Osteosarcoma (OSA) is a rapidly growing, destructive neoplasm of bone that accounts for 80% of all malignant bone tumors in dogs.
Osteosarcoma is most common in older giant and large breed dogs.
- Ionizing radiation, chemical carcinogens, foreign bodies (including metal implants, such as internal fixators, bullets, and bone transplants)
- Pre-existing skeletal abnormalities such as sites of healed fractures
- Viruses such as polyomavirus, SV-40 and type C retrovirus
OSA can be found in both the appendicular and axial skeleton, and originates commonly in the metaphyses of the long bones, arising in the medullary cavity, penetrating the cortex, and extending into the subperiosteum, causing the formation of a large soft tissue mass contiguous to the bone.
Extraskeletal OSA is rare but may originate in the spleen, adrenal gland, eye, testicle, vagina, kidney, intestine, mesentery, liver, skin, and mammary gland.
OSA metastasizes primarily via hematogenous routes and rarely through lymphatics. The lung is the most common site for visceral metastases. Other metastatic sites include liver, kidneys, amputation stump, and, rarely, adjacent bones.
Clinical Signs include lameness, limb swelling and pain. Neoplasms involving the skull and nasal cavity can result in neurological deficits, dyspnea, nasal obstruction, and a bloody to purulent nasal discharge.
A prospective diagnosis can be entertained from clinical signs but definitive diagnosis requires radiography evidence of irregular lysis and periosteal new bone formation. Ultrasonography may assist interpretation but definitive diagnosis requires bone biopsy and histopathological interpretation.
Treatment include radical excisional surgery, chemotherapy, and radiation therapy. Survival without treatment is usually 2 - 6 months. Amputation is palliative but rarely increases survival time. In cases of considerable bone pain and possible pathological fracture, transtumoral plating can be palliative option.
Chemotherapy administered after amputation helps to control metastatic disease and may increase survival time significantly. The most successful chemotherapy protocol is a combination of cisplatin and doxorubicin that is administered following amputation. Novel tyrosinase kinase inhibitor masitinib has shown promise in experimental use and may be considered in future trials. Adjunctive therapies such as acemannan have also been tested with variable responses.
A poor prognosis is associated with high serum alkaline phosphatase activity, tumor extension into soft tissue, tumor origin in a hindlimb, and the presence of pulmonary metastases. The best prognosis is associated with the fibroblastic subtype of OSA that has the lowest grade of malignancy.
Dogs between 7 and 10 years of age have greater survival times than younger and older dogs.
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