Ultrasonography of Osteosarcoma in Dogs
Sonography Atlas of Dogs

5. Diagnostic work-up: metastatic disease

Approximately 10% of dogs will have gross evidence of metastatic disease at diagnosis. Fewer than 15% of cases have detectable pulmonary metastasis and less than 8% have metastasis to other musculoskeletal sites. Three view thoracic radiographs are recommended to evaluate for pulmonary metastasis; however, metastases cannot be detected radiographically until the nodules are 6–8 mm in diameter. This underscores the belief that in dogs, as with humans, the presence of metastatic disease on presentation is likely underappreciated. Advanced imaging (e.g., CT, MRI, PET/CT) may be used for patient staging [32, 33]. A thorough orthopedic examination with palpation of long bones and the accessible axial skeleton is necessary to evaluate for sites of bony metastasis. Bone survey radiographs, including lateral views of all bones in the body and a ventrodorsal projection of the pelvis, have been useful in detecting second skeletal sites of OSA [34]. Whereas whole-body bone scans are standard in human medicine, there are conflicting reports on the utility of nuclear scintigraphy for clinical staging of dogs with OSA [35–39]. It can be useful for detection and localization of bone metastasis in dogs presenting for vague lameness or signs of back pain. Although it is a very sensitive imaging modality, it is not specific for identifying skeletal tumors.

6. Prognostic factors

Tumor size, determined as an actual tumor volume or the percentage of bone length affected by tumor, has been found to be a prognostic indicator in dogs. Large tumors have been found to have a poorer prognosis [11, 27, 40, 41]. Tumor location is also a prognostic factor. In general, tumors of the mandible and scapula carry the most favorable prognosis. Other than the mandible and scapula, tumors of the axial skeleton carry a poor prognosis with survival times often less than 6 months. Tumors of the appendicular skeleton are intermediate in prognosis. Specifically, tumors of the appendicular skeleton (radius, ulna, humerus, femur and tibia) are associated with a median survival time of 1 year when treated with aggressive surgery and chemotherapy [21]. Median disease-free intervals (DFIs) and disease- free survival (DFS) times for skull OSA are 191 days and 204 days, respectively. In one study, dogs with mandibular OSA treated with mandibulectomy alone had a 1-year survival rate of 71% [18]. Dogs with maxillary OSA have been found to have a median survival time of 5 months following maxillectomy [42, 43]. A study of orbital OSA reported long-term survival following complete surgical excision [44]. Median survival time for dogs with rib OSA is reported to be 3 months for cases treated with rib resection alone and 8 months for cases treated with resection and adjuvant chemotherapy [45–48]. DFI and median survival time in dogs with scapula OSA was 210 days and 246 days, respectively, with the use of adjuvant chemotherapy increasing DFI and survival time [49]. Survival time of dogs with OSA distal to the antebrachiocarpal (equivalent of human wrist) or tarsocrural joints (human ankle) has a median survival time of 466 days, which is longer than the survival time of dogs with OSA of more common appendicular sites. However, OSA of these sites is aggressive, with a high potential for metastasis [50]. Extraskeletal OSA has an aggressive systemic behavior with a high metastatic rate, with short median survival times ranging from 1 month to 5 months [27, 28].

As in humans, elevated alkaline phosphatase (ALP) has been associated with a poorer prognosis for dogs with appendicular OSA [27, 51–54]. A preoperative elevation of either serum ALP or the bone isoenzyme of ALP (>110 U/L or 23 U/L, respectively) is associated with a shorter disease-free interval and survival. Dogs that have elevated preoperative values that do not return to normal within 40 days following surgical removal of the primary tumor also develop earlier metastasis. At least one study has suggested that tumor grade, characterized by degree of necrosis, mitotic rate and cell differentiation, is also highly prognostic, with higher grade tumors having shorter survival times and disease-free intervals [55].

7. Treatment

Cure is achieved in fewer than 15% of dogs diagnosed with OSA. For the most effective management, multimodality therapy is required to address both local and systemic disease. Amputation, limb-sparing surgeries, as well as nonsurgical techniques, such as stereotactic radiation therapy (SRT), are highly effective for management of local disease (contrary to management of OSA in humans, in which radiation therapy is rarely used). Amputation is the standard local treatment for appendicular OSA and readily allows for adequate surgical margins. It is well tolerated in dogs and offers significant improvement in pain relief and survival relative to palliative treatment. Pelvic tumors are treated with amputation and hemipelvectomy and these patients generally have good levels of function. Rib tumors are treated with thoracic wall resection; mandibular tumors are treated with hemimandibulectomy and maxillary tumors are treated with partial maxillectomy and/or orbitectomy. Limb salvage surgery, most commonly using bone allografts or metal endoprostheses, can be performed in some dogs with appendicular OSA. The most suitable patients for limb salvage are dogs with tumors in the distal radius or ulna [15], but tumors of the scapula, diaphyseal radius and ulna, metacarpus, metatarsus, diaphyseal humerus, femur and tibia and distal tibia have also been treated with limb salvage surgery [21]. Limb function following limb salvage surgery is found to be good to excellent in most dogs and survival is not adversely affected by removing the primary tumor with marginal resection. Intraoperative radiation therapy and extracorporeal intraoperative RT (IORT) have also been utilized in a small number of canine OSA patients. Stereotactic radiation therapy (SRT) is a nonsurgical alternative method which is used on an increasingly common basis for facilitating canine limb salvage. This technique can provide adequate local control of disease in the context of a life expectancy for dogs that is almost always shorter than 2 years, even with systemic therapy.

