Bone tumors in dogs can be non-cancerous or cancerous. Cancerous tumors can be classified as benign (non-spreading, local) or malignant (invasive and capable of spreading to other sites). Non-cancerous bone tumors are rare in dogs and mainly due to abnormal development. Benign tumors are also rare. Most bone cancers (80% - 90%) are malignant. Osteosarcoma is by far the most common malignant tumor, particularly in large dogs. In these animals, it is usually in the appendicular skeleton (bones of the legs, hips and shoulders) and often spreads to other parts of the body (metastasis). Approximately 5% - 10% of bone tumors occur in the inside of the nose or in the ribs, mandible and other bones.
The reason a particular pet may develop this, or any cancer, is not straightforward. Cancer is often seemingly the combination of a series of genetic and environmental circumstances that come together for the unfortunate individual. Non-cancerous bone tumors have a genetic basis. The precise cause of malignant cancers of bones in dogs is unknown, but abnormal bone cell growth and unusual hormone stimulation have been implicated. A history of previous fracture at the site of osteosarcoma can result in excessive proliferation of cells to heal the fracture giving greater opportunity for mutation to a cancerous form. Body size and sex are also important in the development of bone cancers in dogs implicating that genetic factors also have a role.
The median age of affected dogs is seven years but tumors can occur in young dogs. The youngest recorded case is a 3-month-old dog with a mandibular tumor. Bone tumors are common tumors in dogs, particularly large breed dogs. The most commonly afflicted breeds include Great Danes, German Shepherds, Rottweilers, Saint Bernards, Boxers and Irish Setters. Male dogs are more likely to develop these tumors than females. In small dogs, bone tumors are more likely to occur in bones other than the limbs (axial). Less than 50% of axial bone tumors in small dogs are osteosarcoma. Large dogs with axial tumors are usually female. Approximately 5% - 10% of bone tumors occur in the inside of the nose or in the ribs and other bones. Rib tumors are more common at an early age.
Owners often first notice their pet limping or a painful swelling over a portion of the leg. This lameness and pain are due to the tumor weakening the bone and causing microscopic fractures. All bone tumors look similar on x-rays, therefore, a biopsy is the only way to determine the type of tumor in a particular patient. In large breed dogs, lameness associated with certain sites and hallmark clinical presentations and X-ray (radiography) patterns can be assumed to be cancer until proven otherwise. Whenever your pet is showing signs of a health issue your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from a bone tumor or another serious condition, a veterinary specialist is available at an ExpertVet certified hospital.
Sometimes, the accurate diagnosis also requires microscopic examination of bone biopsy samples (histopathology). The samples for this are usually small parts of the cancer taken by special needle or surgical biopsy. The samples are specially prepared on slides and stained for microscopic observation by a veterinary pathologist at a specialized laboratory. The veterinary pathologist usually adds a prognosis (the likely outcome). This may include information on local recurrence or metastasis (distant spread). Specialized imaging such as MRI (magnetic resonance imaging) or CT (computed tomography) can demonstrate the extent of spread of the tumor(s).