Primary bone tumors in dogs

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Primary bone tumors arise directly from bone and may include osteosarcoma, chondrosarcoma, and fibrosarcoma, among other types of neoplasia. Osteosarcoma is the most common primary bone tumor in dogs, representing 85% to 98% of all bone tumors.^1,2 Bone tumors may arise in either the appendicular or axial skeleton, with 75% of tumors arising in the appendicular skeleton.^1,3 Further, primary bone tumors have a predilection for metaphyseal bone, with the most common sites being the distal radius, proximal humerus, distal femur, and proximal tibia.^1,2 Primary bone tumors most commonly present in large- and giant-breed dogs, in the middle-aged to older population, and have a predilection for male dogs.^3,4

Clinical presentation

Most dogs presenting with a primary bone tumor will have a history of acute or progressive lameness, a palpable bony mass, and varying degrees of localized pain.^1,2 Lameness may initially be subtle but can quickly progress to non–weight-bearing lameness due to bone pain or the presence of a pathologic fracture.^2,5,6

Diagnosis

A tentative diagnosis of a primary bone tumor may initially be made based on history, signalment, and physical and orthopedic examination findings. However, further evidence to support the diagnosis is achieved through local radiographic diagnosis of an aggressive bone lesion.^1,2 It is important to consider other causes for aggressive bone lesions such as bacterial or fungal osteomyelitis, relative to your regional disease risk.

A definitive diagnosis requires direct sampling of the affected tissues, either by way of fine needle aspirates and cytologic evaluation or bone biopsy for a histopathological diagnosis. Cytology has a high likelihood of correctly diagnosing bone tumors, with a 97% sensitivity for diagnosing a sarcoma and a 100% sensitivity for diagnosing an osteosarcoma when an alkaline phosphatase (ALP) stain is applied.⁷ Fine needle aspirates should be performed with image guidance when possible; samples should be collected from the center of the lesion, not at the periphery.⁸ Bone biopsy will provide the most definitive diagnosis, as it can not only determine the tumor type but also can provide histopathological grading, which can influence the overall prognosis.⁹ A pretreatment histopathological diagnosis should be considered when a cytologic diagnosis has been unsuccessful, if there is a high risk of a bacterial or fungal osteomyelitis diagnosis (as final treatment recommendations may differ), or if prognosis would impact client’s decision-making.

Beyond the local disease, further oncologic staging tests should be performed prior to considering treatment options. Staging tests are aimed to look at general systemic health, metastatic disease, and other concurrent disease processes. As most animals with primary bone tumors are older, even concurrent disease may alter a client’s decision-making regarding treatment. Baseline diagnostics including a complete blood count, a biochemistry profile, and urinalysis are recommended. The lungs, local lymph nodes, and bone should be assessed for metastatic disease. Lungs can be assessed by 3-view thoracic radiographs or alternatively by CT scan. Local lymph nodes can be aspirated for cytological evaluation. Bones can first be assessed by a thorough orthopedic examination to detect bone pain and sites of concern can be further assessed by targeted radiography. Abdominal imaging may also be considered to rule out concurrent disease processes.

Osteosarcoma is a highly metastatic tumor, with the lungs being the primary site for metastases and the lymph nodes and bone being less common sites.^1,2,10,11 Despite this highly metastatic disease, only 15% of dogs have gross pulmonary metastases at the time of initial diagnosis.^2,12 However, because of the subsequent development of pulmonary metastases following treatment of the primary tumor, it is estimated that 90% of dogs have pulmonary micrometastases at the time of diagnosis.^2,12 Local lymph node metastases are estimated to be 5% at the time of osteosarcoma diagnosis.¹³ Bone metastases have historically been reported in about 8% of dogs; however, with more advanced imaging modalities and the use of combinations of imaging modalities, bone metastases have been reported in up to 27% of dogs with osteosarcoma.^14-16

Treatment options

Treatment options can be divided into curative intent vs palliative treatments. Curative intent options include limb amputation, limb-sparing surgery, curative intent radiation therapy, and adjuvant chemotherapy, with the standard being limb amputation followed by adjuvant chemotherapy.^1,2,17 Palliative treatments are aimed at controlling bone pain and are often employed in the face of orthopedic or neurologic disease that precludes amputation, metastatic disease, or when treatment is not acceptable to a client for personal reasons. Palliative treatment options may include limb amputation alone, palliative radiation protocols, analgesics, aminobisphosphonates, and metronomic chemotherapy.^1,2

Prognosis

Prognosis for osteosarcoma is overall poor, but survival times vary based on prognostic factors and treatments elected. Increasing age; elevated serum ALP; and proximal humeral, distal femoral, and proximal tibial tumor locations are all negative prognostic indicators for mortality and metastases.^18,19 Dogs with preoperative metastatic disease have a median survival time (MST) of 76 days, with those having bone metastases experiencing increased survival time compared with those with soft tissue metastases.²⁰ Unfortunately, lymph node metastases hold an even worse prognosis with an MST of 59 days compared with an MST of 318 days for dogs without lymph node metastases.¹³

When comparing treatment outcomes, dogs treated with amputation alone have an MST of 134 to 175 days, whereas dogs treated with amputation and adjuvant chemotherapy have a MST of 258 days to 18 months.^21-24 Treatment with limb-sparing surgical techniques followed by adjuvant chemotherapy has resulted in similar survival times (MST = 289-430 days) compared with amputation and chemotherapy.^25,26 However, when dogs experience surgical site infections associated with their limb-sparing surgery, MST has been reported as being increased.^25,27,28 Further, dogs that undergo a secondary limb amputation following a limb-sparing surgical technique have a further increased reported MST of 604 days.²⁹ Comparatively, palliative radiation therapy has a much-reduced MST of 125 to 312 days, with pain relief lasting 59 to 129 days.^30,31

Although the prognosis for osteosarcoma remains poor, research on immunotherapies, biotherapies, chemotherapies, and advanced surgical treatment options is ongoing. For the best outcomes, treatments must be aimed at local disease control and distant metastatic control.

Katie Hoddinott, DVM, DVSc, BSc, DACVS-SA, is originally from Nova Scotia, Canada. She graduated with her DVM from the Atlantic Veterinary College, University of Prince Edward Island, in 2012. She then completed 2 internships and a surgery residency at the Ontario Veterinary College, University of Guelph. She became a diplomat of the American College of Veterinary Surgeons – Small Animal in 2019. Hoddinott is currently working at the Atlantic Veterinary College as an assistant professor in small animal surgery, where she enjoys teaching undergraduate veterinary students, interns, and residents. Her professional interests lie mainly in surgical oncology and minimally invasive soft tissue surgery. Her current research focuses include advances in clinical teaching for surgery residents and surgical site infections.

References

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