Osteosarcoma, a bone tumour, usually occurs 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. Not all bone tumours are osteosarcomas. These other bone tumours are less common than osteosarcomas and may be less aggressive.

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 and palpable where the tumour is located.

Metastases are common and develop early in the course of the disease. Of note, they initially 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, micrometastases are likely present in about 90% of dogs. This tumour type usually metastasizes 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 other bones or soft tissues.

Prevalence

This tumour type is most prevalent in middle-aged to older dogs (most commonly seen at 7 years), but a (usually very aggressive) form has been reported in very young dogs (1-2 years) as well.

Breed predisposition

A higher risk for development of this tumour type was reported for large and giant breeds (Saint-Bernard, Great Dane, 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.

Some oncologists recommend thoracic X-rays every 2-3 months. If a single lung metastasis is noticed, surgical removal may be considered in some cases. Scintigraphy could also be used to identify metastases.

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, but they are usually not tumour specific.

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). The dog should be restaged every 3-6 months or when considering changing therapy.

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 very few 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).

A survey evaluated the outcomes of dogs and owner satisfaction and perception of their dogs’ adaptation following amputation of a thoracic or pelvic limb. 58 of 64 (91%) owners perceived no change in their dog's attitude after amputation; 56 (88%) reported complete or nearly complete return to preamputation quality of life, 50 (78%) indicated the dog's recovery and adaptation were better than expected, and 47 (73%) reported no change in the dog's recreational activities. Most (55/64 [86%]) respondents reported they would make the same decision regarding amputation again, and 4 (6%) indicated they would not; 5 (8%) were unsure. In general, smaller dogs will do better than large ones and recovery from amputation of a front leg will be more difficult than a hind leg.

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 (within 10-14 days) after surgery, even when no visible metastases were detected at the time of diagnosis, because this tumour type metastasizes very frequently. According to some studies, micrometastases are present in 90% of dogs at the time of diagnosis. It has been shown that intravenous treatment with chemotherapeutics positively impacts survival.

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. The primary goal of palliative radiotherapy is pain relief and not inhibition of tumour growth.

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.

Immunotherapy

Several research groups in Europe are actively investigating immunotherapy as a treatment option for osteosarcoma. This treatment option is still experimental and not widely available.

Palliative treatment

For dogs not undergoing surgery for pain relief, palliative therapy may include a combination of radiation therapy, nonsteroidal anti-inflammatories, opioids, bisphosphonates, and drugs to treat neuropathic pain.

• 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 aminobisphosphonates can be added to the pain relief plan (aminobisphosphonates are 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, but is not widely available for dogs.
• Cementoplasty is a minimally invasive procedure in which a bone substitute is injected through the skin into a region of damaged bone. Its goal is to increase the bone’s mechanical strength, to reduce pain and the risk of pathological fractures and thus provide an alternative to amputation. It offers a palliative solution, improving the quality of life of dogs suffering from osteosarcoma. This procedure is currently being investigated and is not yet widely available.

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.

For non-metastatic osteosarcomas located on the limb:

• Palliative pain relief: a 2.5 months survival can be expected.
• Radiotherapy: survival of 4 months.
• Limb amputation: survival of 4.3 months; 12% of dogs will still be alive after 1 year; 2% after 2 years.
• 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.

For osteosarcomas located on the limb with metastases at diagnosis:

• Radiotherapy and chemotherapy: median survival time of 130 days
• Surgery alone: median survival time of three days
• Surgery and chemotherapy: median survival time of 78 days

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

For osteosarcomas located:

• Distal to carpal and tarsal joints, the median survival time is 15 months after surgery alone.
• On the rib(s), the median survival time is 3 months after surgery or 8 moths after surgery combined with chemotherapy.
• On the scapula, the median survival time is 7 months after surgery combined with chemotherapy.
• On the vertebrae, the median survival time is 4 months with surgery combined with radiotherapy and chemotherapy.
• Elsewhere than bone (extraskeletal), the median survival time is one month for non-mammary locations and 3 months when the tumour is located in the mammary gland. The survival time has been reported to increase up to 5 months with surgery and chemotherapy.

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