Bone tumours usually occur in the bones of the limbs (average of 75%). This tumour type is known to appear twice as much on the front limb as on the hind limb and has a preferential location per bone. Twenty-five % of bone tumours appear in the skull and spine (27% at the lower jaw, 22% the upper jaw, 15% spine, 14% skull, 10% ribs, 9% nasal cavity/sinus near the nose, 6% hip bones). Although rare, bone tumours can also be found in mammary glands, subcutaneous tissue, spleen, intestines, liver, kidney, testicle, vagina, eye, stomach ligament, the delineation of the articulation (synovium), meninges, adrenals.

Behaviour

Osteosarcomas are very aggressive tumours. They invade the bone locally, cause bone loss and/or produce inferior bone. Hence, this process could lead to a spontaneous fracture of the involved bone. Usually, a swelling of the tissue is visible where the tumour is located.

Metastases are common and develop early during the course of the disease. Of note, they usually don’t cause any symptoms and therefore often go unnoticed (= subclinical). Although at the time of diagnosis less than 15% of dogs will show detectable metastases on radiographs, about 90% of dogs dies within one year with metastases in the lungs (when treatment was limited to surgery). This tumour type metastasizes usually via the blood, but in rare cases it can also be found in the local lymph nodes. The lungs are the most reported location for metastases, followed by the bones or soft tissues.

Prevalence

This tumour type is most prevalent in average-aged to older dogs (most commonly seen at 7 years), but it has been reported in young (1-2-year-old) dogs as well.

Breed predisposition

A higher risk for development of this tumour type was reported for large and giant breeds (Saint-Bernard, Danish Dog, Irish Setter, Dobermann, Rottweiler, German shepherd, Golden retriever). As a general rule, large dogs have a greater risk of developing on bone tumour on the limbs, whereas smaller dogs, a greater risk of developing one in (the extension of) the spine (= axial skeleton).

To be more precise:

1. Dogs with a weight above 40 kg represent 29% of the dog population with bone tumours. In these dogs, only 5% of bone tumours occurs in the skull or spine (axial skeleton).
2. Dogs weighing less than 15 kg represent 5% of the canine population with bone tumours. In these dogs, 59% of the tumours are located in the skull and the spine.

Symptoms

The symptoms depend on the tumour location. If an osteosarcoma is situated on a limb, there usually will be a local swelling that can be painful and lead to limping. If it’s situated on the axial skeleton, the symptoms can vary and include a local swelling with or without limping, difficulties eating (if in the upper or lower jaw), mild protrusion of the eye and showing discomfort when the mouth is opened (if in the orbit and the lower jaw), facial deformation and nasal discharge (if in the sinus and nasal cavity), being overly sensitive with or without neurological symptoms (if in the spine). Respiratory problems rarely occur, even if radiographs of the chest clearly show the presence of metastases.

Furthermore, a lower energy level can occur. An increased level of calcium in the blood (hypercalcemia) can be present in the dog’s bloodwork, but is very rare.

Imaging

The veterinarian can collect a lot of information via radiographs about the possible presence of a bone tumour. In case of osteosarcomas, bone destruction is expected to be visible on the radiograph, as well as disorganized bone production due to irritation of the bone membrane. However, in a very early disease stage, these lesions may not be visible via radiographs. In that case, a CT-scan could be more appropriate.

Histology

Via a biopsy, the type of bone tumour can be confirmed. Via scintigraphy and aspirates of (local) lymph nodes, the veterinarian can visualize the distribution of the osteosarcoma. Other options to map the tumour distribution include a combination of radiographs and/or CT scan, ultrasound of the abdomen to detect metastases towards the lymph nodes and internal organs.

Blood analysis

Some blood values can indicate an increased bone activity.

The treatment approach depends on the tumour location and the presence of metastases. Furthermore, an osteosarcoma can irritate the bone membrane, which is painful and requires appropriate painkilling. The chances of therapeutic success increase as more treatment options (surgery, chemotherapy, radiotherapy) are combined. Osteosarcomas often require local (surgery, radiotherapy) as well as general (chemotherapy) treatment (in case of metastases).

