Size of animal: breeds weighing >35 kg are 60 times more at risk compared with breeds <10 kg.
Breeds 20-35 kg are 8 times more at risk compared with breeds <10 kg.
Pathophysiology
Affect metaphyseal areas of long bones. Most common site is distal radius (27%) followed by proximal humerus (27%), distal femur (14%), proximal (14%) or distal (7%) tibia. Thus the most common sites are 'away from the elbow, close to the stifle'.
Malignant tumor of bone cells.
Fast growing, rapidly metastasize to lungs , spread to local lymph nodes is uncommon.
Radiographic evidence of metastasis to other bones in 6.4% of cases at time of diagnosis.
If treated with surgery and chemotherapy, 50% of cases develop bone metastases and 50%, pulmonary metastases .
Timecourse (incubation, duration)
Untreated - most cases are euthanased withing weeks due to unremitting pain.
Thoracic radiography for evidence of pulmonary metastases . Requires good radiographic technique. Anesthetize patient to allow manual inflation of lungs during radiographic exposure. Examine left and right lateral views carefully. Dorsoventral view may be useful if abnormalities detected on lateral projections.
<10% show radiographic evidence of pulmonary metastases at time of diagnosis, but >90% will have micrometastatic disease.
Limb radiography - lesions may be primarily osteolytic or osteosclerotic or mixed (most common).
May have 'sunburst effect' and Codman's triangle (lifting of mineralized periosteum off the underlying cortex). No radiographic features are specific for osteosarcoma so a definitive diagnosis cannot be made on the basis of radiography alone.
OSA rarely crosses the joint.
Bone survey radiography (all long bones, spine and pelvis) has been shown to demonstrate evidence of bone metastases in 6.4% of all cases at the time of diagnosis.
Can present with pathological fracture . No correlation between radiological appearance and prognosis.
Lymph node
Palpate local lymph node and take fine needle aspirates for cytology or a biopsy for histopathology, if enlarged.
Histopathology
Bone biopsy - take multiple cores through central core and zone of transition for accurate diagnosis. If limb-sparing is likely, biopsy sites should be on craniolateral aspect of radius.
Use Jamshidi needle (8-11 gauge) or a trephine through a key-hole incision. Jamshidi needle core biopsy has an accuracy rate of 92% for differention of neoplasia from other disorders and an accuracy rate of 82% for specific tumor type. Once core has been obtained it should be extracted from needle (using probe) in a retrograde manner to avoid crushing sample in tapered cutting end of needle. Trephine associated with an increased risk of fracture post-biopsy.
Active tumor is in center of lesion; periphery will be periosteal proliferation.
Needle cores can also be submitted for culture to rule out osteomyelitis .
Confirmation of diagnosis Discriminatory diagnostic features
Radiography and signalment.
Definitive diagnostic features
Histopathology.
Gross autopsy findings
Swelling with bone proliferation/destruction in metaphyseal region.
Metastases in lungs, liver, kidney or bone.
Histopathology findings
Varying proportions of cartilage, osteoid and bone in spindle stroma. Must see osteoid for the diagnosis .
Excision of tumor (ie amputation) will remove source of pain. However, due to high incidence of micrometastatic disease at time of diagnosis, amputation alone is only a palliative procedure.
Survival times only increased if micrometastatic disease is controlled along with management of primary tumor. Most readily achieved by amputation with adjunctive chemotherapy. In select few cases, limb sparing surgery with adjunctive chemotherapy may be indicated and will provide survival rates comparable to amputation and chemotherapy.
Amputation should include the whole affected bone. Amputation at level of proximal humerus is advised for radial OSA and at level of proximal femur for tibial OSA. Humeral OSA requires forequarter amputation and distal femoral OSA requires amputation by coxofemoral disarticulation. Proximal femoral OSA will require amputation and hemipelvectomy to achieve clean margins of exision.
Limb amputation - most dogs cope very well (hindlimb amputation may pose fewer problems than forelimb , but both are well tolerated). Must assess other orthopedic problems BEFORE amputation to ensure that dog will tolerate surgery.
Limb salvage by specialist surgeon: resection of affected bone and replacing with cortical allograft . Only recommended for distal radial OSA.
Must be combined with chemotherapy to justify procedure.
Chemotherapy with the platinum containing drugs (cisplatin or carboplatin ) can be started prior to, or soon after, definitive surgical treatment of primary lesion. Implement standard safety precautions when handling any cytotoxic agent. Wear gloves, aprons and goggles and it is advisable to prepare the drugs over a plastic tray, with an absorbent liner, in a fume cupboard. NB These drugs, like most cytotoxic agents used in veterinary medicine, are not licensed for veterinary use.
Cisplatin :
Intravenous infusion of cisplatin 60-70 mg/m2 IV every 3 weeks for 4 doses (although ideal cumulative dose is not clear). Cisplatin has several potentially serious and life-threatening toxicities. Will cause renal failure if diuresis is not used. Renal function must be monitored with every treatment. Cisplatin causes intractable vomiting during administration. Must pretreat with butorphenol 0.4 mg/kg IM 30 min prior to treatment.
Agressive (18.3 ml/kg/h) saline diuresis needed 4 h pre-cisplatin and 2 h post. Cisplatin is administered slowly over 30 minutes and concurrently with the infusion. Pre-existing renal insufficiency is an absolute contraindication for cisplatin use.
Carboplatin :
300 mg/m2 slow bolus IV over ten minutes every 3 weeks for 4 treatments.
Similar survival to cisplatin.
Renally excreted, so renal function must be normal prior to administration.
Diuresis not required, as carboplatin is not nephrotoxic.
Cisplatin and carboplatin are excreted through the kidneys. Advise owners to keep children away from pets' urine for at least 48 hours following cisplatin or carboplatin treatment. Additionally, dog should be encouraged to defecate and urinate away from areas frequented by children during this period and gloves should be worn to clean up accidental urine or fecal spillages in the home. Waste should be double-bagged prior to disposal.
Both cisplatin and carboplatin are myelosuppressive; carboplatin more so. Neutrophils and platelets are most commonly affected, with a nadir at 10-14 days following carboplatin treatment and a double nadir at 9 and 16 days following cisplatin treatment. Check neutrophil counts prior to each dose and delay treatment if neutropenia occurs. Treat clinically significant neutropenia aggressively with broad spectrum intravenous antibiotics .
Palliative radiotherapy shows an 80% response rate/improvement in limb function within 1-3 weeks. Median response duration,130 days. Pathologic fracture may occur (10-20% of dogs) with increased weightbearing.
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Vetstream contributor(s)
Dr Laura Garrett DVM DipACVIM, School of Veterinary Medicine, Kansas State University, Manhattan, KS 66506-5606, USA.
Andy Moores BVSc CertSAS MRCVS, Department of Clinical Veterinary Science, University of Bristol, Langford House, Langford, Bristol BS40 5DU, UK.
Requires good radiographic technique. Anesthetize patient to allow manual inflation of lungs during radiographic exposure. Examine left and right lateral views carefully. Dorsoventral view may be useful if abnormalities detected on lateral projections.
No radiographic features are specific for osteosarcoma so a definitive diagnosis cannot be made on the basis of radiography alone.