Bone tumors can involve either the appendicular (limbs) or axial (spine, ribs, pelvis, scapula, and skull) skeleton. These tumors are classified as either primary (i.e., arise directly in bone) or secondary (i.e., spread from an adjacent site, such as multiple myeloma of the bone marrow or metastasize from a distant site, such as transitional cell carcinoma of the urinary bladder). The four primary bone tumors are osteosarcoma, chondrosarcoma, fibrosarcoma, and hemangiosarcoma. Osteosarcoma is the most common primary bone tumor and accounts for over 95% of all bone tumors. In dogs, appendicular osteosarcoma is a highly aggressive disease and curative-intent treatment involves surgical resection of the tumor followed by chemotherapy to minimize the risk of tumor cells spreading (metastasizing) to other areas, particularly the lungs and other bones.

The majority of primary bone tumors, particularly osteosarcoma, arise spontaneously with no known or apparent cause. Scottish Deerhounds are genetically predisposed to developing osteosarcoma and this tumor also occurs frequently in other large breed dogs, particularly the Rottweiler. Large or giant, and particularly tall, dogs are at a greater risk for the development of osteosarcoma compared to the general dog population, although small dogs (less than 15 kg) can also be affected. Older dogs are most commonly affected; however, bone tumors can occur in young dogs as well.

Primary bone tumors are uncommon in cats. Unlike dogs, where the majority of primary bone tumors are malignant, up to a third of feline bone tumors are benign. Osteosarcoma is also the most common bone tumor in cats, but the behavior of this tumor type is less aggressive than in dogs.

Tumors can occur at sites of previous bone damage. The types of bone damage, which have been linked with the development of primary bone tumors include fractures, orthopedic implants (used for fracture repair and total hip replacement), radiation therapy, and bone diseases (i.e., benign bone tumors, bone cysts, and infarcts [areas of bone without a blood supply]). It must be stressed, however, that the risk of developing a bone tumor after fracture, fracture repair, or total hip replacement is rare and the vast majority of primary bone tumors develop spontaneously with no apparent predisposing cause.

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