Page 546 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

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Tumors of the Skeletal System

NICOLE P. EHRHART, NEIL I. CHRISTENSEN, AND TIMOTHY M. FAN

Osteosarcoma in Dogs common locations. In the pelvic limbs, tumors are fairly evenly
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Incidence and Risk Factors distributed between the distal femur, distal tibia, and proximal
tibia, with the proximal femur a slightly less common site. Pri-
2
Osteosarcoma (OSA) is the most common primary bone tumor in mary OSA distal to the antebrachiocarpal and tarsocrural joints
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dogs, accounting for up to 85% of malignancies originating in the is relatively rare in dogs. In 116 cases of canine primary OSA
skeleton. 1–5 OSA is estimated to occur in more than 10,000 dogs in the axial skeleton, 27% were located in the mandible, 22%
each year in the United States; however, this is probably an under- in the maxilla, 15% in the spine, 14% in the cranium, 10% in
estimation because not all cases are confirmed nor recorded. The ribs, 9% in the nasal cavity or paranasal sinuses, and 6% in the
6,7
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demographics of canine OSA have been well reported. 8–19 It is pelvis. Clinically documentable multicentric OSA at the time
largely a disease of middle-aged to older dogs, with a median age of initial diagnosis occurs in less than 10% of all cases. OSA of
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of 7 years. There is a large range in age of onset, with a reported extraskeletal sites is rare, but primary OSA has been reported in
case in a 6-month-old pup and a small early peak in age incidence mammary tissue, subcutaneous tissue, spleen, bowel, liver, kid-
at 18 to 24 months. Primary rib OSA tends to occur in younger ney, testicle, vagina, eye, gastric ligament, synovium, meninges,
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adult dogs, with a mean age of 4.5 to 5.4 years. 20,21 OSA is classi- and adrenal gland. 26–31
cally a cancer of large and giant breeds. In a review of 1462 cases of
canine OSA, dogs weighing more than 40 kg accounted for 29% Etiology
of all cases and only 5% of their tumors occurred in the axial skel-
eton. Only 5% of OSA occurred in dogs weighing less than 15 kg, The etiology of canine OSA is generally unknown. Some have
but 59% of their tumors originated in the axial skeleton. Increas- speculated a viral cause because OSA can occur in litter mates and
ing weight and, more specifically, height appear to be the most may be experimentally induced by injecting OSA cells into canine
predictive factors for OSA in dogs. The breeds most at risk for fetii. However, an etiologic virus has not been isolated.
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OSA are Saint Bernard, Great Dane, Irish Setter, Doberman Pin-
scher, Rottweiler, German Shepherd, and Golden Retriever; how- Physical Factors
ever, size seems to be a more important predisposing factor than A simplistic theory based on circumstantial evidence is that,
breed. 1,2,4,8,9,13,17,19,22 A hereditary basis for the formation of OSA because OSA tends to occur in major weight-bearing bones
has been suspected based primarily on the breed prevalence of the adjacent to late closing physes and heavy dogs are predisposed,
disease as well as the subjective assessment of increased incidence multiple minor trauma and subsequent injury to sensitive cells
in some related families. With regard to gender, males are slightly in the physeal region may occur. This may initiate the disease
more frequently affected than females (1.1–1.5: 1) 2,3,9,12,13,16 with by inducing mitogenic signals increasing the probability for the
the exception of the Saint Bernard, Rottweiler, and Great Dane, development of a mutant lineage. One in vitro study comparing
and for dogs with primary OSA of the axial skeleton (except rib the incidence of microdamage in cadaver radii of small and large
and spine), in which affected females outnumber males. 2,21 Intact breed dogs found no difference between the two groups. OSA
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males and females have an increased risk for OSA. However, in has been associated with metallic implants used for fracture repair,
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Rottweilers, male and female dogs that underwent gonadectomy chronic osteomyelitis, and fractures in which no internal repair
before 1 year of age had an approximate one in four lifetime risk was used. 34–37 OSA has also been reported at the site of a bone
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for bone sarcoma and were significantly more likely to develop allograft used for fracture repair 5 years previously. Exposure to
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bone sarcoma than dogs that were sexually intact. There was ionizing radiation can induce OSA. 31,39–47 In plutonium-exposed
a highly significant inverse dose-response relationship between people, 29% and 71% of the OSAs were in the appendicular
duration of lifetime gonadal exposure and incidence rate of bone and axial skeleton, respectively, with the spine having the most
sarcoma independent of adult height or body weight. tumors (36%). An almost identical distribution of plutonium-
Approximately 75% of OSAs occur in the appendicular skel- induced OSA was reported for dogs injected with 239 Pu as young
eton with the remainder occurring in the axial skeleton. 2,21 The adults in experimental studies. This distribution of OSA is quite
metaphyseal region of long bones is the most common primary different from the distributions of naturally occurring OSA for
site, with thoracic limbs affected twice as often as pelvic limbs both species and appears to be related to bone volume and turn-
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and the distal radius and proximal humerus as the two most over. Similar findings were seen for dogs injected with 226 R.

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