Surgery

If possible, the veterinarian will surgically remove the tumour. This is relatively simple for osteosarcomas located in the limbs, but less evident for tumours in the skull or spine. When the tumour causes pain due to its size and location in the spine, a part of the tumour can be removed (decompression) to decrease pressure and offer more comfort to the dog. Usually, surgical removal of a tumour in the limb results in the amputation of the paw. In certain cases, limb-sparing surgery can be considered, but this is a complex procedure requiring a team of veterinarians. Amputation of the limb can be considered for large dogs if the remaining 3 paws are healthy (e.g. do not harbour advanced arthrosis). Metastases can be surgically removed to somewhat prolong life or to improve the quality of life for dogs in whom -as a consequence of metastases towards the lungs- hypertrophic osteopathy occurs (abnormal widening of the bone as a result of new bone production). When the metastases are removed, the symptoms associated with hypertrophic osteopathy disappear as well.

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Chemotherapy

Chemotherapy is usually advised after surgery, even when no metastases were detected at the time of diagnosis, because this tumour type metastasizes very frequently. At this time, chemotherapy isn’t very satisfactory in the treatment of metastases. Presumably, this is due to the limited access/distribution of this medication within the tumour and metastases. This approach will possibly become more important in the future when it can be combined with medication that enhances the access to the tumour and metastases.

Receptor tyrosine kinase inhibitors such as Masivet® and Palladia® are tablets that can be administered orally and block growth receptors within the tumour. As with chemotherapy, receptor tyrosine kinase inhibitors only work well when they can efficiently be distributed within the tumour. So far, this treatment has not led to a cure, but during a study with 23 dogs, a growth delay was observed in half of the tested dogs.

Radiotherapy

Radiotherapy (at higher doses) is often combined with limb-sparing surgery and done prior to surgery. High doses can also be used to cause necrosis within the tumour when the tumour’s location does not allow its surgical removal. In general, radiotherapy can contribute to the temporary control of the tumour growth.

Scintigraphy

Radio-isotopes are radioactive substances attached to a component that binds to a specific tissue, e.g. bone. 153Sm-EDTMP is a bone-seeking isotope that is used to treat primary and metastasized bone tumours as it is preferentially absorbed by the bone cancer cells. The treatment consists of intravenous administration of the radio-isotope.

Overall, this treatment will not lead to a cure, but rather a prolongation of life. Depending on the situation and/or treatment given, the dog’s life can be extended for months to even years.

Palliative treatment

In case of osteosarcomas, different stages and forms of pain can be experienced by the dog.

• Initially (local) anti-inflammatory drugs can be sufficient. When the tumour grows, nerve tissue can become involved which may require other types of pain relief such as morphine-based drugs (used for e.g. nerve pain). When the tumour starts to grow uncontrollably and destroys bone, bone protection such as aminobiphosphonates can be added to the pain relief plan (aminobiphosphonates is used as treatment against bone destruction, as is also the case for bone metastases in humans).
• Radiotherapy is one of the most efficient treatment options for the relief of pain caused by bone destruction in humans with cancer. This technique is also used to temporarily stun the tumour growth and thus reduce bone pain (the latter is the case for 74-93% of dogs with bone cancer). The relief is temporary and remains effective between 53 and 103 days.
• Radiopharmaceuticals are an option as well, although more information needs to be gathered on its use in dogs with metastasized bone tumours. Radiopharmaceuticals consist of radio-isotopes that emit local radiation and thus temporarily control the tumour. This pain relief technique is successfully used in humans with metastasized bone tumours.

Mental health

The mental health of the dog is a factor that should not be underestimated: offering continued attention and love, can reduce feelings of pain in humans and animals. It’s important to avoid boredom, fatigue and isolation in your dog, as this lowers the pain threshold. A regular scheme and environment, sufficient sleep/rest, environmental enrichment, company, sympathy and avoiding unpleasant environments can considerably decrease the inconvenience associated with cancer.

The prognosis depends on the chosen treatment approach and tumour location.

1. Palliative pain relief: a 2.5 months survival can be expected.
2. Radiotherapy: survival of 4 months.
3. Limb amputation: survival of 4.3 months; 12% of dogs will still be alive after 1 year; 2% after 2 years.
4. Limb amputation and chemotherapy: depending on the metastases a survival of 10-11 months can be expected; 35-50% of dogs will still be alive after 1 year, 20-28% after 2 years.

Osteosarcomas of the skull/lower or upper jaw are locally aggressive but metastasize less than an osteosarcoma on the limb (37%).

Pre-operative high blood levels of alkaline phosphatase are associated with a shorter survival.