Osteosarcomas are highly metastatic cancers. Almost all cases of osteosarcoma will eventually develop metastasis. Most commonly metastasis develops in the lungs, however other sites such as other bones or abdominal organs are also possible. Even if metastasis can’t be detected at the time of diagnosis it is highly likely to develop in future. However dogs that have no visible signs of metastasis at diagnosis (based on scans) have a significantly better prognosis than those that already have signs of metastasis.
Osteosarcoma (OSA) is a malignant cancer arising within the bones from the malignant transformation of bone cells called osteoblasts. Osteoblasts normally function in the skeleton to produce bone. OSA is the most common primary bone cancer in dogs. OSA most commonly occurs is large and giant breed dogs, but any breed of dog can be affected. OSA can arise at any age.
Like other cancers, older dogs tend to get more OSAs, however it is one of the few cancers that is relatively common in younger dogs at around 1.5-2 years of age. The breeds at highest risk for developing OSA include Rottweilers, Greyhounds, Great Danes, Irish Wolfhounds, German Shepherd Dogs and Golden Retrievers.
The most common sites for OSA to develop is the limbs, however it can develop in any bone and rarely in non-bony tissues. Most dogs with OSA will initially present with a progressive lameness or pain associated with the tumour. As the tumour grows it weakens the bone and in some cases the first sign of the cancer is when the bone breaks, causing a sudden severe lameness.
Diagnostics for limb OSA in dogs
Often the first test performed is an X-ray of the affected leg. X-ray changes which are suggestive of an osteosarcoma include:
- Bone loss (called “osteolysis”)
- Abnormal bone formation (called “osteoproliferative change”)
A presumptive diagnosis of osteosarcoma is often made when a dog has: 1) a history consistent with osteosarcoma, and 2) X-ray changes in the bone suggestive of osteosarcoma.
Can be performed to plan treatment and give prognostic information
Scans of the lungs:
Chest X-rays are the most common type of scan performed and give a good overview of the lungs. A CT scan can also be performed which is more sensitive than X-rays at picking up nodules in the lungs (so nodules can be seen earlier than on X-rays).
Scans of the bones and abdomen
It is far more common for OSA to spread to the lungs than other sites, so these scans are less essential than the lung scans, however some osteosarcomas will spread to sites other than the lungs. So if we want to fully assess a dog we ideally also scan the bones (ribs, spine, and pelvis) and abdomen.
A CT scan is the best way to assess the bones and, especially for larger dogs, the abdominal organs. Alternatively X-rays of the bones of the chest (included in the X-ray of the lungs) and an ultrasound of the abdomen can be performed.
Scans of the OSA
These usually do not need to be repeated after the initial X-ray. Times when a repeat scan might be recommended include:
- The lameness suddenly gets worse and we want to check the bone for a fracture.
- If we are considering performing limb-sparing surgery we need to CT the limb to assess the extent of the cancer on the inside of the bone.
Fine needle aspirate of the mass
This may be performed to make sure the cells in the affected area look like osteosarcoma cells and not like some other rare cause of the bone changes (such as a different type of cancer or a bone infection). This test does not 100% confirm the diagnosis but is a screening test for rare diseases before the dog proceeds to treatment.
Routine blood tests
These are often recommended to assess the overall health of your pet, especially if surgery is being considered. These also give some prognostic information, as the level of “ALP” (a specific enzyme in the blood) can give some prognostic information if it is increased.
Treatment options for limb OSA in dogs
There are 2 main aims of treating osteosarcoma in dogs
1. Stop the pain from the mass within the bone
The most common way to do this is surgically removing the painful area of bone. This is the effective way of fixing the pain. For most osteosarcomas this involves amputation of the leg. This is the only option if the bone has fractured due to the tumour. Our canine patients function incredibly well after limb amputation. Most dogs able to be as active as before surgery.
A typical 3-legged dog
In rare cases we can consider “limb sparing surgery” which involves removing only part of the bone and replacing the bone with an implant.
2. Slow down the development of metastasis
This involves giving chemotherapy. Chemotherapy is most commonly used after surgery, once the surgical site has fully healed (usually ~2 weeks after surgery). This approximately doubles the survival time of dog compared to those who do not receive chemotherapy. The most common chemotherapy protocol involves giving a dose of IV chemotherapy once every 3 weeks for a total of 4-6 treatments. Most dogs tolerate this very well with few side effects.
In general, for dogs with tumours of the limbs that are treated with amputation:
- If metastasis is visible on scans before surgery, the average survival is ~2 months.
- If metastasis is not visible on scans, dogs treated with amputation alone have an average survival time of ~5 months before metastasis leads to euthanasia.
- If metastasis is not visible on scans, dogs treated with amputation AND chemotherapy alone have an average survival time of ~10 months before metastasis leads to euthanasia.
Dogs that do not have their cancer surgically removed will usually eventually be put to sleep because of worsening pain at the site of their cancer, usually within a few months of diagnosis.