the thoracic limb; distal femur and proximal tibia in the pelvic limb. The tumour usually respects the articular cartilage and does not cross the joint, but it invades the cortex and the surrounding soft tissues.

Clinical signs

Progressive lameness that responds poorly to anti-inflammatories, and a firm, painful swelling at the metaphysis. It may present suddenly as a pathological fracture. In axial bones (skull, ribs, vertebrae, pelvis) the signs depend on the location.

Diagnosis

• Radiographs of the affected bone: mixed lytic and proliferative metaphyseal lesion, loss of cortex, "sunburst" periosteal reaction, Codman's triangle at the margins, soft-tissue swelling and sometimes a pathological fracture. The transition zone with healthy bone is wide and poorly defined.
• Cytology or biopsy: fine-needle aspiration or core biopsy from the centre of the lesion, not from the reactive periphery, to confirm the diagnosis and rule out fungal osteomyelitis or other sarcomas.
• Staging: thoracic radiographs or CT to look for metastases, blood work (a raised alkaline phosphatase worsens the prognosis) and, where available, scintigraphy or whole-body CT.

Treatment

Amputation of the limb removes the pain and the primary tumour; combined with chemotherapy (carboplatin or doxorubicin) it prolongs survival markedly compared with amputation alone, although most dogs eventually die of pulmonary metastases within months to a little over a year. Limb-sparing surgery is mainly reserved for the distal radius. Radiotherapy, bisphosphonates and multimodal analgesia are used for palliation.

In the cat

Primary bone tumours are uncommon in the cat, but osteosarcoma is still the most common of them. It affects older cats, without the size predisposition seen in dogs, and occurs both in the appendicular skeleton, more often the pelvic limb (femur and tibia), and in the axial skeleton (skull, pelvis, vertebrae). Extraskeletal osteosarcomas also occur, some at injection sites.

The most important clinical difference is its behaviour: feline appendicular osteosarcoma metastasises less often than the canine tumour, although recent series report metastases in 30–46% of cats. As a result, limb amputation alone is associated with longer survival than in the dog, with medians of over a year, in cats without metastases at diagnosis; the benefit of adding chemotherapy is not settled. Diagnosis and staging follow the same steps as in the dog; in the axial skeleton the prognosis depends on whether the tumour can be removed completely.

Key studies in the cat

• Heldmann et al. (2000), 145 feline osteosarcomas: 62% skeletal (50 appendicular and 40 axial) and 38% extraskeletal, most of them subcutaneous and with an apparent preference for common vaccination sites. Mean survival was shorter for axial tumours (6.1 months) than for appendicular (11.8) and extraskeletal (12.7) tumours. Survival data were available for only 74 cats.
• Dimopoulou et al. (2008), 62 cats with osteosarcoma, compared with dogs with tumours of the same location and subtype. Histology was similar in both species, but prognosis was not. In the cat, histological grade, mitotic index and surgery influenced outcome.
• Nakano et al. (2021), 67 cats with appendicular or scapular osteosarcoma treated by amputation. Distant metastasis occurred in 46.3% overall and in 41.9% after amputation, with a much higher risk in the humerus (6 of 7 cases). Local recurrence (9%) and lymph node metastasis (3%) were rare. Median survival was 527 days.
• Marconato et al. (2024), a multicentre study of 56 cats with appendicular osteosarcoma, mostly of the femur. The overall metastasis rate was 30%. Operated cats lived longer than untreated ones: 66% were alive at 1 year and 55% at 2 years, compared with 25% and 0%. Only 4 received chemotherapy, so its role remained unclear.
• Kimura et al. (2026), a multicentre study of 76 cats with primary appendicular bone tumours treated by amputation, most of them osteosarcomas. For osteosarcoma, median survival was 469 days and the metastasis rate 36.2%; scapular tumours metastasised most often (87.5%). In cats without pulmonary metastasis before surgery, adjuvant chemotherapy was associated with much longer survival (1466 versus 440 days). It is retrospective and chemotherapy was not randomly assigned.