What is it?
Osteosarcoma is a cancer involving bone - in fact, 85% of cancers involving bone are osteosarcomas. Common sites for bone cancer are the end of the radius in the foreleg (the ‘wrist’ area), the end of the femur in the hindleg (the end of the thigh bone), the top of the humerus in the foreleg (just below the shoulder joint) and the top of the tibia (just below the knee joint) in the hindleg.
Osteosarcoma is most common in large breed dogs. Breeds weighing more than 35kg are 60 times more at risk of osteosarcoma than breeds of less than 10kg body weight.
This type of bone cancer mostly affects middle-aged to older dogs.
Why is it important?Osteosarcoma is a particularly aggressive form of cancer. It can grow rapidly and it can spread from the primary site to other bones, and to the internal organs. It often causes severe pain in the affected limb. Osteosarcoma is a serious welfare concern and must be managed appropriately.
What is the risk?50% of osteosarcomas will spread to other bones, and 50% will spread to the lungs. The bone is seriously weakened by the cancer and fractures can occur with only slight injury (e.g. jumping down from a car). These are known as pathological fractures, and they are intensely painful, Without treatment, affected dogs almost always need to be euthanased within weeks of diagnosis due to extreme pain.
What happens to the dog?In its earliest stages, there may be no apparent symptoms. However, as the tumour grows, there will be swelling associated with one of the bones described above. Dogs affected by osteosarcoma will usually be lame on the affected limb, and there is usually a marked pain response when the limb is examined.
How do you (the vet) know what is going on?
The dog is fully examined. X-rays of the affected limb are carried out, usually under sedation. Osteosarcoma shows as a particular pattern of destruction (called “lysis”) of bone, and new bone formation at the same site.
The diagnosis may be confirmed with a bone biopsy. This involves taking a sample of the bone tumour under general anaesthetic and sending it to the laboratory.
If the X-rays show that osteosarcoma is likely, staging of the cancer can be carried out. X-rays of the chest and ultrasound scanning of the abdomen can detect if the tumour has spread. Only around 10% of osteosarcomas will show signs of spreading (metastasis) to the lungs at diagnosis, but more than 90% will have micrometastasis (small cells from the cancer that have spread to the organs but cannot be seen on scans). These small cells go on to form secondary tumours in the lungs, other bones and internal organs.