Osteosarcomas in dogs: diagnosis and therapeutic options
Osteosarcomas in dogs are a malignant mesenchymal tumour of primitive bone cells that primarily affects large or giant breed dogs.1
Introduction
Osteosarcoma accounts for approximately 85% of skeletal tumours in dogs, with a prevalence of > 0.01%.1-3 It usually affects middle-aged or senior dogs (mean 7 years), although it has even been described in puppies. Some authors describe a bimodal presentation with a lower peak in incidence at 18–24 months of age and a higher one at 7–9 years.2
When it occurs in the ribs, osteosarcomas usually affect younger animals (4.5–5.4 years).1 It is a malignant tumour and frequently causes metastasis. Metastases occur early on and are often subclinical. Less than 15% of dogs have radiological evidence of pulmonary metastasis at diagnosis, but 90% die within a year of metastatic lung disease, which is the most commonly affected organ.1
Aetiology
The aetiology of osteosarcoma is poorly understood. A viral origin has been proposed (but remains unproven), as well as various factors including physical (microtrauma, metal implants, resolution of fractures without internal repair, exposure to ionising radiation), genetic and molecular aspects.1 In some breeds, osteosarcomas are suspected to be hereditary.1,2 A higher incidence has been reported in intact males and females, although in Rottweilers there appears to be an inverse relationship between length of exposure to gonadal hormones and the risk of developing osteosarcoma.2
Approximately 75% of osteosarcomas in dogs affect the appendicular skeleton, while the other 25% affect the axial skeleton. More often than not it develops in the metaphysis of the long bones of the forelimbs (twice as frequently as in the hindlimbs); especially the distal radius and proximal humerus, that is, distant from the elbow. The location seems to be more uniform in the hindlimbs. The jaw is the most commonly affected bone in the axial skeleton.
Clinical picture
Dogs with appendicular osteosarcoma usually shows signs of lameness and local inflammation. The pain is due to microfractures or disruption of the periosteum because of osteolysis of the cortical bone following tumour spread from the medullary canal.
Large dogs with lameness and localised inflammation in the bone metaphysis should be assessed for possible osteosarcoma.1,2 The clinical manifestations at an axial level depend on the area affected.1
Diagnosis
Diagnostic imaging
The initial basic diagnostic tests are plain X-rays, always from two views for the appendicular skeleton and three when studying pulmonary or other metastases.1,2 Bone lesions are usually monostotic and characteristically aggressive. Some individuals have pathological fractures due to bone lysis.1,2 Codman’s triangle is frequently observed, but it is not pathognomonic of osteosarcoma.1
A mediolateral X-ray of the left hindlimb of a dog diagnosed with femoral osteosarcoma, revealing multiple lytic areas in the distal femoral diaphysis with discontinuity of the periosteum and associated amorphous periosteal reaction.
Given the high rate of metastasis, an X-ray or, if possible, a chest CT scan is recommended to search for any tumours. Nuclear scintigraphy is the most sensitive diagnostic technique if bone metastasis is suspected.2
The signalment, history, clinical picture and radiological study can be used to reach a presumptive diagnosis, but other causes1 of lytic or proliferative bone lesions should also be considered.
Cytology and biopsy
Fine needle aspiration cytology with alkaline phosphatase staining is extremely accurate and minimally invasive in the diagnosis of osteosarcomas in dogs.2
A bone biopsy should be performed on any atypical lesions or those in unusual locations, but not as a matter of routine procedure for all cases. However, if the tumour is removed surgically, histological confirmation of osteosarcoma is essential. Unlike with soft tissue tumours, it is preferable to obtain samples of bone tumours from the centre of the lesion, avoiding the peripheral periosteal reaction that often returns samples of no diagnostic value.2
Comprehensive physical examination
The diagnostic assessment is completed with a physical examination, which should include an orthopaedic and neurological examination, especially if the intention is to amputate the limb. Haematology, blood chemistry and urinalysis are also recommended.
Prognosis
Of the multiple prognostic indicators studied in the literature, those considered most useful for the clinician as negative survival markers are proximal humerus involvement and elevated alkaline phosphatase (preoperative values > 110 U/L).1,2,3
Osteosarcoma in dogs: treatment
Surgery
The goal of treatment is the complete resection of the primary tumour and to constrain the metastatic process.
The first treatment option is amputation of the affected limb. Most dogs can enjoy a good quality of life following surgery, although the presence and severity of any prior orthopaedic or neurological problems must be taken into account.
Another option is resection and reconstruction of the affected bone segment, preserving the limb. This technique is indicated for patients who are not expected to tolerate amputation or whose owners reject it. Potential complications of limb-conserving surgery include infections, implant failures and tumour recurrence. Ideal candidates for this procedure are patients with a tumour located in the radius or distal ulna, with no evident macroscopic disease in another location, no other comorbidities and a tumour measuring less than 50% of the size of the radius. Dogs with pathological fractures or tumour infiltration around the entire circumference of the soft tissues of the limb and those with oedematous lesions are not considered good candidates as they have a high risk of local recurrence.2
Cytotoxic therapy
Surgical treatment is usually accompanied by adjuvant therapy with cytotoxic drugs as they prolong survival. The drugs used are cisplatin, carboplatin and doxorubicin, although cisplatin has fallen out of use due to its side effects.1,2 None of these agents have shown a superior efficacy over the others, nor have drug combinations proven more effective than monotherapy. Similarly, no differences in survival have been reported between patients receiving the first dose of chemotherapy preoperatively, intraoperatively or up to 3 weeks after. Therefore, the choice of the drug depends on each particular case, assessing any of the patient’s concomitant diseases, the side effects and costs of the drug.
Analgesia
One of the most important aspects of managing osteosarcomas in dogs is pain control, because the pain associated with bone destruction is one of the most unbearable.2 Nonsteroidal anti-inflammatory drugs and opiates are administered initially, but it may be necessary to add other drugs such as gabapentin, amantadine and bisphosphonates. Although they are not analgesics in themselves, their mechanism of action suppresses bone resorption by inhibiting osteoclast activity, increasing bone density and reducing pain.1,2 If available, hypofractionated palliative radiotherapy is considered the most effective method of rapid pain control, and also slows disease progression.2
Conclusions
Osteosarcomas in dogs are malignant tumours. They have a high metastatic potential, so early diagnosis is essential. As such, osteosarcoma should be included in the differential diagnosis for large breed dogs with lameness, inflammation and pain in the metaphysis of long bones.