Although amputation and limb-sparing surgeries, as well as nonsurgical techniques such as SRT, have proved highly effective for management of local disease, the ability to control the progression of OSA metastases remains a clinical challenge. Systemic chemotherapy is the essential component for management of OSA metastases. Protocols that have shown significant improvement in survival include doxorubicin, cisplatin and carboplatin, with medial survival times of approximately 1 year [18, 40, 56, 57]. Lobaplatin has also been used but provides a median survival time of 7 months [21]. No survival advantage has been found when using combination chemotherapy. Despite the aggressive treatment approach, more than 50% of dogs do not live beyond 1 year postamputation and 90% die of disease by 2 years [15]. Further research is needed for discovering effective new combination therapies for improving the long-term prognosis of canine OSA.

9. Elucidating the genetic origins of osteosarcoma

The genetic and molecular origins of OSA remain poorly understood. Any given tumor may demonstrate tremendous genetic variation and complexity, characterized by abnormal genetic structural and copy number changes, irregular karyotypes and gross aneuploidy, making elucidation of common pathways more difficult [31, 63, 64]. Dogs with spontaneously occurring OSA present a unique alternative to humans for identifying conserved genetic drivers of OSA. The dog genome was mapped a decade ago and is publicly accessible via the CanFam genome browser, making genetic comparisons with the humans relatively simple [59, 65, 66]. Early genetic studies have demonstrated that canine and human OSAs are virtually indistinguishable. A study by Paoloni et al. used parallel oligonucleotide arrays to compare OSA expression signatures between human and canine samples; interestingly, hierarchical clustering could not segregate the samples by species of origin [67]. Studies such as this demonstrate the tremendous capacity for genomic analysis using comparative oncology; a number of shared molecular targets have been identified using this comparative analysis between canines and humans, including RTK MET, STAT3 and others [68–72].

The breeding of dogs has created artificially selected and refined gene pools. Some breeds of dog, such as Rottweilers and Irish wolfhounds, have also developed a reduced intra-breed genetic diversity and disproportionately higher rates of OSA [17, 73]. This presents an opportunity to examine genetic risk factors for the development of OSA. One study examining the Scottish deerhound used whole-genome mapping to find a linkage for the OSA phenotype. A novel locus was identified on chromosome 34 in a region homologous to human chromosome 3q26, demonstrating a potential genetic basis for the disease in both canines and humans [74]. Another study examined the molecular profiles of dogs with OSA in order to identify subtypes and stratify this disease based on gene expression profiling and its biological behavior. This study was able to identify groupings associated with G2/M transitions and DNA damage checkpoints that correlated with biological activity; additionally, these groupings translated to orthologous human molecular subtypes with similar biological activity [75]. Finally, Angstadt et al. used high-resolution oligonucleotide assays to identify common genome-wide copy number aberrations in both human and canine samples of OSA [76]. Their study reaffirmed a number of known aberrant genes and identified several new genes in regions of instability pointing toward possible players in the genetic origin of OSA. Taken as a whole, these studies demonstrate the capacity for genome-wide analysis and comparisons between dogs and humans to identify possible genetic precursors and drivers of OSA.

Numerous somatic genetic similarities have been identified as a commonality between human and canine OSA, which may help identify new and important insights into OSA biology. p53 and RB1, tumor suppressor genes, are possibly two of the best-known and well-described genetic aberrations in both human and canine OSA. With p53, genetic abnormalities in canine OSA are typically the result of missense mutations, which result in an overexpression of this gene and are present in 41–67% of primary OSA tumors [29, 30, 77]. While p53 is often mutated in primary cases of human OSA, it is found at a much lower rate, with only about 20% of cases showing this abnormality [78]; unlike canine OSA, mutations in the p53 gene in humans are often point mutations [30]. RB1 is a genetic abnormality that is common in human forms of OSA, estimated to be present in 30–75% of cases [79]. While initial studies questioned the putative role of RB1, more recent studies indicate that mutations of this gene are present in 29% of cases of OSA, typically resulting in a reduction or complete absence of the gene product [31, 80]. PTEN is another tumor suppressor gene that has been implicated in both human and canine OSA. Early findings in canine OSA cell lines implicated PTENdeletions and a reduction in gene expression in the development of this cancer [29]; this was confirmed using a comparative genomic hybridization (CGH) genomic analysis in primary canine OSA, which found that PTEN mutations were present in 30–42% of tumors examined [31, 76]. Human studies have similarly identified PTEN as a common mutation present in primary cases of OSA [81].

10. Advances in surgical technique and adjunct therapy

11. Conclusion: challenges and limitations

OSA remains a challenging diagnosis with little change in the prognosis for several decades. Recognizing the utility of studying animals with diseases shared by humans, Rudolf Virchow stated in the mid-nineteenth century, “Between animal and human medicine there is no dividing line, nor should there be.” Yet despite a desperate need to investigate OSA in greater numbers and scope, dogs with naturally occurring OSA were not seriously studied until the late twentieth century. Man’s best friend provides us an opportunity to study one of our most dreaded diagnoses in larger numbers than we could ever hope to study in humans alone. With similar disease features and treatment algorithms, research may be carried out in dogs in a more facile manner than in humans. Continuing to expand our understanding and awareness of canine, OSA will be a critical step in developing novel and better therapeutic strategies.

Several challenges do remain [93]. While canine models show an accelerated disease progression as compared to humans, this timeline is still significantly longer than murine models. While funding for mouse models of cancer is widespread, funding for the investigation of canine cancer remains sparse. Some novel therapies, such as species-specific antibodies and proteins, may not translate from canines to humans and vice versa. Finally, regulatory oversight and ethical care mandate that some study designs are simply not feasible in pets. Despite these limitations, this review demonstrates that canines are an ideal—and underutilized—model to deepen our understanding of osteosarcoma and to translate exciting developments from the laboratory into the clinic.