Affects: Cats, Dogs
An osteosarcoma (OS) or osteogenic sarcoma (OGS) is a cancerous tumor in a bone. Specifically, it is an aggressive malignant neoplasm that arises from primitive transformed cells of mesenchymal origin (and thus a sarcoma) and that exhibits osteoblastic differentiation and produces malignant osteoid.
Osteosarcoma is the most common histological form of primary bone sarcoma. It is most prevalent in teenagers and young adults.
Signs And Symptoms: Many patients first complain of pain that may be worse at night, may be intermittent and of varying intensity and may have been occurring for a long time. Teenagers who are active in sports often complain of pain in the lower femur, or immediately below the knee. If the tumor is large, it can present as overt localised swelling. Sometimes a sudden fracture is the first symptom because the affected bone is not as strong as normal bone and may fracture abnormally with minor trauma. In cases of more deep-seated tumors that are not as close to the skin, such as those originating in the pelvis, localised swelling may not be apparent.
Clinical Presentation: The most commonly affected bones are the proximal humerus, the distal radius, the distal femur, and the tibia, following the basic premise "far from the elbow, close to the knee". Other sites include the ribs, the mandible, the spine, and the pelvis. Rarely, osteosarcoma may arise from soft tissues (extraskeletal osteosarcoma). Metastasis of tumors involving the limb bones is very common, usually to the lungs. The tumor causes a great deal of pain, and can even lead to fracture of the affected bone. As with human osteosarcoma, bone biopsy is the definitive method to reach a final diagnosis. Osteosarcoma should be differentiated from other bone tumours and a range of other lesions, such as osteomyelitis. Differential diagnosis of the osteosarcoma of the skull in particular includes, among others, chondrosarcoma and the multilobular tumour of bone.
Causes: Several research groups are investigating cancer stem cells and their potential to cause tumors along with genes and proteins causative in different phenotypes. Radiotherapy for unrelated conditions may be a rare cause.
A small supernumerary marker chromosome or a giant rod chromosome is present in the tumor cells of low grade OS including low grade central OS and paraosteal OS (see below Variants section), carry various potentially pro-cancerous genes, and are thought to contribute to the development of these OS. (See Small supernumerary marker chromosomes and giant rod chromosomes in osteosarcomas)
Familial cases where the deletion of chromosome 13q14 inactivates the retinoblastoma gene is associated with a high risk of osteosarcoma development.
Bone dysplasias, including Paget's disease of bone, fibrous dysplasia, enchondromatosis, and hereditary multiple exostoses, increase the risk of osteosarcoma.
Diagnosis: X-rays is the initial imaging of choice to diagnose osteosarcoma. Some characteristics of osteosarcoma on X-rays are sunburst appearance and Codman triangle (elevation of bony cortex by the tumour that caused new bone formation). CT scan is helpful in defining the bony anatomy, the integrity of the bony cortex, detecting pathologic fracture, and assessing ossification (laying of new bone materials) and calcification of the cartilage. On the other hand, soft tissue and medullary cavity is better imaged by MRI scan.
Most times, the early signs of osteosarcoma are caught on X-rays taken during routine dental check-ups. Osteosarcoma frequently develops in the mandible (lower jaw); accordingly, dentists are trained to look for signs that may suggest osteosarcoma. Even though radiographic findings for this cancer vary greatly, one usually sees a symmetrical widening of the periodontal ligament space. A dentist who has reason to suspect osteosarcoma or another underlying disorder would then refer the patient to an Oral & Maxillofacial surgeon for biopsy. A biopsy of suspected osteosarcoma outside of the facial region should be performed by a qualified orthopedic oncologist. The American Cancer Society states: "Probably in no other cancer is it as important to perform this procedure properly. An improperly performed biopsy may make it difficult to save the affected limb from amputation." It may also metastasise to the lungs, mainly appearing on the chest X-ray as solitary or multiple round nodules most common at the lower regions.
Treatment: A complete radical, surgical, en bloc resection of the cancer, is the treatment of choice in osteosarcoma. Although most patients are able to have limb-salvage surgery, complications—particularly infection, prosthetic loosening and non-union, or local tumor recurrence—may cause the need for further surgery or amputation.
Mifamurtide is used after a patient has had surgery to remove the tumor and together with chemotherapy to kill remaining cancer cells to reduce the risk of cancer recurrence. Also, the option to have rotationplasty after the tumor is taken out exists.
Patients with osteosarcoma are best managed by a medical oncologist and an orthopedic oncologist experienced in managing sarcomas. Current standard treatment is to use neoadjuvant chemotherapy (chemotherapy given before surgery) followed by surgical resection. The percentage of tumor cell necrosis (cell death) seen in the tumor after surgery gives an idea of the prognosis and also lets the oncologist know if the chemotherapy regimen should be altered after surgery.
Standard therapy is a combination of limb-salvage orthopedic surgery when possible (or amputation in some cases) and a combination of high-dose methotrexate with leucovorin rescue, intra-arterial cisplatin, adriamycin, ifosfamide with mesna, BCD (bleomycin, cyclophosphamide, dactinomycin), etoposide, and muramyl tripeptide. Rotationplasty may be used. Ifosfamide can be used as an adjuvant treatment if the necrosis rate is low.
Prognosis: Prognosis is separated into three groups.
Stage I osteosarcoma is rare and includes parosteal osteosarcoma or low-grade central osteosarcoma. It has an excellent prognosis (>90%) with wide resection.